- What criteria do conservators use to determine how and when to treat an artifact?
- Horn, tortoiseshell, baleen
- Rubber and plastics
- Textiles, paper
- Composite artifacts (made of more than one material)
- Architectural materials, brick, stone
- Environmental remains
The criteria that conservators commonly use to decide on treatments are as follows:
- The treatment must not cause the object any further damage. Whether a treatment will damage an artifact is dependent on the treatment materials used, the skills with which it is carried out and the condition of the piece. It is particularly important to remember that a treatment that may be fine for one artifact may be extremely detrimental to another artifact made of the same material.
- The treatment should be as reversible as possible and should cause minimum intervention to the structure and informational content of the artifact.
- Treatment should not impede future analysis if possible.
- All of the treatment materials should be at least as stable as the artifact itself.
Other factors that influence conservation decisions are:
- The intended use or disposition of the artifact.
- The skill and knowledge of the conservator.
- The treatment facilities available.
- A conservator usually chooses a method or combination of methods taking into consideration all these factors for each object.
The general characteristics of metals are that:
- They are ductile-they can be deformed easily
- They transmit heat and carry an electrical current
- They can be melted and cast into a shape
- They can be melted, sprayed , electrically plated or rolled onto each other to create sandwich-like coatings, such as tinning, silver plating and gilding
- They can be mechanically joined to each other by folding or riveting, and they can also be joined together by soldering.
- Two or more metals can be blended together to alter the properties of a given metal creating an alloy. For example, copper is often found alloyed with silver to make the silver less ductile.
Individual metals have individual characteristics that are important in their identification. For example, iron is magnetic and lead is unusually dense.
The condition of archaeological metal is complex and varies widely, depending on the nature of the metal or alloy and the conditions in the burial environment. Metals are rarely found in a pure metallic state. They are most often encountered naturally in the form of ores (metal ions that are bonded with other materials such as oxygen, water, sulphur containing compounds or hydroxides). Metals are extracted from ores by a process known as smelting in which heat is used to break down chemical bonds. The newly produced metal is not very stable. It will begin reacting with oxygen, water and other environmental contaminants almost immediately. This process (the reversion of the metal form to the more stable ore form) is known as corrosion. Different metals corrode at different rates. Some metals, known as base metals, corrode more rapidly than others, known as noble metals. Corrosion products come in many forms and colors. They can occur as a thin, distinct layer or patina, which is usually stable. They may also occur as voluminous, hard, or powdery accretions and can be associated with pitting and surface loss. Rough and uneven corrosion layers on archaeological metals are not necessarily indicative of active corrosion but areas of flaking and cracking often are. Archaeological metals are often quite brittle as a result of corrosion. Oxygen, chlorides, sulphides and acidic conditions are all damaging to archaeological metal and its alloys. The degree of preservation is dependent on the amount and the combination of these variables in the burial environment. Changes that occur during the corrosion process often alter and obscure the original surfaces of the artifact. However, original surface details, applied materials (such as silvering or gilding) and organic remains (or pseudomorphs of organic materials) may be preserved within the corrosion layers. It is important to take this into consideration when developing a treatment approach so that important information within the artifact is preserved.
X-Radiograph and image by H. Wellman. Used by permission of the Maryland Archaeological Conservation Laboratory
Careful examination and/or analysis of the corrosion products can reveal important information about the burial environment, the nature of the artifact, and its state of preservation. X-Radiography can be a very useful tool in identifying technological information and the condition of metal. Careful investigative cleaning and microscopic examination are important tools used to aid in understanding the nature of the artifact and its state of deterioration. These techniques may also reveal the presence of surface detail and preserved organic remains within corrosion layers. Understanding the complex relationship of these factors will affect conservation treatment decisions.
Treatment methods are usually determined by the stability of the artifact and the end goal of treating the artifact. Decisions are based on the analytical value of the artifact and its relationship to the assemblage as a whole. Treatment decisions are also based on whether the artifact will be placed in storage and what the storage conditions will be. Other considerations to take into account are whether the artifact will be part of a study collection or whether it will go on display. All of these factors should be part of an ongoing collaboration between archaeologists, conservators and curators or other museum professionals.
The primary components of metals treatments are usually examination, corrosion removal, desalination, the application of corrosion inhibitors and the application of moisture barriers. Corrosion removal tends to be an aesthetic decision as much as it is a research tool. There are two approaches that are often taken. Each has pitfalls. One approach is to remove all the corrosion layers and attempt to stabilize the remaining metal. This can be problematic because valuable information is often preserved in the corrosion layers and is lost in this technique. Another is to maintain and stabilize the large part of the corrosion layers while using limited corrosion cleaning to enhance the appearance of the piece and reveal information trapped in the corrosion layers. With either approach, corrosion removal must be controlled. Corrosion removal is often done using mechanical (i.e. with hand tools), chemical or electrolytic methods. Chemical and electrolytic methods can be more intrusive and can potentially change the character of the metal and result in the loss of important archaeological and technological information. Mechanical methods may also be damaging if a metal is soft. Depending on the stability of the artifact, metal artifacts may be desalinated, treated with a corrosion inhibitor , and/or given a protective coating . Desalination is necessary for all iron excavated in the Mid-Atlantic area but is not always necessary for other metals, unless they come from marine environments. Metals may also be stabilized using appropriate preventive measures, such as storage in controlled environmental conditions with appropriate enclosures.
All metals should be stored using archival quality materials. It is important to avoid using acidic materials. Generally, archaeological metals should be stored in low RH conditions (see How do I store different types of excavated materials? and How do I create a desiccated microclimate for storage?). Actively corroding metal should be stored in RH below 35%. Iron artifacts should be stored in an RH of 20% or lower. Artifacts should be kept in closed containers or spaces with a desiccant to control the RH and to limit accumulation of dust on the surface of the metal.
Archaeological iron is perhaps one of the most difficult materials to treat due to the size and number of artifacts made with this material, as well as to the peculiarities of the material itself. Iron corrodes (rusts) when it is exposed to water and oxygen. This process is catalyzed by the presence of salts. During burial, iron objects are changed partly or entirely to corrosion products, which can incorporate other materials and obscure the object’s original appearance. Significant traces of the original surface may lie within this corrosion crust, though they can be more difficult to reveal than with other metals. The purpose of treatment is to prevent further deterioration and to reveal technological information. Iron treatments are usually assessed based on their ability to remove agents causing deterioration (in the case of iron, these are mainly chloride salts), their ability to reveal and preserve the technology of the artifact and their ability to protect the iron from further corrosion.
Prior to treatment a number of factors must be considered in order to determine the condition of the artifact. These include: determining how the piece was constructed, assessing whether it was made of cast or wrought iron, determining whether other materials were also employed in the construction, and assessing the degree of corrosion that has taken place on the artifact. X-radiography is a powerful technique for addressing these questions and most conservators employ it as a first step. X-radiography will show whether cracks or casting flaws, are present, it will give a good indication of how much of the core metal remains in the artifact, and will reveal applied materials such as gilding, paint, enamel or tinning that may be obscured by the corrosion layers. Because of the rapidity with which iron corrodes there is a high probability that organic materials may have become incorporated into the corrosion products. These materials often provide valuable information about the way in which the artifact was used, the materials buried with it, and the burial environment itself. The presence or absence of mineral preserved organics can only be assessed by careful examination of the surface of the artifact under low magnification. Occasionally this assessment must be combined with small amounts of corrosion removal. Another important component of iron conservation is determining the free chloride content of the artifact as chlorides contribute to most of the forms of “run away” corrosion of iron.
All these condition factors will help to determine how the object needs to be treated in order to stabilize it and preserve the information it contains. If mineral preserved organics or applied materials are present on the surface of the piece or if the object is in delicate condition the treatment that is adopted will most likely take the least intrusive and most passive approach possible. If, on the other hand, there are no applied materials or mineral preserved organics on the surface and the piece is in good condition overall, it may be possible to adopt a more interventive and/or aggressive approach.
Electrolytic reduction is a conservation treatment that can be used on some metal artifacts. It is commonly used on iron excavated from marine environments, but it has occasionally been used on other metals, including lead and silver under special circumstances. It should not be used on all metal artifacts, particularly highly corroded ones, as it has the potential to be a very damaging treatment if used incorrectly or inappropriately.
The corrosion of metals in an archaeological environment occurs as an electrochemical process. When dissimilar metals come into electrical contact in the archaeological deposit, less noble metals corrode preferentially to more noble metals. (This process also occurs on a microscopic level within metal alloys). Serving as the anode, the less noble metal donates electrons to more noble metal, or the cathode. This exchange of electrons is similar to the flow of electricity through a battery and can result in a variety of complex corrosion products.
Electrolytic reduction (ER) is a process that reverses the flow of electrons in the galvanic cell, ultimately converting corrosion products to a more stable or easily removed form. In short, the electrolytic reduction treatment involves establishing a galvanic cell in which the archaeological metal serves as the cathode, and a more noble metal serves as the electron donor, or anode. A positive rectified current is induced in the anode and a negative rectified current in the cathode while both are immersed in a conductive medium or electrolyte. A control panel allows manipulation of the current flow that the anode and cathode receive. Variables include the condition of the artifact, the electrolyte used, the anode metal chosen, the configuration of the anode relative to the artifact surface, the method of establishing electrical contact between the artifact and the anode, the intensity of current applied and the regularity of monitoring. Control over these variables is vital as electrolytic reduction has the potential to damage artifacts irreversibly if carried out in an uncontrolled manner.
During electrolytic reduction, a number of different processes occur:
- Corrosion products are converted, or reduced, to a more stable form (such as magnetite in iron) or to a metallic form (in the case of lead and silver). If the metal artifact is highly corroded this conversion may result in the loss of surface detail, an undesirable consequence. Metallic products that are formed on the surface of the artifact are sometimes beneficial and aesthetic, at other times they can obscure surface details or can be somewhat unnatural looking (such as those formed when silver is electrolytically reduced).
- Negatively charged chloride ions are attracted from the object to the anode, thus removing them from the artifact. Chloride ions are a particularly aggressive form of ion that contaminate many buried metal artifacts, especially those recovered from marine environments. If left within the artifact, chlorides can react with moisture and oxygen to form acidic byproducts, which cause further cyclic corrosion. Removal of chloride ions is often the main aim of an electrolytic reduction treatment.
- Hydrogen gas in the form of bubbles is released at the surface of the metal core of the artifact, beneath the corrosion layers. This reaction can help to mechanically loosen marine encrustations on artifacts from marine sites. However, it can be very damaging to heavily corroded artifacts that have only residual core metal. To preserve the corrosion layers in such cases, the hydrogen bubbling must be carefully limited through control of the applied current (or an alternative treatment may be chosen).
Electrolytic reduction can be a useful tool in cleaning and stabilizing metals recovered from marine archaeological sites. Iron artifacts from marine environments contain extremely high chloride levels and often have surfaces that are too fragile to clean by other mechanical methods.
Electrolytic reduction is economically feasible and can be accomplished with minimal health and safety risks. However, the risks to the artifact must not be taken lightly. Electrolytic reduction can be an aggressive treatment and is not recommended before taking many considerations into account and weighing the alternatives. Before beginning an ER treatment of iron, it is important to determine the extent of the corrosion, the degree of chloride contamination, and the reason for removing the corrosion layers. ER is not recommended for highly corroded artifacts or for general removal of surface rusting. If not implemented properly, electrolytic reduction can cause structural damage, such as loss of surface details, and ruptures along existing internal flaws, and can generally leave a pitted, stripped metal surface. Thus, it is not recommended that electrolytic reduction be undertaken without careful consideration of all the factors involved. Consultation with a conservator is strongly advised.
It is unnecessary and often highly undesirable to polish archaeological metals. Even modern or antique metals such as a silver service should not be regularly polished, as this action eventually wears down the surface, removing engravings, imprints, tool marks, and other details.
Metals that have been in a burial environment have changed considerably from their original condition, both chemically and physically. They have lost some or all of their original metal to corrosion. They have acquired corrosion products that may protect the object or provide information about the composition of the object or its burial environment. The corroded surface may preserve traces of organic materials that were originally part of the object, such as a wooden handle for a tool, or the leather scabbard of a sword. Therefore, quite often, it is best to leave corrosion products in place.
