- What is Preventive Conservation?
- Temperature & Relative Humidity
- How do I assess whether my storage site is good or bad for my collection?
- How do I create microclimates for storage?
- How do I know my plastic bags are safe for artifact storage?
Deterioration is a continuous, natural process. It can, however, be slowed. Indeed, science has suggested ways in which the natural lifespan of most museum objects can be extended. Preventive conservation is a process that seeks to prevent, reduce or mitigate the effect of all the factors that threaten an object’s continued survival. It requires a holistic approach and continual assessment of how collections are stored, handled, displayed and maintained. A successful preventive conservation approach should involve all people who work for a cultural heritage firm, not just conservators. Preventive conservation is a strategic enterprise driven by a museum’s primary purpose. It requires planning, teamwork, and some initial funding to institute a successful preventive conservation policy but its benefits are significant over the long-term.
Conservators often use the term “agents of deterioration” to address those factors which cause the most significant damage to artifacts. The most common agents of deterioration are: light, relative humidity, temperature, pollutants, man, and pests. Preventive conservation seeks to mitigate the effects of these processes. Many of the techniques used in preventive conservation are based on common sense and good housekeeping. However, these must inevitably be reinforced by the results of current research, and access to specialist information is vital if an informed approach is to be taken. Appropriate and continuing training, to ensure consistency in approach over time and between individuals, is also vital.
See: Storage Environments: Packing and Labeling Requirements Storage Environments: Monitoring and Control
TABLE 8a – Preventive Conservation – a summary of the threats (and their possible remedies). (Adapted from a table created by Fergus Read, North West Museums Service, Lancashire, England http://www.meaco.com/preventativeconservation.htm
|THREAT||SPECIFIC||DAMAGE||POTENTIAL CAUSE||PREVENTIVE ACTION|
|People||Staff -Visitors -Intruders||Breakage -Abrasion -Crushing-Theft||Unnecessary handling
– Open displays
– Poorly organized storage
– Poor labeling
-Poor handling. Poor cleaning
-Inadequate physical or electronic protection
|Prevent visitors from touching objects
-Label objects clearly
-Handle only as required, using approved procedures
-Upgrade physical and electronic security-Train and motivate staff
|Relative Humidity||Incorrect humidity level (high or low)
-Rapid fluctuations in RH
|-High humidity causes mold growth and corrosion — Low humidity causes embrittlement of organic materials — Fluctuations cause splitting, cockling, warping||Changes in the weather
-Floods and leaks
-Wet-cleaning of floors
-Poorly insulated building
-Inappropriate heating source or temperature control regulation
|Regularly measure and record RH
-Reorganize collections to put most sensitive material in locations best suited to them
-Improve air circulation-Improve insulation
-Attempt to impose RH control (by use of humidifiers/ dehumidifiers)
-Rapid fluctuations in temperature
|-Heat causes increase in degradation
-Fluctuation causes splitting, cockling, warping
|-Changes in the weather
-Poor building insulation
-Poor heating control
-Radian-Regularly measure and record temperature
-Mount lights externally to display cases
-Control temperature (by air-conditioning or use of heating/humidity control)
|Light||-Exposure to excessive light especially
– high intensity light
– short wavelength light
|Natural and artificial light:
-Too many windows
-Objects poorly positioned
-Inappropriate artificial light sources
-Lack of filters or blinds
|Measure light intensity and ultraviolet (UV) levels-If appropriate, set annual lux hours exposure limit
-Filter to reduce UV light
-Reduce light intensity in display areas–Block out all light from storage areas
-Reduce display times
-Rodents; rats, mice
|-Gaps in building shell
-Damp (high relative humidity)
Infested acquisitions or loans
-Attractants and food sources used in displays (such as plants and display props)
|-Regularly monitor with sticky traps
-Inspect all new acquisitions
-Regularly inspect vulnerable items-Isolate infested items immediately
-Maintain building shell and keep storage areas cool
-Avoid damp or humid areas
-Regular and thorough cleaning
|Pollutants||-Wind-borne gas and particle pollutants, especially
– oxidant and sulphiding gases
– dirt and dust
|-Degradation of materials||Close proximity to road
-Lack of air-filtration
-Poor door/window fitting
-Inappropriate cleaning methods and agents
|-Identify the type & sources of gaseous & particulate pollution, determine risk and reduce ingress and/or;
– use mechanical air- filtration
-reduce impact on objects
– box or wrap objects in store
– use cleaning techniques that do not involve excessive use of chemicals
|Storage & Display
|-Gaseous, chemical and particulate migration from materials used in construction of display cases, mounts and frames, storage racks, boxes and packaging||Corrosion
-Physical damage (e.g-staining)
|Use of inappropriate materials, with high acidic characteristics especially: boards and papers, composite woods, many paints, many glues, some plastics, and felt amd other woolen material||Ideally, use only those materials approved and tested for a particular situation
-Avoid use of known problem materials
-If possible test unknown materials before use; otherwise seal, cover or vent to mitigate possible effects
|Flood causes staining, ink and dye run, mold growth, warping, swelling, corrosion, disintegration
-Fire causes incineration, scorching, chemical deposition
|-Floods may be caused by burst pipes, leaking roof, fire-fighting water, hurricanes and high wind
-Fire may be caused by electrical faults, arson, accident and lightning
|– Formulate a comprehensive Disaster Plan, include checklists for all situations, such as:
– contractors (special care to be taken during construction)
– maintenance checks (internal & external)
– Do not allow smoking
– Train, and test staff
Temperature and relative humidity are important agents of deterioration. Relative humidity (RH) is expressed as a percentage and represents the ratio of water vapor in the air to the amount the air could hold if fully saturated. Low RH values means dry conditions since the air is capable of taking up moisture; high values mean the air is humid or wet and unable to hold much additional moisture. Temperature is measured with a thermometer; RH is measured with a hygrometer; a hygrothermograph measures both.