A conservator may remove corrosion and other deposits from the surface of a metal object in order to stabilize the object, expose surface details, or gain more technological information. Often, however, only a portion of the corrosion is removed. If a metallic surface is exposed, it may be weaker, pitted, and of a different shape and composition from the original metal (for example, enriched or depleted in a certain element) due to corrosion. It may contain faintly preserved tool marks or other traces of manufacture or use. Polishing (or any kind of aggressive cleaning) changes the character of the surface, causes loss of metal surface and original detail, and may leave chemical residues on the metal. It is not expected that an archaeological metal will have the appearance of a modern, shiny metal. The corroded appearance of an archaeological metal is part of its history, and should not be altered by polishing.
Typically a corrosion inhibitor is a chemical that reacts with the surface of a metal to create a protective coating. This coating is unlike the one formed by a plastic or wax, as it is formed by a chemical reaction between the inhibitor and the metal ions at the surface of the object. In other words the protective coating becomes part of the surface of the object.
Two corrosion inhibitors used commonly in archaeological conservation are benzotriazole (BTA) and tannic acid. Benzotriazole (BTA) has proven to be very successful for stabilizing copper alloys. Although BTA has been in use since the 1960’s, research is ongoing to find out exactly how this inhibitor actually works. The one disadvantage of using BTA is that it is a suspected carcinogen, so safety precautions should be taken when treating the object and handling it after treatment. BTA is also useful for stabilizing silver objects that have been alloyed with copper. Tannic acid is used to inhibit iron corrosion. Brushed onto the surfaces of iron artifacts, it inhibits corrosion through the formation of an insoluble iron tannate complex on the surface of the iron.
Canadian Conservation Institute (1997) “Tannic Acid Treatment.” CCI Notes 9/5, Canadian Conservation Institute: Ottawa.
Madsen, H. (1985) “Benzotriazole: A Perspective.” UKIC Occasional Papers No. 4- Corrosion Inhibitors in Conservation: pp. 19-20. Merk, L. (1981) “The Effectiveness of Benzotriazole in the Inhibition of the Corrosive Behavior of Stripping Reagents on Bronzes.” Studies in Conservation 26:73-76.
Skerry, B. (1985) “How Corrosion Inhibitors Work.” UKIC Occasional Papers No. 4- Corrosion Inhibitors in Conservation: pp. 5-12. Turgoose, S (1985) “Corrosion Inhibitors for Conservation.” UKIC Occasional Papers No. 4-Corrosion Inhibitors in Conservation: pp. 13-17.
Moisture barriers are coatings that are applied to some conserved metal objects to protect the surface from water vapor. Additionally, they usually offer some protection against the deleterious effects of dust and air pollution. Examples of moisture barriers are waxes (such as microcrystalline wax and carnuba wax) and acrylic resins (such as Acryloid B-44, the main resin component of Incralac lacquer, and Acryloid B-72). Waxes are less efficient at protecting an object from moisture vapor and some research indicates that in humid regions such as the Mid-Atlantic, waxes may fail (resulting in pin prick corrosion) up to four times more often than acrylic resins, when all other aspects of the treatment were identical. Use of a moisture barrier with archaeological metal does not negate the need to provide low humidity storage, since even the best coating is slightly permeable. When using a moisture barrier with archaeological iron in particular, one should always store the object in a low humidity environment in order to maximize the efficiency of the coating.
Almost all coatings fail over time and will require reapplication at some point. The failure rate will depend on a number of factors including the way in which the coating was originally applied, the amount of light the coating receives, the amount of moisture in the environment, and whether the coating is abraded by handling or other wear. Additionally, the efficacy of the moisture barrier depends in large part on the quality of its application-small pinpricks or gaps in the coating or areas of excessive pooling will cause the coatings to fail faster. A visibly failing coating should be removed, as it can cause further problems by setting up differential corrosion cells through the presence of areas of exposed surface next to areas of coated surface.
The application of a moisture barrier is not a substitute for necessary conservation treatment; a coating applied to an untreated metal artifact will almost certainly fail and thereby exacerbate corrosion problems.
Johnson, R. (1984) “The Removal of Microcrystalline Wax from Archaeological Ironwork.” Adhesives and Consolidants IIC: London. pp. 107-109.
Keene, S (1984) “The Performance of Coatings and Consolidants Used for Archaeological Iron.” Adhesives and Consolidants. IIC: London pp. 104-106.
Pascoe, M. (1982) “Organic Coatings for Iron: A Review in Methods.” Conservation of Iron: National Maritime Museum Monograph, No. 53:56-7.
Williams, E. (2002) “Conservation Assessment of the Archaeological Collection at Colonial Williamsburg.” Journal of Middle Atlantic Archaeology 18: p97-103
All ceramics are made of fired clay. Kaolinite (Al2O3.2SiO2.2H2O) is the most predominant clay mineral, but other minerals commonly found in clay are quartz (SiO2), feldspars, calcite (CaCO3), iron compounds such as iron oxide (Fe2O3), as well as other materials such as shell (CaCO3) and straw. Fillers and fluxes are added to clay to help with shape retention and control the hardening of the body. Different types of ceramics are distinguished from one another by the initial mineral content of the clay, the fillers and fluxes used, and the conditions and duration of firing.
Photos by M. Myers. Used by permission of the Virginia Department of Historic Resources
Before firing, a ceramic must air-dry so that most of the water has evaporated from the body. Once sufficiently dry, it is ready for firing, which may involve one single firing or a series of firings to achieve different effects. The hardness of the ceramic body is determined by the temperature at which it is fired-the higher the temperature, the harder, or less porous, the body. Glazes are often applied to earthenware ceramics in order to make them impermeable to liquids. A glaze is essentially a thin layer of glass that is fired onto the body of the ceramic. In addition to their functional use, glazes are often highly decorative and a number of colors can be achieved by the addition of metal oxides to the glaze. Ceramics may also be decorated prior to firing by painting, by gilding, by scoring and by applying a slip of diluted fluid clay to the surface of the body.
Generally, the most common problems seen in archaeological ceramics are cracking and flaking of the surface (either the paint or glaze), crumbling of the ceramic body, salt damage, and staining from metals or other associated materials. Often these problems are seen in ceramics with more porous bodies rather than harder ceramics. For example, surfaces that are especially prone to flaking are those found on low-fired, porous ceramics, such as delftware, due to the fact that the surface glaze is much harder than the body. Cracking and spalling of the harder surface will occur as temperature and relative humidity fluctuate, both within the burial environment and later in storage. Under-fired or low-fired ceramics are also prone to crumbling in moist soil as the ceramic body begins to re-hydrate back to clay.
Soluble salts perhaps have the greatest potential for damaging ceramics. Porous earthenwares are most likely to be affected by soluble salts, as opposed to harder ceramics, such as porcelain. When salt-contaminated ceramics are excavated from a moist burial environment, as the ceramic dries, the salts crystallize in the artifact. Damage occurs as the crystals grow and expand within the pores of an artifact, exerting great pressure. The repeated dissolution and recrystallization of soluble salts during periods of fluctuating relative humidity in artifact storage is particularly damaging (See Question 4c for further information about soluble salts).
Again, porous ceramics are most likely to be affected by staining from associated materials, such as iron. This staining will not cause any further damage to the ceramic itself, but may obscure any decoration and make the artifacts less pleasing aesthetically.
Ceramic vessels and shards are generally robust and are not at risk of being damaged during or after excavation. There are, however, several instances where fragile ceramics must be lifted using additional supports, and specially packaged in order to prevent them from incurring further damage until they can be cleaned and stabilized.
Important points to remember:
- Ceramics with flaking glaze or paint must be lifted carefully and the attached soil should remain on the surface until the ceramics can be cleaned in a lab.
- Low-fired ceramics, especially those from wet contexts, may be fragile and must be lifted using supports. There are various effective techniques that can be used for a lift, which should be carried out by a conservator or after consultation with a conservator, to ensure that the proper materials and techniques are being used (see Question 3c and Lifting below).
- Porous ceramics from wet contexts must not be allowed to dry out until they are assessed for soluble salt content. If the ceramics are known to contain soluble salts, contact a conservator to determine a proper treatment plan (see question 4ci-v).
- If a vessel is complete, it must be lifted with the contents intact. Once lifted, the contents can be excavated separately.
- Package fragile and damp/wet ceramics carefully for transfer to the lab. Use acid-free tissue, Ethafoam and polyethylene bags to package fragile, dry ceramics. If ceramics are only slightly damp, store them in perforated bags so that water does not condense inside the bag.
- Never use acid-free tissue to package damp/wet ceramics, because as the tissue dries out it will stick to the ceramic, causing further damage later. Use damp Ethafoam to keep damp/wet ceramics wet temporarily during transfer to the lab where they can be properly cleaned and dried.
Important points to remember for a lift (defined as any process for removing the artifact from the excavation context with special support):
- A lift should be conducted either by a conservator or by an archaeologist trained in lifting techniques.
- Always remember to use a barrier to protect the ceramic from the supporting material (polyethylene cling film or aluminum foil will work).
- Before executing a lift, make sure that there is a plan and that all necessary materials are readily available. These include the materials needed for the lift as well as the packaging materials needed for transporting the ceramic from the field to the lab.
Most ceramics can be cleaned gently with soft-bristled brushes and deionized water. However, DO NOT clean ceramics with brushes and water if they have very soft, crumbling bodies or if they have flaking paint, gilding or glaze. Consolidation may be recommended by a conservator. Never attempt to remove stains on ceramics. Stains cannot be removed without chemical cleaning, and often this will result in further damage.
Do not attempt to mend ceramics that are particularly fragile or are crumbling and weak. For information on mending and reconstructing ceramics, see question 6ci-ii.
Once clean and dry, ceramics do not require very strict storage conditions. Damage occurs when ceramics are stored before they are completely dry or without being desalinated. Like all other materials, ceramics should be kept in an environment free from fluctuations in RH and temperature, and free from vibrations. Handling also poses a great risk to ceramics, and all sherds and artifacts should be handled with clean hands over a padded surface to avoid staining and damage.
Buys, S. and V. Oakley. (1993) Conservation and Restoration of Ceramics. Oxford: Butterworth-Heinemann.
Mended ceramic sherds being supported during the adhesive cure in a box of wax beads. Photo by M. Myers used by permission of the Virginia Department of Historic Resources.
The most important factor in choosing any adhesive to use on archaeological artifacts is the long-term stability of the adhesive. Adhesives that discolor, become brittle, fall apart, or cannot easily be reversed (re-dissolved in order to undo the join) are not appropriate for archaeological materials. Many adhesives readily available from hardware stores, craft stores and other suppliers may be appropriate for fixing a broken coffee mug or other household item where preservation is not a factor but they are not appropriate for use in museums or other cultural institutions. Many such adhesives have been used on archaeological ceramics in the past and have caused more harm than good.
Adhesives for all archaeological artifacts should be conservation-grade and should be purchased from conservation suppliers for the following reasons:
Mended ceramic sherds being supported during the adhesive cure in a box of wax beads.
- The adhesives sold by conservation suppliers tend to be purer mixtures than those sold elsewhere. They contain only the resin and solvent whereas many commercial adhesives include plasticizers and fillers that may not be archivally stable over the long term.
- Most commercial manufacturers use proprietary formulas to make up adhesives. These formulas may change from batch to batch with no warning so that an adhesive that may once have been a suitable material for preservation use is no longer suitable. Unless you have the resources to test each batch for suitability prior to use, it is better to avoid commercially manufactured products altogether.
Another factor in choosing an adhesive for ceramics is that the adhesive should not be stronger than the ceramic fabric. If there is going to be a mechanical failure, or physical shock to the object, you want the adhesive to fail at the old join before the ceramic cracks in a new place.
Cellulose nitrate adhesives (Duco Cement, HMG Blue Tube) tend to be stronger than most earthenwares. They also do not age well – they darken over time, become brittle, and become less soluble. Cyanoacrylate adhesives (such as Super Glue) are also very strong and are not stable in the long term (many were developed for short term purposes such as adhering skin together after operations). Cyanoacrylate adhesives are not recommended for use on artifacts.
Epoxy adhesives create very strong bonds, yellow as they age, and are difficult to reverse. Conservators tend to avoid using them, except in special circumstances such as mending glass and porcelain, where there are no pores to help the ceramic bond. In such cases conservation-grade varieties, such as HXTAL or Epotek 301, are used. Epoxy adhesives should never be used on earthenwares or low fired ceramics.
Acryloid B-72 (also called Paraloid B-72) is the preferred conservation adhesive for most conservators, since it does not change color, and remains easily reversible with an appropriate solvent. Solidified B-72 does begin to “flow” at 40° C (104° F) so in very hot climates alternatives may be used to avoid the collapse of mended areas.
Acryloid B-72 is sold by conservation suppliers in the form of solid beads which can be custom-mixed with solvent or as premixed adhesive in tubes. Although it is possible to make up one’s own adhesive by dissolving solid resin pellets in a suitable solvent, unless one is using very large supplies of adhesive it is often easier and better to simply purchase pre-mixed tubes. The company Herbert Marcel Guest (HMG) makes a number of conservation adhesives. Acryloid B-72 is sold by HMG, available through Conservation Resources International in a purple tube.
Gap-filling is a technique that was once widely used by archaeologists to prepare objects for storage or exhibit. It appears to be used less often now, particularly in historical archaeology, because of the importance many archaeologists place on being able to study the body/ fabric of a ceramic vessel or item. The advantages of gap-filling are that it allows an artifact to be considered as a whole, which is useful for exhibits and for study collections, and it allows support to be provided to otherwise unsupported shards or those that have very small points of contact with each other. The disadvantages of gap-filling are that if it is done poorly it can cause significant damage to a piece, and that reconstructed ceramics require more space to store than shards.
A number of materials are used for filling ceramics depending on the size of the piece, the span of the fill, any texturing on the surface, and the body/ fabric of the ceramic. Plaster of Paris is one of the preferred materials for gap-filling ceramics, especially for earthenware and stoneware; dental plaster is often recommended as it is uniformly fine-grained and does not contain lumps or other impurities. Proprietary calcium carbonate and calcium sulphated- based fillers such as Pollyfilla and Hydrocal are also used. These materials remain workable for longer periods than plaster of Paris, allowing the material to be wet-modeled to a greater extent than with plaster. Additionally they are generally a little softer when dry allowing pieces to be sanded, shaped and cut back more easily. Resin putties or epoxy putties, such as Finebond, are sometimes used for porcelain and harder paste ceramics as they allow for finer molding and modeling and tend to match the translucency and luster of the ceramic better. The color of the fill may be matched to the color of the ceramic body or glaze either by mixing dry pigments into the fill medium or by in-painting the fill once it is dry. It is important to ensure that the fill materials and in-painting never cover the surface of the ceramic and that any techniques used to shape the fills do not cause abrasion to the ceramic. One useful technique for gap-filling, that allows any shaping work to be done away from the ceramic itself, involves creating a separate shard fill that precisely fills a gap and is then mended into place as if it were another potsherd. When in-painting, conservators often use an approach known as the “Rule of Six feet /Six inches,” meaning that when an object is seen from six feet the colors blend well and the object is read as a whole, but at six inches it is easy to discern the fill.
Making a successful gap-fill is time-consuming. It calls for patience and practice in mixing, molding, and shaping the final product. It is also necessary to exercise care in choosing materials and to ensure that the materials are sympathetic to the piece.
Buys, S & Oakley, V. (1993) Conservation and Restoration of Ceramics. Butterworth-Heinemann: London.
Koob, S & Sigel, T (1997) “Conservation and Restoration Under Field Conditions: Ceramics Treatment at Sardis, Turkey” Objects Specialty Group Postprints 5: 98-115
Koob, S (1987) “Detachable plaster restorations for archaeological ceramics.” In Black, J. (compiler) Recent Advances in Conservation and Analysis of Artifacts, Institute of Archaeology Jubilee Conservation Conference Papers. Summer Schools Press: London. Pp. 63-67.
Photo by M. Myers. Used by permission of the Virginia Department of Historic Resources
Glass is a random amorphous material made by fusing silica (SiO2) with an alkali, usually either sodium oxide or potassium oxide, and lime. The most common glass type is soda-lime glass, but there are other types of glass, including lead glass, which contains at least 20% lead oxide. Glass may be colored by the addition of metal ions (cobalt, copper, manganese and iron). Deterioration
Factors that can accelerate decay in glass include: the type of glass and the manner in which it was manufactured, the amount of moisture, alkalinity, and salts in the burial environment and physical wear or abrasion prior to burial.
Water is the main cause of environmental decay. As water reacts with the glass, causes ions to leach out opening up the glass network and eventually leading to dissolution of the glass structure. Highly acidic or highly alkaline environments will also cause glass to decay.
The most common forms of visible decay seen in glass are listed below:
- Iridescence: Glass decays from the outer layers inward, forming thin, onion-like layers that appear opaque and/or different in color from the original glass because the way in which the light is refracted through the glass has been altered.
- Exfoliation/Spalling: As the decayed layers separate, they flake off, causing the original surface of the object to be lost.
- Devitrification: Deterioration resulting in a crystalline and iridescent appearance.
- Pitting: Small isolated holes or indentations in the glass surface or on broken edges.
- Surface Abrasions: Damage to the glass surface as a result of use or burial.
- Cracking: Cracks running throughout the glass structure.
Although most glass can be lifted without additional support, during excavation, treat all glass as though it is fragile. Remove as much loose dirt from around the artifact as possible before lifting it. If there is dirt sticking to the glass surface, do not try to remove it in the field, as it may be helping to hold fragile layers of the glass together.
If glass artifacts are found dry, pack them inside sealed polyethylene bags in well-padded containers for transport to the lab. If glass is found wet, it must not be allowed to dry out or else the surface layers will begin to shrink and delaminate. Do not attempt to remove dirt on the surface of wet glass. Store wet glass in sealed polyethylene bags and transport the glass as soon as possible to a lab. An alternative solution is to store wet glass in punctured, sealed polyethylene bags in plastic bins containing damp polyethylene foam. Store these containers of glass in a cool place until they can be properly cleaned in a lab. Glass should not be stored wet for a long period of time.
If the glass is dry and in good condition, dirt can be removed with a soft, dry brush and soft wooden picks. Dirt removal is easiest while the soil is still damp. If the glass is very decayed, iridescent, or exfoliating, do not attempt to remove the dirt, as the outer layers of glass may flake off during cleaning. The opaque layers of glass may contain decorative elements, such as hand painting and etching. They also preserve the original size and shape of the glass piece. Removing these outer layers of glass, irrevocably alters the surface of the piece and removes important information.
Iridescent, weathered glass, either wet or dry, may need to be consolidated to prevent the opaque layers from exfoliating further. Consolidation should be left to a conservator, or undertaken only after consultation with a conservator. Consolidation reinforces the layers by adding a material between the layers to replace lost material. It will aid in keeping the opaque layers intact, without altering their appearance, morphology or history. Acryloid B-72 is often used as a consolidant for archaeological glass. This particular resin is very clear and remains so after treatment, so it does not interfere with the appearance of the glass. It is also hard upon drying, without being too brittle, and has been shown to be most like a glass object compared to other adhesives.
Consolidation can occur in one of two ways: by surface application or by immersion. Techniques for surface consolidation include brushing the consolidant onto the surface and injecting or dripping it into the artifact. It is a useful technique for very fragile artifacts, those with loose paint or enamel, and large objects; however, the drawback is that the consolidant may not fully penetrate the artifact, consolidating only the surface layers. This results in a strengthened exterior and a weak core. As the artifact is handled, the exterior may separate from the interior. Consolidation by immersion involves immersing the artifact in the consolidant, if the artifact is not too fragile to withstand it. In order to avoid tide lines the artifact must be fully covered. Sometimes this technique is carried out under a vacuum to ensure that the consolidant is drawn fully into the artifact. Consolidation by immersion requires larger amounts of consolidant than surface applications and can pose health and safety hazards depending on the consolidant used. Vacuum impregnation should only be carried out on robust artifacts as the vacuum can damage very fragile pieces.
Proper cleaning, documentation and drying of the glass pieces must be performed in coordination with any consolidation efforts. Care must be taken when drying the artifact to avoid surface pooling of the consolidant.
The consolidation of waterlogged glass is complicated by the need to replace the water absorbed by the glass with a suitable solvent system. Only a conservator should undertake it.
Once clean and dry, glass artifacts should be kept in an environment free of fluctuations in RH and temperature, and free from dust and vibrations. Glass must not be packed too tightly, but should be sufficiently padded in storage with either Ethafoam or with acid-free tissue. Handling also poses a risk to glass, and all fragments and artifacts should be handled with clean hands over a padded surface to avoid damage.
Newton, R. and S. Davison. (1989) Conservation of Glass. Oxford: Butterworth-Heinemann.
The most important factor in choosing any adhesive to use on archaeological artifacts is the long-term stability of the adhesive. Adhesives that discolor, become brittle, fall apart, or cannot easily be reversed (re-dissolved in order to undo the join) are not appropriate for archaeological materials. Many adhesives readily available from hardware stores, craft stores and other suppliers may be appropriate for fixing a broken coffee mug or other household item where preservation is not a factor however they are not appropriate for use in museums or other cultural institutions. Many such adhesives have been used on archaeological glass in the past and have caused more harm than good.
Adhesives for all archaeological artifacts should be conservation-grade and should be purchased from conservation suppliers for the following reasons:
- The adhesives sold by conservation suppliers tend to be purer mixtures than those sold elsewhere. They contain only the resin and solvent whereas many commercial adhesives include plasticizers and fillers that may not be archivally stable over the long term.
- Most commercial manufacturers use proprietary formulas to make up adhesives. These formulas may change from batch to batch with no warning so that an adhesive that may once have been a suitable material for preservation use is no longer suitable. Unless you have the resources to test each batch for suitability prior to use, it is better to avoid commercially manufactured products altogether.
- Another factor in choosing an adhesive for glass is that the adhesive should not be stronger than the glass itself. In the case of physical shock to the object or mechanical failure of the join, it is better for the adhesive to fail at the old join than for the glass to break in new place because the mend is so strong.
Adhesives traditionally used to mend archaeological glass such as cellulose nitrate (Duco Cement, HMG Blue Tube) and cyanoacrylate (Superglue) tend to be stronger than most glass structures. They also tend to be irreversible and do not age well, yellowing over time and becoming brittle and insoluble as they begin to cross-link from age. Most proprietary adhesives are intended for temporary use, and therefore are not suitable for conservation purposes.
Acryloid B-72 is often suitable for archaeological glass, since it does not change color, and is easily reversed with solvent, if that becomes necessary. There are some instances, however, when it is not the best choice for glass. If the glass is in excellent condition, there may be no pores to help the glass piece bond with the B-72. In these cases, stable conservation-grade epoxies, such as Hxtal NYL-1, can be used. The working properties epoxies vary, and must be used properly in order to get a good bond. Hxtal, for instance, takes 7 days to fully set.
Any material used for filling gaps in glass must have the same properties as an appropriate adhesive for glass. See What Adhesive do you use to mend glass? A glass fill, however, also needs to be transparent or translucent. The more transparent the artifact, the more difficult it is to create an acceptable fill. The condition of the glass has an effect on the choice of gap filling material as well. For example, for small holes or line-chips on a mended artifact where surface deterioration has rendered the glass translucent, Acryloid B-72 to which an inert fill material has been added, such as fumed-silica or glass balloons may be used. However, this may not be suitable in all cases. Acryloid B-72 shrinks slightly when it cures, so it is impossible to use it as a clear casting material for larger missing areas.
Most clear synthetic resins that are designed as casting materials and are available to the consumer will yellow with prolonged exposure to light and are difficult to reverse. Hxtyl-NYL is a conservation grade epoxy (the hardener has been purified more thoroughly than most epoxies, so it’s light fastness is much better than most epoxies, but it still not as light-fast as Acryloid B-72). For mended glass artifacts that are in very good condition (dense and clear), it is possible to create an acceptable fill with Hxtyl-NYL Hxtyl-NYL requires special handling and safety precautions for both its use and reversal, as well as excellent manual dexterity and considerable practice in using it for this purpose.