Although both temperature and RH affect collections, RH is the more important factor of the two for most archaeological collections. High RH can lead to corrosion on metals, swelling of organic materials, growth of mold and insect infestation. Low RH can lead to shrinking and embrittlement of organic materials. High temperatures increase the rate of chemical reactions and provide a welcome environment for many pests. In general, high RH and temperature levels serve to catalyze undesirable chemical reactions in materials, particularly if they are already unstable.
Conditions of extreme or rapidly fluctuating relative humidity present major risks, particularly to organic materials. The hygroscopic nature of organic materials (they easily absorb and release water) means that they can expand and contract with fluctuating RH. This generates stresses that may be manifested as cracking and splitting. It is important to avoid rapid changes of RH (no greater than +/- 5% in 24 hours) while staying within the safe range (40-65% RH for organics and composites, below 50% RH for metals and less than 20% RH for archaeological iron).
If relative humidity is controlled, temperature control is generally less crucial. Ironically, however, since the human body is far more sensitive to heat and cold than to humidity, it is temperature control that is more often seen as the priority in public buildings. Public areas are usually kept between 17-19oC (63-66oF). For collections care, a range of 15-25oC (59-77°F) is acceptable for most collections, while storage areas can be kept at cooler temperatures to both save energy and reduce decay rates.
TABLE 8bi Optimum and acceptable RH levels for archaeological objects (adapted from a table created by Fergus Read of the North West Museums Service, Lancashire, England available at http://www.meaco.com/preventativeconservation.htm)
Controlling temperature and relative humidity is a key aspect of preventive conservation. Although monitoring alone cannot prevent damage, becoming aware of what is occurring in a collections space, through monitoring, is the first step to preventing damage. Data from environmental monitoring is often an important form of supplementary information when applying for funds to improve collections storage. In order to be the most effective, the monitoring program should be carefully planned and should continue throughout a change of seasons. It is desirable to record both the seasonal and daily fluctuations that occur in the storage or exhibition space. Many different types of monitors are available for providing temperature and RH data.
Direct Read Thermometers and Hygrometers
These systems are the simplest and least expensive environmental monitoring equipment available. They are also subject to the greatest number of limitations. They do not record any data, so the person responsible for the collection must regularly inspect the monitors and record the readings. It is best to create a regular schedule that that includes readings at different times of the day, and to record the data on a table. Since there will undoubtedly be periods when no readings can be taken, there is a chance that changes in the environment may go unnoticed and unrecorded. Direct read hygrometers and thermometers can be broken into a number of categories.
Humidity indicator cards are small cards with chemically impregnated spots that change from blue (dry) through lavender to pink (humid) depending on the humidity in the location. This is the simplest form of a hygrometer and the least expensive (approximately $21 for a pack of 100 cards). They have a long shelf or use life, but do not have great precision or accuracy.
Psychrometers measure the ambient (dry-bulb) temperature and the temperature of an evaporating water source (wet-bulb, or a moistened wick surrounding the thermometer bulb). Using a psychometric chart, the two temperatures are compared and a relative humidity reading is produced. There are two versions available, a digital psychrometer and a sling psychrometer, ranging in price from $50 to $200. The digital psychrometer uses a small battery powered fan to provide the air flow over the wet bulb, whereas the sling psychrometer requires several minutes of manual work to achieve the required air flow while the wet bulb temperature is obtained. While these are often used to calibrate other forms of monitoring equipment, a false reading can occur if they aren’t correctly maintained by staff members, if the wick becomes contaminated, or if the batteries become weak thereby causing an inadequate air flow for the wet-bulb reading.