The scholarly examination of archaeological glass and many exhibit situations may necessitate mending the artifact, but rarely requires the filling of larger gaps. If however, for aesthetic reasons or display purposes, a glass artifact requires fills, it is highly recommended that a conservator with experience in this area be contracted to perform this kind of work. Creating appropriate and effective fills for glass of any kind requires significant experience with the materials in order to be successful. Experimentation is not encouraged, as unsuccessful fill attempts subject the artifact to additional unnecessary reversal procedures and can create damaging stresses within the artifact.
Bone, antler and ivory are all classified as bony skeletal material. These materials are related because they are all basically composed of two main components: an inorganic mineral, hydroxyapatite, and an organic protein, collagen.
Generally, bone is found on archaeological sites in the form of worked artifacts, dietary remains, and/ or human remains. All bones consist of compact and cancellous bone, which is made up of inorganic and organic matter.
As seen in the cross section of a limb bone, the dense, compact bone is found in the shaft, while the spongy, cancellous bone is found at either end (the epiphyses). Generally, compact bone is carved to create tools and decorative artifacts. These worked bone artifacts can be easily distinguished from those carved from ivory if any traces of cancellous bone remain.
Antlers are outgrowths of bones on the skulls of even-toed ungulates (including deer and antelope.) These outgrowths are essentially fast-growing bones and consist of an outer compact surface that is characteristically rough and channeled from numerous blood vessels, with an inner cavity of spongy cancellous bone. Antlers are shed annually and are initially covered with skin (velvet), and each have a visible feature called a burr, where the antler attaches to the skull. Tougher than bone and usually appearing browner and denser under the microscope, antler is often carved to make tools, cutlery handles and buttons.
True ivory is from the upper incisors (tusks) of elephants and mammoths, but the word ivory is often used to describe similar materials including walrus and hippopotamus teeth. Ivory has a marked laminated structure, created by the build-up of layers during growth. In cross-section, elephant ivory has a structure that looks like a checkerboard, or has an “engine-turned” appearance. Ivory artifacts can range in color because ivory bleaches with exposure to light. Ivory appears yellow/brown if frequently handled and gray when burned.
Bone, antler and ivory are all anisotropic and hygroscopic, meaning that moisture changes exert stress that can lead to cracking and splitting of the materials. Teeth and ivory are particularly responsive to changes in moisture and are known to split even further. All of these materials are composed of two components that are preserved at opposing pH, and pH is essentially one of the largest influences on the loss of these materials. The inorganic hydroxyapatite deteriorates in acidic environments, leaving the organic materials rubbery, while the organic collagen deteriorates in alkaline environments, leaving the inorganic components chalky and brittle. These materials are subject to staining from metals, either from the burial soil, nearby artifacts, or attached metal components. For example, bone and iron can form a blue iron-phosphorous compound called vivianite and bone can be stained orange due to contact with iron, or green from contact with copper. These stains are generally harmless to the organic material and stain removal should not be attempted without consultation with a conservator.
When bony material is exposed during excavation, if it appears to be in good condition, gently remove as much surrounding dirt as possible. If it is slightly damp, allow it to dry out in situ so that it is not as fragile to lift. Once the artifact is sufficiently dry, it should be carefully undermined and placed in a polyethylene bag, along with any fragments that may have detached or broken off-it is important not to leave any fragments behind as they may be important for identification and research. If possible, it is best to put only one bone and its fragments in each bag as mixing fragmentary pieces can complicate identification and mending. If a bone is particularly weak and fragile, it may be necessary to do a block lift or to consolidate the bone in the field in order to keep the bone from completely falling apart during removal from the field. These procedures should only be undertaken with the advice/supervision/guidance of a conservator.
Before attempting to clean any bony material, it is important to ensure that it has been properly identified. Worked bone often is confused with ivory, wood, ceramic, or fired clay pipe fragments. All bony artifacts must also be examined for micro-cracks, fractures and abrasions, and for evidence of paint, gilding, attachments, and other features. These features may be difficult to see without a small amount of cleaning. Ivory is more vulnerable than bone and antler and needs to be handled with greater care and in a different manner. Bone and antler can be treated similarly. If there are no traces of applied surface features, these artifacts can be carefully washed and dried. For dry bone and antler, if the artifact is fairly robust and in good condition, minimal wet brushing/swabbing with a solution of 1:1 ethanol/water may be appropriate. However this technique should not be used for ivory. Do not apply even slightly damp swabs to ivory during cleaning-this moisture will lead to swelling, cracking and delaminating. For more fragile artifacts, dry brushing should be used. If necessary, wooden probes may be used to loosen large clumps of dirt. Do not use metal tools to clean fragile bony materials as they can score the relatively soft surfaces. Remember that when cleaning bones, it is not necessary to remove every bit of dirt. If the bone or antler artifact is damp or waterlogged, it is easier to remove the soil before it dries out. If an artifact is wet and there is even a small suspicion that it is ivory, keep it wet until a conservator can examine it. Treat thin antler and bone artifacts similarly to ivory-if they are wet, they must not be allowed to dry and should be stored in a damp environment or a refrigerator until they can be treated.
In the case of damp bone and antler artifacts (but not ivory), it is important to allow them to dry slowly, and out of direct sunlight. Once a damp artifact is clean, it should be laid out on a table or rack and then turned frequently and monitored. If new cracks develop or other physical changes occur, the artifact can be covered with a polyethylene sheet to reduce the evaporation rate. Waterlogged artifacts will need to be assessed carefully to determine if they can be air-dried successfully.
Conservators sometimes use solvent drying for waterlogged bone and antler if there is any indication that air-drying will cause the object to crack. Typically the artifact is placed in successive baths of water/ethanol mixtures. The ethanol content is increased with each successive bath until 100% ethanol is reached. Bone that has pigment, gilding, staining or other materials on the surface should not be solvent dried.
Consolidation is a preservation technique that can be used on bony material, but that requires consultation with a conservator in order to ensure that it is suitable for the material in question. A common substance used for consolidation of fully dried bony material is Acryloid B-72, which is an acrylic resin valued for its long-term stability and used for a variety of conservation techniques.
Consolidation is often carried out by applying a solution of 5-10% Acryloid B-72 in a solvent such as acetone or ethanol directly to the objects with a brush, or by immersing the objects in the solution. Vacuum impregnation of the B-72 solution into the bones is often employed in order to ensure maximum penetration of the consolidant into the bone structure. Although consolidation may be a desirable treatment for bone that is very friable and/or breaking apart, it is a technique that should not be undertaken without the consideration of future analysis of the bone. While this may not be a consideration with bone artifacts, in the case of human remains, the use of consolidants may affect future analytical work such as DNA analysis and Carbon 14 dating. Additionally, some dietary studies rely on bone weights in order to calculate the biomass equation, which is used in zoo-archaeological studies to estimate dietary contribution. The use of a consolidant may add to the weight of the bones. If the bone weights are altered, the biomass equations will be altered as well, thereby changing the results of the dietary studies. It is important to understand how the bones could be used in future analysis, and why the bones may be breaking apart, before deciding to use consolidation as a preservation technique.
If the surface of the bone is spalling (breaking off) and/or a white substance appears on the surface, the bone may contain soluble salts, which it has absorbed from the ground. If these salts are not removed, they will cause considerable damage to the bone. Desalination of the bone may be necessary. Because of the way in which bone and ivory are structured, some damage may occur during desalination and drying. It is therefore best to consult a conservator before embarking on this process.
Bone, antler and ivory artifacts should be stored in a stable environment with a relative humidity to between 45-55% RH, and minimal fluctuations in RH. Below 40% RH cracks may appear in the bone, which may destabilize it through time. Above 65% RH mold may form on the bone. All light/UV levels should be low, especially for ivory artifacts, and the temperature should be kept cool and stable. Storing ivory artifacts in a drawer, away from direct heat and light, is most suitable.
The excavation, preservation and analysis of human remains are subjects that provoke strong reactions both within the scholarly community and outside it. Ethical and legal issues play an important role in this debate, especially where the burials or remains of first peoples are concerned. Of additional importance are the roles of the conservator, archaeologist and bio-archaeologist/physical anthropologist in the study and conservation of this material. Each specialty approaches the material from a slightly different viewpoint and has the potential for both aiding and obstructing the other specialties involved. Archaeological constraints and techniques can affect the amount of information that a conservator may be able to retrieve from a burial and its associated materials, conservators may compromise the osteological work through their choice of treatments, particularly if they are not aware of the techniques that may be used for analysis, and osteologists may not always think about the long-term stability of the remains and their careful curation. Opportunities to discuss the excavation, treatment and analysis of human remains in a substantive, interdisciplinary manner have historically been rare and infrequent.
Therefore, when human remains are encountered it is important to initiate a dialogue as soon as possible to ensure that the information recovery is maximized and that it is carried out in the least invasive way possible. There are many ways, other than interventive treatment, that conservators can aid in the recovery and analysis of human remains. These include helping to lift the remains, creating housings that minimize the potential for damage, arranging analysis or in some cases (with all relevant approvals) taking samples for analysis, and the study of any related materials such as clothing and or artifacts. As with most conservation actions, these processes will be most effective if they are supported by as much information as possible.
Cassman, V. & Odegaard, N. (2004) “Human Remains and the Conservator’s Role.” Studies in Conservation 49 (4): 271-283. Williams, E (ed) (2001) Human Remains: Conservation, Retrieval and Analysis. BAR International Series 934. Oxford: Archaeopress.
Shell is the hard exterior covering of mollusks such as clams and oysters. Shell is built up on layers and is composed primarily of calcium carbonate and a protein similar to collagen called conchiolin. Whole shells are often found in midden deposits or shell is made into decorative objects, jewelry, inlay, buttons and beads. Shell beads which have been coated with a lacquer or resin in the past which did not have good light-fast properties and has now turned brown.
Shell beads which have been coated with a lacquer or resin in the past which did not have good light-fast properties and has now turned brown. Photo by M. Myers. Used by permission of the Virginia Department of Historic Resources.
Shell survives fairly well in alkaline environments. In acidic environments, it becomes friable and powdery as the calcium carbonate dissolves.
Once excavated, if the shell appears to be in good condition and has no evidence of surface decoration, it can be washed in water with a soft brush. Metal and any other hard tools, even hard-bristle brushes, will easily scratch the surface of the shell. To remove dirt on soft, friable shell, Use a soft brush; if the soil is compact, first slightly soften it with a swab of water and then remove it with a wooden pick-do not try to rub, pry or chip the dirt off, as this will cause the surface of the artifact to come off with the dirt. Soft, friable shell artifacts may need consolidation, and can be consolidated using a 10% solution of Acryloid B-72 in acetone by brushing it on the surface of the completely dry artifact.
Post excavation, shell is prone to a form of deterioration known as Byne’s disease. It manifests itself as a crystalline deposit (calcium acetate) on the surface of the shell and is associated with storage in environments rich in organic acid, particularly plywood, chipboard, cardboard, newspaper, other wood products and formaldehyde resins. Therefore, shell artifacts should be stored in acid free boxes or in polycarbonate plastic boxes or polyethylene bags. Acid free tissue or inert polyethylene foam, such as Ethafoam® or Volara, should be used to cushion individual pieces.
Under the best conditions (an anaerobic context with waterlogged sediments), wood will be remarkably well preserved. Markings, coatings, tool marks, and other use wear will be evident on the surface, even though the surface may be soft and spongy. It is critical to record the wooden object thoroughly and completely before undertaking any conservation work, since there is always the chance that the surface may change during conservation treatments.
During all initial stages of examination and cleaning, you must always keep the wood wet, ideally immersed. Waterlogged wood is particularly sensitive to changes in temperature and moisture. Any drying at all will lead to irreversible cell collapse and shrinkage, which will destroy the original form of the object. The ideal storage environment is wet, dark and cool. Storage
In general, you want to keep the wood immersed in water, and if you must take it out of the storage tank for cleaning or investigation, keep a slow water flow, mister or other moisture source handy to keep the surface damp. You can also keep the exposed surface covered with damp rags or open cell foam sheets. Do not use rapidly flowing or splashing water, as this may erode delicate surfaces. Tap water is sufficient for most storage purposes. If the wood is coming from a saline environment, it should not be put directly into fresh water – the differential in salt content between the salt water in the wood cells and the fresh water in the tank will cause osmotic shock that can damage the cell walls and increase the degradation of the wood. You can prevent this by slowing increasing the fresh water content of the tank by 25% increments over several weeks.
Biological growth is a common problem in long-term storage tanks. It is important to remove burial sediments as soon as possible. In an ideal world, you should not use biocides or fungicides to control algae, bacteria, or fungi that may grow in the tank and on the wood. Chilled water, or cold storage will help control growth, as will frequent water changes, and constant water circulation with filtration. (A simple filter can be built out of a garden-pond submersible pump, a perforated 5-gallon paint bucket, and polyester filter material). But even chilled water must be changed frequently and the containers thoroughly rinsed, since some fungi can grow at very low temperatures. Constant circulation will also help prevent the growth of anaerobic bacteria which can grow in small stagnant pockets of water under and between tightly packed objects, or in sealed containers. These bacteria will generate iron sulphides, which can stain the wood black. Small containers that can be sealed to prevent evaporation can be treated by adding 10% ethanol or rubbing alcohol, but be careful to use this technique only in locations where sparks and flames are not a hazard.
Snails, goldfish or koi have also been used to keep down biological growth in long-term storage tanks. Equipped with standard pond filters and air bubblers, a population of fish can keep tank and wood surfaces free from algae, fungi, and bacteria. Snails can suffer population boom and bust cycles, which may cause long-term cleanliness problems in the tank.
Ultimately, all waterlogged wood will require conservation for its long-term preservation. While you can keep wood in water indefinitely, there will always be on-going degradation, and the cost of maintaining the storage tanks and their contents will add up over the long run.
The treatment of waterlogged wood focuses on two basic problems: the natural pore spaces of wood have been increased by degradation (either biological decay or chemical hydrolysis), which has reduced the physical strength of the wood structure; and all those pore and decay spaces are filled with an excess of water which is partly supporting the weakened structure, but is not physically or chemically stable. There may be other problems as well, such as mineral staining (usually from metal fasteners) that change the appearance of the object, and contribute to the breakdown of the wood.
Since dimension changes are possible during treatment of waterlogged wood, careful recording of size prior to treatment is extremely important. Photo by L. Young, used by permission of Alexandria Conservation Services.
The basic premise of treating waterlogged wood is the removal of excess water while preventing the damaged wood structure from collapsing. This is usually accomplished by introducing some bulking or supporting material while the wood is still wet, and then drying it in a controlled fashion. In all cases, the fine details of a treatment will be determined by the degree of wood degradation, the intended disposition of the object, and the resources available for treatment. Other components of wood treatment may include the removal of foreign materials such as metal staining. Treatments are usually assessed based on their ability to dry the wood without dimensional change, preserve associated materials and provide aesthetically acceptable results.
Since dimension changes are possible during treatment of waterlogged wood, careful recording of size prior to treatment is extremely important.
- Bulking Agents
- Sugar: There has been considerable experimentation using natural sugars (sucrose) and artificial sugars (mannitol, lactitol) to replace the missing wood cellulose, which is itself a long-chain sugar polymer. Results have been varied, and the precise conditions which lead to a successful treatment are difficult to define or reproduce. While sucrose is non-toxic and relatively cheap (and is therefore a good option in some developing countries), there are considerable problems with preventing fermentation of the early stage (low concentration) solutions, and calculating the proper final concentration and time of treatment. The artificial sugars do not ferment, but are much more expensive.
Wheel in PEG. Photo by H. Wellman. Used by permission of the Maryland Archaeological Conservation Laboratory.
Polyethylene Glycol (PEG): PEG is the standard treatment for most waterlogged wood, and has been studied extensively by conservation scientists. It is non-toxic, and available through most chemical suppliers. It can be applied by immersion, or by spraying. It is a wax-like long-chain polymer that comes in a variety of molecular weights (MW); -common grades used in conservation are PEG 200 or 400 (viscous liquids), and PEG 3350 or 4000 (flaky solids). The low and high molecular weight PEGs are often used in combination – the PEG Wheel in PEG.
- 200 or 400 is introduced first to penetrate the smaller spaces and bond to the cell walls, while the PEG 3350 or 4000 is introduced later to fill the larger spaces in the deteriorated wood. There are different opinions as to whether this should be done in two separate applications, or if it can be done in one blended bath. The degree of degradation of the wood determines the final concentration of each grade used, and the length of treatment. Arguments against using PEG include the long treatment times, and the potential for a dark, greasy wood surface, but both can be controlled by proper application. PEG also remains hygroscopic (attractive to atmospheric moisture), but some argue that this helps buffer the treated wood from swings in relative humidity.
- Solvent-based treatments:
- Acetone-Rosin: This treatment involves immersing the object in a heated bath of colophony resin dissolved in acetone. It was a popular treatment from the 1950s, now mostly discontinued because of serious hazards from the heated acetone baths. It also seemed to work well only on a limited number of species of wood.
- Silicone Oils: This is a relatively new procedure using cross-linking silicone oils. The wood is immersed in baths of acetone to replace the water, then immersed in the silicone polymer under vacuum. When the polymer has penetrated fully, it is exposed to a cross-linking agent, and allowed to dry. The cross-linked polymer cannot be removed from the wood. The process (also known as plastination) has long been used to preserve forensic and medical specimens, but the application to waterlogged archaeological artifacts is relatively recent. Many conservators feel that it requires more study to evaluate its long-term effects. It may never be appropriate for large objects (longer than one meter) because of its expense and the toxicity of the cross-linking agent.
- Other polymers: A variety of other solvent-based polymers have been tried, but none have ever worked well, or they developed long-term problems.
- Dehydration: the most critical stage of treatment is dehydration. If the first phase of treatment does not replace water with some other solvent, you must take great care. As water droplets evaporate, they shrink. Water has a very high surface tension, and water in the wood cells is bound to the cell walls by hydrogen bonds – as the water evaporate, the shrinking water droplet pulls the cells walls inwards with it, causing cell collapse and irreversible shrinkage. The various bulking agents, such as PEG, help to prevent this, but will not stop it completely.
- Air-drying: in some very limited cases, it is possible to air-dry wood without introducing any bulking agents. This is usually only successful where the wood is very robust, with little degradation. The rate of drying and the environment in which the object is dried must be carefully controlled to prevent drying too fast. It usually requires a chamber where both temperature and relative humidity can be tightly controlled and monitored, and usually takes several months.
- Freeze-drying: freeze-drying, either under vacuum or at ambient air pressure, has been proven a gentle method of drying. Because under certain conditions ice can sublime directly to water vapor without passing through a liquid phase, you do not have the water-tension shrinkage problems. This can occur most efficiently under a vacuum (thus the creation of vacuum freeze-driers, used commercially to prepare foods and pharmaceuticals), but it can also happen in a very low-temperature freezer (“freezer burn” of frozen meats is an example of ambient pressure freeze-drying), or in Arctic environments. PEG is commonly used as a pretreatment for freeze-drying because it will form a solution of ice and PEG that does not expand like pure water on freezing, and the PEG is left behind to bulk the cell walls when the water is removed.
- Solvent-drying: This involves replacing the water in the wood structure with a solvent of lower surface tension. One theory behind this treatment using solvent replacement is that solvents such as acetone have a much lower surface tension than water, and should not cause as much shrinkage as the wood dries. In reality, the acetone can replace ALL the water in the wood, including that chemically bonded inside the cellulose molecules, so shrinkage occurs anyway.
Recently, researchers in the treatment of waterlogged wood have focused their attention on the role of elemental sulphur and iron sulphides in the long-term deterioration of archaeological wood. Sulphur compounds are often deposited in the wood during burial, especially in polluted and anaerobic environments, and they are difficult to remove. As the sulphur compounds are exposed to oxygen and moisture during and after treatment, they become oxidized and sulphuric acid is produced, which attacks the wood. The reaction is catalyzed by iron, which is also commonly deposited in waterlogged wood. So far the phenomenon has been recorded primarily in timbers from large ships such as Batavia, Mary Rose and Vasa, but there are implications for the treatment, storage and long-term stability of all waterlogged wood. No simple test for sulphur content has been determined yet. It is usually necessary to send a sample to a laboratory for analysis.
Storage after treatment
No matter how it is treated wood that has previously been waterlogged should be kept in a stable environment. The relative humidity should be maintained between 45-60% and large or rapid changes in humidity should be avoided. Light levels should be controlled and dust excluded as much as possible.
Wood that has been treated with a bulking agent or other treatment materials may experience a significant gain in weight during the process. Depending on how they will be displayed and/ or stored fragile wood fragments may require some form of external support system to prevent long-term damage or deformation.
How do you assess archaeological leather objects and decide on a conservation treatment?
Leather is an animal skin -generally, pig, sheep or cow, but not limited to these-that has been tanned, tawed or otherwise chemically treated in order to make it more resistant to putrefaction, reasonably resistant to water, and more supple. Leather is made up of collagen fibers that group together to form fiber bundles-a feature that also imparts great strength to the material. Leather was used to make a number of objects, ranging from clothing pieces, such as shoes, scabbards and jerkins, to household items such as vessels, and furniture coverings. Leather artifacts are often found in association with other materials such as textiles, metals and wood (in the form of stitching, shoe pegs and nails, for example). Paint and gilding may be found othe surfaces of leather as can embossed and piercdecorations.
Treated leather shoe. Photo by M. Myers. Used by permission from the Virginia Department of Historic Resources.
In archaeological settings, leather generally only survives in burial environments that are either very dry (deserts) or very wet (anaerobic environments with waterlogged sediments). In the Mid-Atlantic region the most common places to find leather archaeologically are in sealed contexts such as coffins or in privies, wells and marine settings. Additionally, small fragments of leather may survive in close proximity to copper alloy objects such as grommets and upholstery tacks or in contact with iron objects, such as buttons or rivets. The four main factors affecting the deterioration of leather are:
- Physical factors: –wear and tear while in use, abrasion in a burial environment, mishandling, or physical stress after burial and excavation.
- Biological attack: –insects, molds, bacteria. Some insects-carpet beetles in particular-and bacteria can digest collagen.
- Chemical deterioration: –oxidation and hydrolysis. Oxidation is most apparent in storage settings and leads to embrittlement, weakening, discoloration and fading. Metal salts, usually iron, cause blackening and other staining and can cause embrittlement and other problems. Hydrolysis occurs in damp environments or if there is an increase in pH or temperature. Leather survives best in an environment of pH 3-6.
- Moisture and temperature changes: –in buried material this may include changes in groundwater levels or inappropriate drying or wetting upon excavation.
Dry leather is likely to be quite fragile and brittle. It should be stored dry between 40% and 50% RH. It is best to place it in a rigid container, such as a Tupperware® or Rubbermaid® container, to avoid damage when handling. Do not wet the leather as this can cause staining and can cause the leather to become sticky and lose cohesion. It is best to consult a conservator as soon as possible.
Wet leather must be kept wet at all times: drying will cause curling, cracking, shrinkage and brittleness. To remove burial soil, silt or dirt from wet leather, the object should be placed under a very light stream of water and brushed very gently with soft brushes (such as paint brushes). Do not attempt to remove any harder concretions or lumps as this can cause damage. Do not attempt to flatten out leather or to unfold it, as this can cause significant damage. The flesh side (or fibrous side) is often weaker than the grain side and individual pieces of leather may delaminate along the interface between the two sides-sheepskin is particularly prone to this. If this appears to be occurring, stop washing the piece immediately.
Keep wet leather wet!!! Cover it completely with water and store it in a refrigerator, if one is available. Refrigeration slows the growth of bacteria and mold. Limiting the amount of light that reaches the leather will also cut down on mold growth. Do not store wet leather in a freezer, as the expansion of the water on freezing will damage it. The use of biocides should be avoided. They can affect the efficacy of future analysis on the leather as they can block the chemical reactions that are necessary for some analytical techniques. The use of biocides can also create health, safety and disposal problems and they can alter the pH of the water and creating an environment that is detrimental to the survival of the leather.