Digital thermometers and hygrometers are usually hand-held instruments with digital sensors that give instant readings of temperature, relative humidity or both. Some have a built-in memory function that can record minimum and maximum readings for both temperature and humidity since the last memory check. They are usually small, accurate and relatively inexpensive ranging from $20 to $100 depending on the manufacturer and what additional features they may have. Some may require time for the readings to stabilize.
These instruments incorporate either mechanical or electronic sensors that control two arms, each with a pen on the end, which record the temperature and RH readings as graphs on a moving drum or disk. This provides a written record of the temperature and relative humidity of the space being monitored. There are numerous varieties available, from companies such as Belfort, Cole-Parmer and Dickson. Prices vary from $350 to $700 depending on the size of the charts, the level of accuracy, and the number of accessories included. Some versions offer remote leads for easy monitoring in closed storage areas or in cases without easy access. Others have high and low alarms, digital readout of current conditions and an AC/DC operation with battery back-up. Typically, electronic sensors are better than mechanical sensors, and battery operated drives tend to function better than clock, or wind-up, drives. Also, felt-tip pens tend to be easier to use, and less affected by operator error than inked pens.
In addition to purchasing the hygrothermograph, it is necessary to purchase charts, replacement pens and batteries on a regular basis. Like other environmental measuring devices, hygrothermographs also need to be calibrated every few months to maintain accurate readings depending on the condition surrounding the machine. Depending on the chart available for the recorder, the charts need to be changed on a daily, weekly, or monthly basis. These charts provide a continuous paper record of temperature and humidity conditions without the need for a staff member to visit the location multiple times a day to collect the readings. The charts also provide information on fluctuations, which can be correlated with weather and building systems.
Data loggers are generally small instruments that can be placed in display cases or storage areas. They log environmental data that is then downloaded to a computer and analyzed by a software program to produce easy to read charts. Data loggers are generally more accurate and require less calibration than recording hygrothermographs. They range from a cost of $59 to $700 depending on brand, accuracy and accessories. Brands available include ACR, HOBO, Dickson, Tracker, TinyTag and Nomad. The unit price generally reflects the quality of the sensor, the longevity of the battery, the durability of the casing, the flexibility of the software, and any additional features such as displays or alarms. Unless a data logger is equipped with a digital readout, obtaining real-time data is not an option. Data can be collected after days, weeks, or months depending on the programming and the logger’s capabilities, and then the resulting data can be graphed or further analyzed. While this allows one to track trends and requires less maintenance than a digital hygrothermograph, the down side of this is that one may not always realize that a problem has occurred until after the fact.
Hard-wired systems are used for general environmental monitoring. They provide real-time data, allowing problems to be assessed immediately. A transmitter, which includes sensors and electronics, sends a signal up to 1 km away, measuring temperature and relative humidity. A central computer digests the data, stores it and emits an alarm when the temperature or humidity fluctuate above or below set parameters. The installation price is high, typically $1000 per measuring site. Service and maintenance are additional. In locations where cables can be easily damaged or regular changes need to be made, the connection and re-routing of cables can be problematic.
Radio Telemetry Systems
Radio Telemetry Systems offer immediate real-time data from sensors placed in the location that is to be monitored. Data is collected by battery-powered sensors, which contain a micro-controller that records environmental data at set intervals. The data is then transmitted by low power radio waves until the ‘receiver’ picks it up. The receiver is connected to a controller, which interprets the data to your PC for permanent storage. Since the information can be accessed without disturbing the artifacts or location where the sensor is placed there are real advantages to this type of system over data loggers. The sensors are not limited to a specific spot, as in a hard-wired system, but allow for some location adjustment as the exhibit or storage space changes. Repeaters can also be purchased to increase the distance between the sensors and the receiver.
Installation of radio telemetry systems can be very expensive and the equipment is costly to purchase and repair. Initial radio frequency problems can occur due to radio interference from security or maintenance department radios.
The system chosen for monitoring relative humidity and temperature levels ultimately depends upon the institution’s budget, and the requirements and limitations of the space being monitored. In most cases, using two types of monitoring equipment in one location is preferable. Having a psychrometer to calibrate the recording hygrothermograph, or having a hygrothermograph to double-check the information retrieved from a data logger are two commonly used examples of this technique.