Consult a conservator as soon as possible. Even during storage, chemical processes, such as hydrolysis, can cause significant damage to leather.
For desiccated leather
Most desiccated leather is simply brushed to remove burial soil, and stored with form-fitting supports and strict environmental controls. It is fragile, so supports are usually created that allow lifting and viewing the artifact without touching the leather itself. It is highly sensitive to moisture, so reshaping and re-humidification (as described below) is often not possible, nor are water-based treatments. Numerous analyses may be performed on the leather, such as identifying the animal type and tanning agents. It is preferable not to consolidate dry leather, as the treatment causes noticeable darkening and changes the leather’s texture. A conservator will perform consolidation only after careful consideration of alternatives. Occasionally, it may be necessary to mend or reinforce rips and tears in order to give the object greater structural stability or to prevent further damage. This is often done with toned Japanese tissue and a conservation grade adhesive whose properties match those of leather as closely as possible.
For waterlogged leather
The aim of treating waterlogged leather is to bring the leather to a dry state and prevent the charged fibers from being drawn together and adhering or cross linking to each other resulting in hardening and shrinking of the artifact. Treatments are generally assessed based on their effects on the shrinkage, color and stability of the piece. The typical leather treatment is made up of a number of steps, all of which involve individual decisions.
–Cleaning and desalination
Cleaning is the first step in any waterlogged leather treatment. It is usually done as gently as possible, with soft brushes and wooden tools under a slow trickle of water. Desalination may be carried out if necessary.
This step is optional and can be controversial. Stain removal is easiest to achieve soon after burial rather than after months or years of storage. However, it is primarily an aesthetic matter. It is hard to know what color leather would have been prior to burial in the ground. Additionally, many conservators feel that not enough research has been done into which stains are damaging to leather and which are not. There are concerns that stain removal may damage the leather and affect the potential for future research such as the analysis of tannins. In each case, the desire to remove or minimize stains should be weighed against the potential for information loss. In some cases, such as when an object is required for display, the aesthetic concerns may be most important, while in other cases, the research potential may be of greater value.
–Impregnation with a bulking agent followed by dehydration.
The use of a bulking agent is necessary in order to lubricate the leather fibers and prevent them from cross-linking with each other during drying. The two bulking agents that are most commonly used by conservators are glycerol and polyethylene glycol (PEG) 400. Both materials act as lubricants during drying. They form weak bonds with the polar groups on the collagen molecules similar to those formed by collagen and water, and prevent the fibers from cross-linking and sticking to themselves during drying. Neither material affects the analysis of fatty acids. Some research carried out in France and Canada suggests that PEG 400 may stabilize leather better than glycerol does. The two methods of drying leather that are most commonly used after the bulking step are solvent dehydration and freeze-drying. During solvent dehydration the water is replaced with a liquid of lower surface tension, which is much less likely to cause the fibers to approach each other as it evaporates. During freeze-drying the wet object is frozen and the ice sublimates off as a vapor, preventing the surface tension problems caused by evaporating water. There is a slightly higher shrinkage rate with solvent dehydration than there is with freeze-drying. If either of these drying techniques is used without first impregnating the leather with a bulking agent, significant shrinkage and hardening can occur.
Dressings in the form of various oils and fats have been applied to archaeological leather to improve their appearance and flexibility. However, they tend to be more harmful than helpful. Dressings are less reversible than bulking agents and they may affect future analysis. Often they darken the leather, make the surface of the leather into a dust trap, and have to be reapplied periodically. Also their application can require significant handling and rubbing of the leather, which can damage archaeological pieces. Additionally they may not be compatible with other chemicals that were used to treat the leather, or with any subsequent adhesives, stitching work and supports materials. It is better not to apply leathers dressings, and to focus instead on proper environmental protection: control of RH, temperature, light, dust, pollutants and handling.
— Reshaping and re-humidification
Some wet artifacts may be reshaped during the dehydration phase of their treatment-particularly if the artifact is being freeze-dried-simply by allowing the artifact to dry over forms made up of tissue or foam. Bindings and clamps may be used to facilitate the process. For objects that have already been dried, reshaping must occur with humidification. Generally the object is placed in a high humidity environment for several days to relax the material. Then the forms are inserted and the artifact is slowly dried. It is important to test the acidity of the leather prior to humidification, as if it is acidic (i.e. below pH 3), the high RH may promote hydrolysis. Once hydrolysis begins it is a self-catalyzing process: more acids are released and more damage is done causing yet more acids to be released.
Restoration is usually only necessary for objects that are going to be photographed or displayed. It can cause severe problems for an artifact if the restoration job is not done well, as it can set up stresses within the artifact that may lead to tears and problems. For example, joined leather pieces may move against each other during environmental changes (even minor ones), pulling threads and/or other fastenings against the leather and causing tears. Additionally, rigid gap fills may not move in the same direction as the leather and may create tension in the piece. It is best to use as many sympathetic materials as possible and to use materials that are weaker than the leather. Then, if there are stresses, the mended area will tear before the leather will. Wet-strength Japanese tissue is often used for this reason and because it can be toned with acrylic paints to match the leather. Fine dyed linen thread may be used to stitch pieces back together. Forms made of polyethylene foam can be created on which the leather can rest so there is no strain on individual sections of the piece; a layer of unwashed muslin is often placed between the foam and the leather to make sure that the leather does not catch on any rough areas and to hide the foam and create an aesthetic display. Consolidation of weakened powdered areas may be necessary whether or not the artifact is going on display.
Other treatment methods
- Castor oil-Castor oil has been proved to be a less than ideal method. The oil comes from the castor bean plant. Castor oil is a poor choice of lubricant for leather from a conservation perspective. It tends to oxidize, leading to thickening of the oil and formation of gummy or resinous products. The resinous products may form between fibers preventing them from moving across each other and causing damage. The amount of time that has elapsed since treatment appears to be a major factor in assessing the long-term effect of the treatment, as it takes several years for the oil to begin oxidizing. Thus although initial results may appear promising, significant damage occurs to the leather over time.
- Other oils commonly used on leather include neatsfoot oil, tallow and linseed oil. These treatments have turned out to be quite problematic over the long term. Most oils need to be reapplied on a regular basis but research has shown that excessive lubrication of leather can be very damaging.
- Bavon–Bavon ASAK ABP (solvent soluble) and Bavon ASAK 520S (water soluble) are proprietary compounds based on alkylated succinic acid, which is intended to bond with the polar regions of the collagen fibers and bulk the leather. The problem with proprietary solutions is that it is very difficult if not impossible to find out what is in them and they are subject to change without notice. In the case of Bavon, the formula changed and the later results have never measured up to early results. This treatment is referred to often in the conservation literature (particularly that from the 1980’s) but is seldom practiced anymore.
- Silicone Oils–Recent research has suggested that silicon oils may be used to treat waterlogged materials, including leather. Although this type of treatment has some promise in certain situations where the object is too fragile to survive other forms of treatment, it is not ideal because of the need to dehydrate the leather in acetone prior to the application of the treatment materials and because it is irreversible.
The aim of any good housing or packaging is to promote viewing and study while avoiding direct handling and folding, which may stress fibers. It is important to:
- exclude light.
- exclude dust.
- store at about 55% RH.
- include organic packing materials, such as paper and board to act as buffers to RH changes.
- ensure that any packaging or storage materials used are acid-free.
- exclude sulphur; sulphur concentrations in air and in leather can form sulfuric acid leading to a condition known as red rot that manifests as powdery reddish areas on the surface of the leather and can seriously weaken it.
How do you assess objects made of horn, tortoiseshell or baleen and decide on a conservation treatment?
Horn, tortoiseshell and baleen are all composed of keratin, a proteinaceous complex secreted by the epidermis of the skin of vertebrates. Wool is also composed of keratinaceous fibers but will be covered in the section on textiles. Horn is derived from an outgrowth of the skin of cattle, sheep, goats and antelope. Horn forms over a vascularized core of bone that supports a germinal layer that produces keratin throughout the animal’s life. This gives horn the appearance of growth even though it is dead tissue. It also produces a characteristic cone within a cone structure that makes horn susceptible to delamination. Commercially, horn was manipulated by soaking and heating to soften, shape and reform it and was used for a variety of small artifacts such as powder horns, combs and decorative components on small boxes.
Tortoise, good condition
Courtesy of Sonia O’Connor at the University of Bradford.
Tortoiseshell is derived from the shells of sea turtles. Only three (the Hawksbill, Green and Loggerhead turtles) of the seven species of sea turtles produce a shell that is thick enough to be commercially viable. The Hawksbill’s shell has traditionally been the most highly valued source. A turtle’s shell consists of three key elements: the flat ventral plate or plastron, the domed carapace that covers the back of the turtle, and the bridge, which forms a rigid connection between the two. Each of these elements consists of spongy bone plates, which are covered with an outer shield of keratinaceous scales. The scales are laid down annually, with increasingly broader layers of keratin-containing cells added to the bottom of the scales, producing characteristic protruding scales with sloping laminated sides. These growth rings produce an optical ripple effect, which can be used as a diagnostic tool for identifying tortoiseshell. The color and patterning of tortoiseshell varies depending on the species, the age of the individual and the location on the shell. The Hawksbill’s plastron produces a lightly colored shell, referred to as “Blonde shell,” which was particularly prized in the 17th and 18th centuries. One of the first written references to tortoiseshell, by the Roman author Pliny, records its use as a veneer. It was used in many of the same ways that horn was and in the 18th and 19th centuries elaborate combs were made from it. Due to the rarity of tortoiseshell, other materials such as horn and bone were sometimes dyed and stained to imitate it. One of the first uses of modern plastics was to emulate tortoiseshell.
Tortoise, part decayed
Courtesy of Sonia O’Connor at the University of Bradford.
Courtesy of Sonia O’Connor at the University of Bradford.
Courtesy of Sonia O’Connor at the University of Bradford.
Courtesy of Sonia O’Connor at the University of Bradford.
Baleen is derived from whales of the suborder Mysticeti, such as humpback, finback, minke, sperm, sei, blue and right whales. These whales employ a filtration system for feeding. The system is composed of baleen plates that grow from the upper jaws of these whales. The baleen plates grow continuously from the epidermal layer of the gum and can reach lengths of 13-14 feet. They have a triangular shape and are placed approximately 1 cm apart. Baleen was most commonly used for “boning” in clothing.
Correct identification of horn, tortoiseshell and baleen is critical to ensuring that it receives proper treatment. Often these materials can be confused with bone and ivory or with a variety of plastics that sought to imitate the appearance of the materials (particularly horn and tortoiseshell). Pigment variations in tortoiseshell and horn are often used to tell the two materials apart. The pigment in tortoiseshell is evenly distributed throughout the thickness of the piece and appears slightly granular, whereas the patterning in stained horn is a surface phenomenon. Additionally, horn generally appears to have striations or corrugations in its surface that tortoiseshell lacks. These variations can be seen with a low powered microscope. Sonia O’Connor’s article “The Identification of Osseous and Keratinaceous Materials at York” is the definitive article on the identification of these materials.
Keratin is very sensitive to changes in humidity. It responds by taking up or losing water, which affects its mechanical properties and can cause dimensional changes. As the RH rises the material can become weaker but more pliable. Some bacteria and fungi specialize in digesting keratin. Alkaline burial conditions can disrupt the disulphide links in the keratin, speeding up hydrolysis. Visible changes occur in the translucency of the piece-it will become increasingly opaque as it deteriorates. Additionally, the material will begin to delaminate. Heavy overburden (resulting in a high degree of pressure) in the burial environment can cause deformation of the material.
Unlike other materials where it has been possible to discuss some of the pros and cons of treatments that are in wide usage, there are very few standardized treatments for horn, tortoiseshell and baleen. Due to the relative rarity of these materials, the trade restrictions on tortoiseshell, and the fact that they are usually found individually or in very small groups there has not been a substantial amount of research on their treatment. There are no standard treatments that are used and each object must be treated on a very individual basis. The susceptibility of horn and tortoiseshell to damage from changes in humidity, makes it important to call a conservator as soon as possible after the artifact has been excavated.
When horn, tortoiseshell or baleen artifacts are found in waterlogged conditions, they should not be allowed to dry out, as this will cause them to warp and crack. Either place the artifact in a 4mil polyethylene ziplock bag with water and place that bag within another bag to ensure that if it leaks the water will be captured and remain in contact with the artifact, or place the artifact in a Rubbermaid® or Tupperware® container filled with water (this will provide some rigidity and handling should be minimized as fragile objects can float into the walls and be bent or damaged. If possible, refrigerate the bags or containers to reduce the potential for biological growth.
The storage requirements of treated pieces of horn, tortoiseshell and baleen are similar to those for other organic materials. It is particularly important to keep these materials away from heat, a factor that should be carefully considered should these materials be put on display, as lighting in display cases can warm them too much.
O’Connor, S. (1987) The Identification of Osseous and Keratinaceous Materials at York. In: Archaeological Bone, Antler and Ivory. Occasional Papers Number 5. London: The United Kingdom Institute for Conservation of Historic and Artistic Works of Art. Available at http://www.ukic.org.uk/pubtable.html.
Wardlaw, L. & Grant, T. (1994) “Treatment of Archaeological Baleen Artifacts at the Canadian Conservation Institute”. Journal of the IIC-Canadian Group 19: 31-37.
How do you assess archaeological objects made of rubber and plastic and decide upon a conservation treatment?
Rubber, front. Rubber artifact. Images by Emily Williams, Colonial Williamsburg.
Rubber, back. Rubber artifact. Images by Emily Williams, Colonial Williamsburg.
Plastics include objects composed of both synthetic and natural materials. All plastics are composed of polymers, which are made up of a large number of smaller molecules called monomers. Polymers can be grouped into three broad categories: 1) Natural (amber, gutta percha, and other resins); 2) Semi-synthetic (cellulose nitrate, cellulose acetate) and 3) Synthetic (polyethylene, polyvinyl chloride, and polyester). Most of the plastics manufactured today are made from synthetic polymers, but examples of semi-synthetic objects still exist. Plastics can be grouped into two categories based on how they were manufactured. Thermosetting materials are heated and stay in a fixed shape even after heating. Thermoplastic materials are heated, become firm when they are cold, and soften again upon heating. Additives to the polymers help to form the final shape and characteristic of the plastic. Fillers, colorants, fibers, anti-oxidants, and dyes are all added to modify the plastic and obtain the desired result.
Plastics appear in the archaeological record in a number of forms ranging from toys and personal items to buttons and jewelry. Fragments of rubberized fabric, typically from buckets, tents and ground cloths, are often found in 19th century contexts.
|Celluloid tortoise shell||Plastic fake ivory||Plastic fake ivory, longitudinal veining|
Images by Sonia O’Connor by permission of the University of Bradford.
Although plastics often seem eternal, these polymers break down and deteriorate with age like other materials. Several factors contribute to the decomposition of plastic objects including the environment in which they were used, their composition, and how they were manufactured. Temperature, relative humidity, ultraviolet (UV) radiation and pollutants are a few of the factors that can cause chemical and physical changes in plastics.
- High temperatures cause chemical reactions to proceed at a faster rate and may also cause mold spores and fungal colonies to begin to grow; excess heat causes polymer chains to break effecting the mechanical properties of the material
- Fluctuations in relative humidity can be harmful to plastics. Large fluctuations in RH can cause materials to crack, split and warp. Excess moisture can cause chemical changes
- Ultra violet radiation causes plastic materials to change color (usually manifested as increased yellowing) and will also cause the polymer to cross-link (two or more polymer chains join by a chemical bond) causing the material to become inflexible and brittle, resulting in physical breaks
- Pollutants such as sulfur dioxide, nitrogen dioxide and ozone are harmful to plastics and may cause off-gassing of vulnerable materials, leading to their further degradation
- Physical damage can result to fragile plastics if dropped, handled improperly or packed improperly for transit or long term storage
Plastic degraded by burial
Image by Sonia O’Connor by permission of the University of Bradford.
The degradation of polymers can result in obvious changes to plastic materials. These changes occur for a variety of reasons and may occur slowly in stages over time or quite dramatically overnight. One indication that plastic materials are degrading is that they often produce a very discernible odor. Cellulose acetate, for example, produces acetic acid upon deterioration, which smells like vinegar. Other materials, such as polyvinyl chloride, produce hydrochloric acid upon deterioration, which has a very sharp and acidic smell. Some other common problems to look for with plastics are listed below:
Decaying cellulose nitrate with active corrosion at the contact points between the metal and plastic Image by Emily Williams, Colonial Williamsburg.
- Many plastics change color indicating they are undergoing a chemical change. The most common form of this change is when clear plastics are exposed to ultra violet radiation causing the items to turn yellow over time. This can occur in a variety of materials, but it is most noticeable in polyvinyl chlorides, nylon and plastic adhesives such as epoxy resins and cyanoacrylates. Other color changes can exist where clear and colored objects become more opaque. Occasionally a white bloom may be seen on the surface of the plastic. This is usually the result of plasticizers or other additives leaching to the surface of the material. This is common with both cellulose acetate and cellulose nitrates.
- Decaying cellulose nitrate with active corrosion at the contact points between the metal and plastic
- Some plastics may become chemically distorted over time. The appearance of small bubbles, often trapped beneath the surface, may be an indication that degradation has occurred. These bubbles are most often seen in cellulose acetate and cellulose nitrate materials.
- Physical damage to plastics can occur in a variety of forms. If the plasticizer off-gasses or leaches from the object over time, softer, more flexible plastics such as rubbers and resins will become inflexible and brittle over time. This can result in physical breaks and cracks. In more rigid plastics such as polystyrene or polymethyl methacrylate, cracks may also be the result of either internal stress in the materials from the manufacturing process or the result of an outside force (i.e. dropping the object).
- Crumbling of materials, such as polyurethane foams, is the result of chemical breakdown due to the exposure of oxygen and ozone. Other damage resulting in physical changes may include warping, flaking of layered materials, and fraying of fibers.
- Many plastic materials begin to “weep” as they degrade. This term refers to the formation of tiny droplets of liquid on the surface of the materials. The droplets, often acidic in nature, are sticky and oily. They are a result of the loss of the plasticizer in the material and are most commonly associated with polyvinyl chloride, which produces fine droplets of phthalates on the surfaces. These droplets are not only harmful to the material, but can also be harmful to those handling the artifacts.
Before a conservator treats an object made of plastic, its condition must be assessed to determine several things. First, the plastic must be correctly identified. Many plastics and rubbers appear to be similar but vary in chemical make-up. It is very important to determine the material prior to attempting to treat the object.
There are several types of analytical equipment available to conservation scientists or polymer chemists that can correctly identify the type of plastic and sometimes even the additives present. Additionally, if there is an unknown substance on the surface it should be analyzed to determine what it is prior to removal.
Plastics are difficult to treat because very little attention has been paid to their long term deterioration. Conservators are just beginning to understand how they age and what treatment is best for their long-term preservation. No single solution exists for each material or type of object. The importance of testing the materials before any treatment is performed cannot be overemphasized.
Cleaning and Stabilization
Minimal cleaning and stabilization is all that is necessary unless the object is needed for an exhibition or is deteriorating and is in jeopardy of total loss. All cleaning methods, no matter how small, should be documented and the materials and techniques used should be recorded and kept with the object. Some cleaning of surfaces can be performed using a museum-quality HEPA (high-efficiency particulate air) filtered vacuum cleaner with variable speed control.
Harder plastics, such as polycarbonate, polystyrene and acrylics can be cleaned using a dry, electrostatic cloth. If surface bloom or leaching is evident on the plastic’s surface, it is advisable not to clean the object and to consult a conservator prior to cleaning. Any wet cleaning undertaken on plastics, including cleaning surfaces with water, should be performed only after consulting a conservator. Some porous plastics will be permanently damaged by water. At no time should solvents, detergents, polishes, scratch cleaners, or waxes be used to clean plastics. Solvents in particular can cause dissolution of the plastics.
There has been little success using adhesives to repair broken plastic objects. A variety of adhesives are available and finding the right adhesive for a particular plastic is much more difficult than it appears. Off-the-shelf adhesives are not suitable for the adhesion of historic plastic objects as the solvents used in them may cause further damage to the plastic. Due to the complexity of plastics therefore, a conservator should perform any mending that is needed.
Due to the lack of suitable treatment methods available for plastics, the environment in which the object is stored and exhibited is of primary importance. Some simple rules can be applied to all plastics:
- Handling Plastics–All plastics should be handled wearing gloves. White cotton gloves or disposable gloves (such as latex or vinyl) should be worn at all times. Plastics are very sensitive to the acids found in a person’s hands and can be damaged if handled too often. If decay products are visible on the plastic’s surface, then disposable gloves should be worn and disposed of immediately after use.
- Plastics benefit greatly from being stored and exhibited in a stable environment. Extremes in temperature and relative humidity should be minimized. A drier environment of approximately 35-45% relative humidity is ideal. Plastics should be stored in a well-ventilated area, free from ultraviolet radiation, pollutants, dust, and insects.
- Plastics should be stored and supported using only museum approved acid-free materials. Some plastics benefit from storage with oxygen and pollutant scavengers. These scavengers help to reduce and remove chemicals off-gassed from the objects.
- Lastly, some plastics cannot be stored in contact with other materials as they will chemically react and lead to further degradation of the objects. No objects should be stored in close proximity to polyvinyl chloride. Other plastics such as polyurethane foam, silicone rubber and cellulose nitrate can break down in storage producing acidic byproducts, which may harm other objects or storage materials.
Blank, S (1990) “An Introduction to Plastics and Rubbers in Collections”, Studies in Conservation 35 (1990): 53-63.
Grattan, D., ed. (1993) Saving the Twentieth Century: The Conservation of Modern Materials. Canadian Conservation Institute: Ottawa, Canada.
Morgan, J. (1991) Conservation of Plastics. Museum and Galleries Commission: London.
Quye, A & Williamson, C. (1999) Plastics: Collecting and Conserving. National Museums of Scotland: Edinburgh.
Shashoua, Y & Ward, C. (1995) “Plastics: Modern Resins with Ageing Problems” SSCR Resins: Ancient and Modern, pp. 33-37.
Shashoua, Y (1996) “A Passive Approach to the Conservation of Polyvinyl Chloride” in ICOM-Modern Materials Working Group: 961-966.
Winsor, P. (1999) “Conservation of Plastics Collections”, MGC Fact Sheets: September 1999. Museums and Galleries Commission: London
Young, L. & Young, A (2001) “The Preservation, Storage and Display of Spacesuits”, Collections Care Report Number 5, Smithsonian National Air and Space Museum: Washington, DC.
Textile is a term derived from the Latin word texere, which means to weave. Before the invention of synthetics, textile fibers were derived from plants like flax, cotton, hemp, jute, and sisal and from animals such as sheep, llamas, goats, alpacas, oxen, rabbit, and others. These fibers were spun into yarn, which was then woven to form the textile. The characteristics of the fiber depend on the plant or animal from which it is produced, the processing of the yarn, and the subsequent dyeing, washing and finishing of the yarn and the textile.
Oxidation and biological activity are the greatest causes of textile degradation. Additionally, physical damage from cutting, tearing, and abrasion, plays an important role in the degradation of textiles.
Textiles from archaeological sites generally survive best in desiccated conditions, waterlogged anaerobic conditions or in proximity to metals, which inhibit attack from microbial and fungal organisms. Textiles from wet, anaerobic sites survive very well and can, in protected sites, be virtually intact, although greatly weakened by hydrolysis, oxidation, and other forms of degradation. Cellulosic fibers (those derived from plant fibers) are attacked by weak acids, while proteinaceous fibers (those derived from animal fibers) are attacked by weak alkalis. Strong alkalis and acids attack both. The textiles from drier sites are usually brittle fragments preserved in association with a metal, such as copper or iron.
Archaeological textiles fragments are typically very fragile and require support to lift. If possible, lift them on a block of their surrounding earth. If this is not possible use a piece of thin but rigid plastic (such as a thick gauge Mylar® or other polyester film) to slide under them. Do not attempt to wash textiles or to unfold them if they are folded. Do not use any consolidant on them.