See: Storage Environments: Monitoring and Control
Conserve O Gram number 3/3, June 2001 “Data logger Applications in Monitoring The Museum Environment, Part 1: Comparison of Temperature and Relative Humidity Dataloggers”
Light can cause serious irreversible damage to archaeological collections. Light is a form of energy that causes fading and chemical breakdown of the materials from which an object is made. Light levels can be stated in terms of footcandles or lux. Footcandles are a unit of light produced by one candle at a distance of one foot, or the amount of light equal to one lumen per square foot. Lux is an international measurement of light equal to one lumen per square meter. One footcandle equals approximately 10 lux.
Light causes damage to an object in proportion to its intensity and the exposure time. A light of 500 lux will theoretically cause the same amount of damage to an object in one year as a light of 50 lux will cause over ten years. Thus short exposure to a high lux level(e.g., 2500 lux caused by photographic or laboratory lights, or 200 lux for short exhibitions) need not cause undue damage over the total life of an object, provided this high exposure is compensated by a proportionate period of time in a lower than normal illuminance, or in total darkness.
While it is usual to reduce light levels in museum displays, this need not be the only response; reducing the time an item is exposed to light is equally legitimate. Fitting display lights with a timer, or limiting the overall time an object is on exhibition in a given year, is appropriate. For the most light-sensitive objects, it may be useful to establish annual lux-hours exposure limits. Lux-hours are a measure of exposure (illuminance x time).
TABLE 4 – Recommended Light Exposures For Archaeological Objects and Records on Display (Adapted from a table created by Fergus Read, North West Museums Service, Lancashire, England available at: http://www.meaco.com/preventativeconservation.htm
|MATERIAL||RECOMMENDED MAXIMUM VISIBLE LIGHT LEVEL (lumen per m2, or lux)||RECOMMENDED MAXIMUM ANNUAL LUX HOURS EXPOSURE (illuminance x time)|
|Waterlogged Organics||Total Darkness||Not applicable|
|Archaeological Textiles, Archival materials such as watercolors, prints, drawings, paper items, photographic prints (color), transparencies||50 lux||96,000|
|Plastic (especially Bakelite, Ebonite & polythene)||100 – 200 lux||192,000 – 384,000|
|Undyed leather, wood, horn, bone, ivory Photographic Prints (black & white)||200 lux||384,000|
|Inorganic materials (such as metals, stone, glass, and ceramics)||300 lux. (Material would not be unduly harmed by higher, but this level reduces the eye adaptation difficulties for visitors whereother collections are displayed in darker illumination; for similar reasons a maximum illuminance of 400 lux in the remaining public spaces in a museum might be recommended.||576,000+|
The simplest way to prevent light damage is to reduce the amount of light entering a location where artifacts are stored or displayed. Sunlight has a high percentage of ultraviolet rays, which cause more damage than most artificial light. Therefore, it is best to filter natural light with window coverings, shades, blinds and curtains: this will also reduce solar heat gain. Neutral-density window film reduces the amount of light entering a building’s windows, but still allows people to look out. Widely used on buses and trains, such films can offer an effective alternative to blocking a window or putting up curtains. Turning off interior lights when not in use and storing artifacts in folders and boxes also help to limit long-term exposure and damage.
Most light sources emit some ultraviolet (UV) radiation. This is light beyond the limit of human vision, at the violet end of the spectrum, and is damaging to most museum objects. Since UV is not necessary for seeing objects, and can readily be reduced through the use of filters, removal of the UV element from natural and artificial light sources should be a goal. A meter can be used to measure the proportion of UV in the light source in microwatts per lumen (mW/lumen). A maximum acceptable reading is 75 mW/lumen, although filters should be able to reduce this to less than 10 mW/lumen. A periodic check is needed to test the continuing efficiency of UV filters (especially of window coatings and film), as this declines over time. Various filters are available for fluorescent lights in the form of soft, thin plastic sleeves and hard plastic tubes which fit over the bulb. Research indicates that both varieties of filtering tubes retain their ultraviolet (UV)-absorbing properties for approximately 10 years. UV-filtering window films may have similarly limited life spans.
It is important to remember that the radiant heat from lights, especially in enclosed cases, often poses as much of a problem as the quantity and quality of the light that they issue. Low-voltage bulbs often emit great heat. Fluorescent tubes although relatively cool require diffusers or sleeves to counter relatively high UV emissions. Fiber optics are generally the coolest light source, and are therefore best for mounting inside an enclosed case
For more in-depth information about the types of light bulbs used as sources of light in archival spaces and filtering techniques read Beth Lindblom Patkus’s article Protection from Light Damage copyrighted 1999, Northeast Document Conservation Center
Many materials, particularly organic artifacts, are sensitive to light. Excessive exposure of these materials to light can cause fading and structural damage. In order to protect a collection from light damage, a combination of monitoring and controlling the level of illumination and the duration of exposure is the best approach in order to prevent light damage from occurring. Since it is not always possible to keep a collection in complete darkness, one needs to become aware of the lighting conditions your collection will be exposed to in order to best protect the collection. Using visible and ultraviolet light meters to assess the intensity of illumination upon the artifacts, allows one to apply proper preventive techniques in the most cost-effective manner. Constant monitoring is needed to ensure that light levels do not exceed recommended levels. Records should be kept of when artifacts were exhibited, and under what light conditions, and the annual light dose (Intensity X Exposure time) each artifact receives.
Monitoring for Light Damage
Fading and discoloration are common effects of light damage on objects. There are two tools that can help you determine the degree of fading and discoloration that is caused by light intensity in a certain location.
The Blue Wool Standard test cards are used to monitor the effects of the net exposure to light for an artifact on display, and to alert conservators to rotate an exhibit or to reduce the intensity of illumination. They are relatively inexpensive, about $9.00 each. Each Blue Wool Standard contains eight samples of blue-dyed wool. Sample 1 is extremely light sensitive. Sample 2 takes twice as long to fade as Sample 1, Sample 3 takes twice as long as Sample 2, and so forth. Sample 8 is the most stable dye available which is not permanent. To demonstrate the degree of fading at a particular location, cover half of the card with a light-blocking material to protect it from light damage. Write the date on the card, and place it in the desired location. Check the card every couple of weeks to determine how long it takes for the various samples to fade. The sensitivity of the first few samples corresponds to light sensitive materials, such as paper and textiles, allowing you to gain an idea of the amount of damage expected if materials were exhibited for the same period of time at the current light level in the location.
The Canadian Conservation Institute (CCI) has developed a Light-Damage Slide Rule to estimate the damage that can result from particular intensities of light and lengths of exposure. The Light-Damage Slide Rule is a sliding plastic scale that aligns project light types, light levels, and exposure times to predict the fading of a blue wool card under these conditions. For example, it shows that an artifact displayed at 150 lux for 100 years will fade at the same rate as an artifact displayed at 5000 lux for 3 years. The above-mentioned exposure of 150 lux for 100 years would cause significant fading of Blue Wool standard 4 and below. The Slide Rule also compares damage that would be caused by UV-filtered and unfiltered light.
Visible and Ultra-Violet Meters
Visible light is measured in lux or footcandles, and ultraviolet light is measured in microwatts per lumen. Since a light meter measures all the light hitting an object, an ultraviolet meter must be used to measure the UV component of light, which is the most energetic and destructive form of light to artifacts and art objects. If UV levels exceed 75 microwatts/lumen (mW/M2), filtering material is required between the light source and archaeological object.
For lux or footcandle meters, it is best to get a meter that will read clearly down to 5 footcandles, or 50 lux. Some suppliers of light meters include: Extech Instruments, University Products, Minolta, Cole-Parmer, The Cooke Corporation, Gaylord Brothers, Emund Optics, International Light Inc., and Littlemore Scientific Engineering Co. The price range varies depending on what type of light you want measured, and the extra accessories that are available with the meter.
The most basic light meters measure only visible light, either in foot-candles, lux, or both. They usually have a digital display, are portable in size, come pre-calibrated, are battery-powered, and measure a minimum of 0 to 2,000 foot-candles. Some have remote sensors, while others offer auto zero adjust, or a data hold function. These instruments range from $225 to $365.
UV light meters are generally more expensive ranging from $515 to $2,000. Most UV light meters can also measure visible light, while a few are also able to measure thermal radiation (IR), the amount of solar heat coming in through windows and the heating effect of lamps, and have capabilities to log data. Thermal radiation meters can measure the performance of heat reflecting films. The basic meters all have digital displays and are battery operated. UV meters should measure the total amount of UV present and the proportion of UV to visible light present.
Using a Camera to Measure Light Levels
It is possible to use a 35mm single-lens reflex camera to estimate light levels. It is not as accurate as using a meter, but gives an estimate within 3 footcandles of the footcandle meter reading. This method requires a 35 mm single lens reflex camera with a built-in light meter, and a white card measuring 12″ X 16″.
- Set the camera film-speed reading at 800 ASA, and set the shutter speed of a 1/60 of a second.
- Have someone hold the white card in front of the artifact, and at the same angle as the artifact.
- Position the camera so that the card just fills the view screen.