Treatment approaches vary according to the condition of the textile. Dry, brittle textile fragments are best preserved by placing them in a stable, moderate environment, and packaging them in a support that allows viewing without handling. Textiles from wet or damp sites will need to be supported by fine netting, a synthetic non-woven fabric such as Hollytex, or silk crepeline as soon as possible after excavation. This support will remain in place throughout treatment to prevent disintegration of the textile due to weakened fibers. Textiles may need cleaning to remove dirt, stains, metal oxides and salts. Polyethylene glycol (PEG) and Ethulose (ethylhydroxyethyl cellulose, a cellulose ether additive and consolidant) may be used when flexibility must be maintained. Freeze-drying works well to gently dry waterlogged textiles. Treated textiles will be fragile and will need supports for storage and display.
Storage of archaeological textiles must be in a clean, dry, pest free environment of 50% RH/ 65oF. Supports to prevent damage from handling must be used. Low light levels with no UV should be maintained.
Allen, N., Edge, M. & Horie, C., eds., (1992) Polymers in Conservation. Royal Society of Chemistry: Cambridge.
Morris, K & Seifert, B. (1978) Conservation of Leather and Textiles from the Defence. Journal of the American Institute of Conservation, Vol. 18 (1): 33 to 43
How do you assess archaeological paper and decide upon a conservation treatment?
Paper made before the 19th century was often made by hand from linen and cotton rag materials, which are excellent sources of high-cellulose, long fibers. Sizing for the paper was made from animal hides. These papers were mildly alkaline and are more stable. At the end of the 18th century wood fiber took the place of cotton and linen. Wood fibers are shorter and have lower cellulose contents than cotton and linen fibers; additionally, wood fibers contain lignin. Producing paper made with wood fiber required mechanical action and bleaching to make it useful.
Survival of paper from archaeological sites is somewhat rare, and is similar to that of textiles. It generally survives due to anaerobic conditions or proximity to other materials that inhibit attack from microbial and fungal organisms or insects. Paper from drier sites is usually brittle, stained, and fragmentary and found in association with a metal such as copper. Paper from wet, anaerobic sites can survive in protected sites, but is weakened by acid hydrolysis and oxidation. Any writing is usually greatly damaged and often illegible.
Paper is an organic material and is subject to deterioration caused by chemical, physical and biological agents. Acid-catalyzed hydrolysis is the predominant mechanism for cellulose degradation and loss of strength. Acidity can come from the paper manufacturing technique. Manufacturing processes used from the end of the eighteenth century to modern times produced a higher percent of acid-rich papers than earlier techniques. Environmental acids from exposure to acidic air pollutants, acidic materials in contact with the paper, and degraded coatings, can also damage papers.
The strength of paper results from the particular chain length. Cellulose is made up of repeating units of glucose monomers and the number of glucose units present provides a measure of degree of polymerization (DP). Acids break cellulose bonds randomly often cutting the cellulose polymer in central regions. These attacks quickly weaken the fibers.
Paper from an archaeological site will be very fragile. Immediately place between supporting layers (e.g. Mylar sheeting), store in a refrigerator, and call a paper conservator. Keep paper in the dark, as exposure to light can cause rapid fading of any inks, dyes or other pigments on the surface of the paper.
Archaeological paper must be stored in a clean, dry, pest free environment of 50% RH and 65oF. Supports to prevent damage from handling must be used. Low light levels, with no UV, should be maintained.
Kundrot, R. & Zicherman, J. (2001). “Paper Permanence” In: Beale, F (ed.) The Encyclopedia of Materials: Science and Technology. Elsevier Science: London
Composite objects are defined as anything that is made up of two or more different material classes. Common examples of composite artifacts are: wooden timbers with iron fasteners, tools with wooden handles and metal components, an iron knife with a bone handle, a textile with metal fasteners, a glass bottle with a cork stopper, or a metal cufflink with a paste glass inset.
Expanding corrosion products on the iron tang are causing the bone knife-handle to crack. Photo by M. Myers. Used by permission of the Virginia Department of Historic Resources.
The same factors in the burial environment that act on individual materials will also affect composite materials. However, the deterioration of composite materials can also be affected by the proximity of different material types to each other. These effects may be detrimental or quite positive. For example, the rapid corrosion of certain types of metal can create casts of organic materials that are in contact with them, preserving morphological information that might otherwise have been lost when the organic rotted. Expanding corrosion products on the iron tang are causing the bone knife-handle to crack. Photo by M. Myers. Used by permission of the Virginia Department of Historic Resources.
Since there are so many variations both in the materials that composite artifacts are made from and the condition of those materials on excavation, there are no blanket treatments for composite artifacts. Instead each treatment must be designed uniquely for that artifact. Composite objects can be found either in wet or dry environments, but in either case they can pose some of the worst possible problems in conservation. The worst problems arise when one of the component materials actively contributes to the degradation of the other components – i.e. decaying wood can release acids that will attack metals and accelerate their corrosion, while metal corrosion products tend to be of greater volume than the original metal, so the expanding metal will put mechanical stress on the organic material.
Additionally, corrosion products will stain organic materials, and the metal salts will cause cellular disruption, accelerating the decay of wood and bone.
Another serious problem is that the standard conservation treatments used for one of the materials may often damage other component materials. For example, waterlogged wood is usually treated with polyethylene glycol (PEG) before being freeze-dried, but PEG is corrosive to iron. Similarly, iron artifacts from marine sites are often desalinated by electrolysis or by soaking in caustic solutions, but these solutions or electrolytes tend to have a high pH, which can accelerate the degradation of wood. Sometimes the object can be disassembled, so that the different components can be treated separately, then reassembled.
If disassembly is not possible, then it is time for an earnest talk about conservation priorities:
- Does the object need active intervention to prevent its loss? Or can it simply be stabilized in its current condition? This may be an option for some artifacts from dry, terrestrial sites.
- If it is a wet composite object, you may have to consider whether one component is less critical to the interpretation of the object than another: i.e. is the timber or the nails more important? The axe head or the fragment of handle?
- If there are no easy answers, there are options for treating all components, or at least minimizing damage to one part while treating the other part. This can be time consuming and, possibly, expensive. It may require additional corrosion inhibitors, neutral pH electrolytes, and constant monitoring of treatment processes.
It is very important to take this type of object to a professional conservator and to discuss all the options with them.
However the object is treated, in the end, it will still be very sensitive to environmental conditions. Since the different components will prefer different conditions, it will need careful monitoring in storage. Consult with your conservator to determine how best to balance the different needs, and set up a way to monitor the object.
How do you assess architectural materials, brick and stone and decide upon a conservation treatment?
Stone falls into three geological groups: igneous, sedimentary and metamorphic. Igneous rock (i.e. granite, obsidian, and basalt) is formed when molten materials cool. They are generally hard, and have a compact and non-porous structure. Sedimentary rocks are formed through the sedimentary deposition of minerals, other rock fragments, and/or skeletal material from marine animals. Over time these sediments become cemented together creating fairly soft and porous stones, such as limestone, sandstone and shale. Metamorphic rocks are sedimentary or igneous rocks that have been changed or recrystallized through the effects of pressure and high temperature. They have a denser structure than sedimentary rocks and include stones like marble and slate.
Plaster is made out of gypsum, hydrated calcium sulphate that has been heated so that all the water is driven off. When water is added again, the gypsum reforms and sets into a hardened mass. Plaster can be cast in a mold or modeled, carved and attached to something else. Plaster can be painted when dry or mixed with pigments when still wet.
Bricks are molded rectangular blocks of clay baked by the sun or in a kiln until hard and used as a building and paving material. They have properties that are similar to those of ceramics.
Architectural materials are often thought of as being chemically and physically resistant to damage because they are robust and are part of our natural outdoor environment. Nevertheless, these materials are not immune to damage from burial, cleaning, handling and storage. Stones with a laminated structure, like redstone or shale, and those with high amounts of calcium carbonate, such as limestone and marble, are particularly vulnerable to damage. They can become softened or pitted and soluble salts may enter the matrix of the stone. Once the salts re-crystallize, they can cause spalling and destruction of the overlying layers destroying any detail or decoration and weakening the stone. Limestone and marble are particularly vulnerable to the effects of weak acids, which can dissolve the calcium carbonate components of the stone. Plaster is vulnerable to the effects of water and can easily be dissolved. It survives poorly in wet burial environments.
Architectural materials are often heavy and can be damaged physically through improper handling and packing. The weight and composition of these objects must also be considered when undertaking treatments on them.
Some of these materials are composites of many different types of clay, minerals, sand, rocks and modern polymers (in the case of more modern building materials). It is important to identify the type of “stone” or material you have before cleaning or treating the object. Artifacts can be composed of different types of rocks, and their porosity and homogeneity can vary dramatically. Some stone is very difficult to clean, as it delaminates easily. Any cleaners containing acid can attack materials made of limestone or marble. Some plasters and mortars will dissolve if they are cleaned with water. By analyzing, or identifying, the type of stone, mortar, brick or plaster you have first, the cleaning method chosen will be more successful in the long term.
It is best to test any cleaning method prior to cleaning an entire object first. Dry brushing the materials with a soft brush post-excavation is acceptable. After analyzing the materials, some wet cleaning may also be performed. Other cleaning methods used in conservation include using organic solvents, erasers, poultices, and laser cleaning. All of these techniques should be tested prior to cleaning an entire object and should be performed by a professional conservator.
Staining on the objects should be considered part of the archaeological history and evidence of the object, and should only be removed after consultation with a professional conservator.
In outdoor environments, biological growth may occur on architectural remains. Sometimes, this growth can be disfiguring to the object, such as is sometimes seen on tombstones. Most of the fungal and microbiological growth can be removed by cleaning the object with water; however, a biocide may be necessary in order to kill the micro-organisms and to keep re-growth to a minimum. The use of biocides is not without its own problems, and should be considered carefully before use. Household bleach, Lysol and other proprietary biocides should not be used on objects as they are often too strong and can leave detrimental residues in the object.
Many architectural materials are subject to damage by salts. Desalination may be necessary in order to remove salts from the interior pores of many types of brick, mortar, and stone. If the objects are not desalinated, salt efflorescence may occur causing further physical damage to the materials once they are dry. Please refer to section 4 Processing Archaeological Artifacts of this document to locate more information on checking for soluble salts, and desalinating. Because of the susceptibility of plaster to dissolution in water, consolidation may be necessary prior to desalination; it is best to consult a conservator prior to attempting to desalinate plaster. After these objects have been surface cleaned and processed, further conservation treatment may be necessary. If the object is broken it may need to be mended. Stone and brick tend to break due to their excess weight and brittleness. It is not always necessary to mend the materials, but if it is desired a suitable adhesive should be used. For small breaks or fractured pieces, Acryloid B-72 may be used. For larger fragments, or heavier materials, a stronger adhesive system, such as an epoxy or a silane, may be required. If structural support is necessary, a more intrusive method may need to be undertaken, such as inserting dowels to attach to very heavy pieces back together or using metal frames or channels to support the piece. All of these treatments have their drawbacks, and should be considered carefully.
Architectural materials should be stored in a controlled environment where there are no rapid or sharp fluctuations in RH. If there are pigments or paints remaining on the surface light levels may need to be controlled to avoid fading; otherwise most architectural materials can tolerate a high degree of light and UV radiation. Depending on the size and shape of the artifact, supports for storage may be required.
Archaeobotanical remains can be anything from a tiny fragment of wood, to seeds, soils, nuts and plant specimens. These materials are generally collected in flotation samples and should be analyzed by a professional archaeobotanist. Generally, archaeobotanical remains do not require conservation and in many instances conservation can impair future analysis. If long-term storage of such materials is necessary it is best to consult an archaeobotanist.
Rose, C. & Torres, A. (1992) Storage of Natural History Collections: Ideas and Practical Solutions. New Haven: Society for the Preservation of Natural History Collections
Copyright © 2006 Colleen Brady, Molly Gleeson, Melba Myers, Claire Peachey, Betty Seifert, Howard Wellman, Emily Williams, Lisa Young. All rights reserved. Commercial use or publication of text and graphic images is prohibited. Authors reserve the right to update this information as appropriate.