- Adjust the aperture setting until the camera’s light meter shows a correct exposure.
The following list shows how the f-stop reading relates to lux and footcandles:
- f4 indicates 50 lux or 4.6 footcandles
- f5.6 indicates 100 lux or 9.3 footcandles
- f8 indicates 200 lux or 18.6 footcandles
- f11 indicates 400 lux or 37.2 footcandles
- f16 indicates 800 lux or 74.3 footcandles
Archaeological materials, particularly organic materials, such as leather, wood, natural fiber textiles, and paper, are vulnerable to damage and deterioration caused by biological organisms such as vertebrate pests, insects and even microorganisms, such as mold. Both physical damage (from burrowing, tunneling and gnawing) and chemical damage (from saliva and other digestive functions and from feces) may occur resulting in a variety of problems from weakening and staining to losses and powdering. The damage is almost always highly disfiguring, irreversible and can progress very rapidly. Preventing pests from accessing the collections is the most effective method to protect the artifacts.
All aspects of preventive conservation support each other and this is particularly true for pest management. Developing and following an Integrated Pest Management (IPM) system is an important step in preventing an infestation. The aim of an IPM program is to protect the museum and its collections from pests, and to reduce the use of pesticides, which can be harmful both to the artifacts and to the staff and visitors. Traditional methods of pest control often include chemicals and/or fumigants which may stain materials produce chemical changes in the object material, and cause short-term and long-term health hazards to staff that handle or work near the materials.
If the exterior of the building is maintained, there will be fewer areas where pests can enter the building and damage the collections. Inspect the building’s exterior for possible entry routes and make repairs where necessary. Exterior doors and windows should be tight fitting and securely closed. All vents and openings should be secured with galvanized wire mesh to prevent animal entrance. Bird nests should be eliminated as well as external accretions of bird droppings, since they are especially attractive to carpet beetle larvae. Make sure there is no stagnant water around the building.
Inside the building, the following steps can be taken to lessen the chances of an infestation. A clean building and a monitored environment will discourage pests from making their home in storage areas. Food and beverages should be limited to designated areas away from collections and garbage should be removed daily from the building. Regularly remove dust from floors and clean light fittings (particularly fluorescent light diffusers) where dead insects can gather, so as to remove any sources of food. Storage areas should be kept cool, clean and uncluttered. As an added precaution, restrict or eliminate live plants and flowers in the building to prevent insect hitch-hikers.
To prevent mold growth, the environment should be kept cool and the relative humidity should be kept between 45-60% with few fluctuations. There should be good air circulation to prevent areas of high humidity forming. Any refrigeration storage units should be regularly checked to ensure they are working properly and not leaking. Artifact and storage areas should be kept clean and free of dust, which can include mold spores. Leaks in the roofing, gutters, windows and pipes should be repaired and dried. If any artifacts or areas become damp or wet they should be promptly dried and cleaned. Artifacts should not be stored on the floor or directly against an outside wall, which often has a higher level of relative humidity and condensation levels.
How to Monitor the Collection
Insect and fungal infestations usually indicate an environmental problem, such as high relative humidity, gaps in the building structure or poor housekeeping. In order to protect your collection you must know which types of pests can damage your artifacts, and how to identify signs of infestation. The use of glue or sticky traps to determine the existence of and to identify insect pests in the collection is recommended. These monitoring devices can be furnished by your local pest control technician or purchased in hardware stores and come in strip form or can be folded into a low profile box shape.
The traps should be placed where insects are more prone to be located. Sticky traps usually catch insects only on the move, such as adult forms of insects that are migrating to breed (usually between April and July). Prime locations for trap placement are along baseboards, under storage and exhibit furniture, on windowsills near floor drains and in cabinets. It is also good to monitor non-storage areas within a building to ensure that pests cannot enter these areas and spread to storage areas. These areas may include mechanical rooms, attics, crawl spaces, kitchens, supply storage rooms, or areas along the building perimeter, which have accumulated plant debris, animal nests or are susceptible to mold and algal growth.
Create a floor plan indicating where traps are located to ensure traps are not forgotten. Set monitoring traps widely and inspect them on a regular basis, replacing when necessary. During each inspection note which traps have caught what insects. Various organizations, such as the Canadian Conservation Institute and English Heritage, have posters of common insect pests found in museums that can aid in the identification of pests. Common museum insects include booklice, silverfish, furniture beetles, powder post beetles, carpet beetles, clothes moths and carpenter ants.
How to Treat an Infestation
Use of commercial pesticides should be avoided, if at all possible. Unless a specific problem must be addressed, regularly scheduled chemical treatments should be avoided since there is no all-purpose chemical for eliminating every pest problem
For vertebrate pests, traps are the preferred method of treatment. Snap traps or live traps should be placed along baseboards and inside cupboards. They should be secured in place, if necessary. A floor plan should be used to prevent misplacing traps. Traps should be checked daily for signs of activity. Poisons should not be used, since it is possible that animals will die in inaccessible places, creating a food source for other pests. If birds nesting outside the building are a frequent occurrence, steps should be taken to discourage nesting. Placing wire screening, sheet metal at a 45-degree angle, or pointed wires over or on potential nesting sites are a few suggestions. Bird feces should also be removed as they attract insects.
The first step after discovering signs of an insect infestation is to identify the insect and the extent of the infestation. The infested artifact, or artifacts, should be isolated from the rest of the collection by placing it in polyethylene sheeting or a polyethylene bag in a freezer (below -20° C or -4°F) for a minimum of 48 hours. After the artifact is removed it should be vacuumed to remove the larva or remaining insects. In addition, the storage areas where the infestation occurred should also be vacuumed and cleaned. The used vacuum bags should be disposed of immediately afterwards in the outside garbage. An infestation of moths or other flying insects can be treated by leaving a low wattage light on overnight and attaching a fly strip near the light to catch the insects attracted to light. Chemical treatments should only be used when all other methods fail or when the infestation is large and effects a large area.
Fungal infestations should be treated with extra care and caution. Artifacts that are affected by mold should be carefully placed in sealed polyethylene sheeting or bags and placed in an isolation room (or refrigerator if the item is wet or damp) until they can be treated in a conservation lab. It is important not to disturb the mold while handling the affected artifact since it can release spores into the air, enlarging the contamination problem. If the artifact is dry, it should be vacuumed thoroughly to remove the mold spores. The used vacuum bags should then be disposed of in outside garbage. After the artifacts are isolated and cleaned, check the environmental conditions of the infested location to make sure proper relative humidity levels and temperature did not promote mold growth. Additional environmental monitoring can help prevent future fungal infestations.
Chicora Foundation (1994) Managing Pests in Your Collections. Chicora Foundation.
Konkright, D (1991) Insect Traps in Conservation Surveys. WAAC Newsletter vol.13 (1): 21-23.
All aspects of preventive conservation support each other and this is particularly true for pest management. Developing and following an Integrated Pest Management (IPM) system is an important step in preventing an infestation. The aim of an IPM program is to protect the museum and its collections from pests, and to reduce the use of pesticides, which can be harmful both to the artifacts and to the staff and visitors. Traditional methods of pest control often include chemicals and/or fumigants which may stain materials produce chemical changes in the object material, and cause short-term and long-term health hazards to staff that handle or work near the materials.
Pollutants can be separated into two categories, gaseous or particulate, based on size, and can result from internal and external sources, including from other artifacts. Both categories of pollutants can damage the collections by causing physical and chemical changes.
Sources of Pollutants
Potentially harmful pollutants can be found in every possible environment meant for storing or exhibiting artifacts. It is important to know what the sources are so the risk of endangering the artifacts is kept to a minimum. In a confined area with poor air circulation, such as a well-sealed display case, the levels of harmful pollutants can rise dramatically. In these environments, significant damage may occur.
Gaseous pollutants are found in various sources within and outside of the museum environment and usually produce strong oxidizing agents, resulting in chemical changes to the collection. Artifacts made from wood naturally contain organic acids, such as acetic and formic acid, which stimulate corrosion in metal artifacts, particularly those made of lead. Adhesives may release damaging gases during curing. The formaldehyde-based resins used as a bonding adhesive for wood and wood panel products release formic acid. Materials containing sulfur, such as wool, leather, parchment, rubber, and certain types of adhesives can tarnish silver, corrode polished copper, and damage photographic prints and negatives, paper, leather and some types of stone. Additional sources of gaseous pollutants include newly applied oil-base paints, cleaning agents, display materials, storage materials, building works, cellulose nitrate film, burning of fossil fuels, industrial emissions, and chlorides from sea air.
Dust and dirt are the most common examples of particulate sources of pollutants and can abrade or disfigure the surface of the artifact, usually through physical contact. They usually have a large organic component, including vegetable matter, human skin, fingernail shedding and hair, all of which are an excellent food source for pests. Dust absorbs moisture from the surrounding atmosphere and keeps it close to the surface of the artifact, becoming a prime location for mold or other fungal infestations to develop. Particulates can become concentrated or trapped close to the object’s surface and overtime dust can become part of the surface of porous artifacts. As the pores expand and contract due changes in temperature and relative humidity, dust can become adhered to artifact finishes. Fibrous materials, including carpeting and clothing, as well as furnaces, fireplaces, kitchen cooking and smoke are sources of particulate matter.
How to prevent pollutant damage
Preventing pollutant damage is similar to other preventative plans of action; maintain a clean and dust free environment, monitor the collection and improve the filtration system of the collection space. No smoking or eating should be allowed in the collection areas where artifacts are housed. The building environment should be climatically sealed with a filtered fresh air intake system. It is also important to change furnace and air conditioner filters on a regular basis to prevent particulate and gaseous pollution. Weather stripping should be installed on windows and doors. Cracks in cases, walls, windows and doors should be repaired in a timely manner, to lessen the amount of dirt entering the collection space. Vacuum cleaners should be equipped with a high-efficiency, particulate air (HEPA) filter, to prevent the redistribution of dust.
Specific measures are required in order not to introduce harmful pollutants to the artifacts themselves. Whenever treated metal artifacts are handled, gloves should be used to prevent contaminants, such as dirt and oil, from transferring from fingers to the metal surface. Materials used for storage and display should be made from stable or inert materials that will not release harmful gases into the environment. Materials used for construction and mounting in exhibits should not emit harmful pollutants. Materials and finishes that are well seasoned can be used as an alternative to freshly manufactured materials. Materials that create pollutants, such as wooden cabinets and shelves, cleaning compounds and some carpeting should not be introduced into the collections or building space. Records or artifacts should not be stored near copying machines, which may produce ozone and dust from the toner used. Concrete floors should be painted or covered to prevent abrasive dust from settling on open artifacts. Wherever possible, enclosures should be used to prevent particulate and gaseous pollutants from coming in contact with the artifacts. Boxes and folders meeting the American National Standards Institute (ANSI) standard create a stable micro-environment for permanent records and for artifacts.
How to isolate pollutants:
It may be impossible to completely prevent a potentially harmful material from affecting an artifact from the collection, and sometimes the artifacts themselves are the source of pollutants. In these cases it is possible to isolate the pollutant with the use of physical barriers such as films, coatings and scavenging materials to achieve short-term protection. Scavenging materials are useful in closed display and storage spaces and are non-interventive. They can be placed in the same space as the artifacts to trap various gaseous pollutants on a molecular level and prevent them from contaminating the artifact. For example, activated charcoal is effective at trapping organic chemicals. Many other chemicals are not attracted to carbon at all, so they will pass right through the filtration system. Once all of the bonding sites are filled, the activated charcoal filter stops working and must be replaced. MicroChamber papers and boards are comprised of alkaline buffers combined with dispersed molecular traps to remove and neutralize a wide range of acids, pollutants, and harmful by-products of deterioration. A similar product called Scavengel is offered by Gaylord Brothers. This is a pollutant control product in polyester sheet form that removes a broad range of common indoor gaseous pollutants. Other products are material-specific, like Pacific Silvercloth® or Corrosion Intercept®. Pacific Silvercloth® is a cloth used to protect stored silver from tarnishing atmospheric gases. Corrosion Intercept® is an enclosure material that offers protection for all metals against corrosive atmospheric gases, such as sulfur, and has also been used to protect textiles, paper documents, books, and works of art on paper. It is composed of highly reactive copper particles bonded into a polymer matrix. When the natural copper color turns black, it is time to change the intercept sheeting or bags.
How to monitor for pollutants
Although direct reading airborne particle counters are available, they are very expensive. It is best to use visual inspection, active sampling, passive monitors, metal coupon and material testing.
“Active sampling” techniques collect pollutant samples either by physical or chemical means for analysis in a laboratory. Typically, a known volume of air is pumped through a collector, such as a filter or chemical solution, for a known period of time, and then removed for analysis.
Passive monitors, or passive sampling devices (PSD), are usually contaminant specific, requiring one monitor per gaseous pollutant, i.e. an organic vapor monitor, or a formaldehyde monitor. A PSD can be a small tube, or a badge, which is opened at the sampling site. Sampling times vary between hours and days. Usually, the longer the sampling time, the lower the detection limit. The sample is then mailed to a laboratory for analysis. Detection limits are dependent on the sampling media, the compound in question, the analytical technique, and the sampling time. It is important to use the monitors correctly or false readings are liable to occur. The advantage of a PSD is that they are small, and able to fit into confined spaces like a drawer or a showcase.
Metal coupons are used as detectors for corrosive vapors specific to metals. The coupon is placed in the same environment in which the object(s) will later be placed. The coupon is generally of the same type of metal as the object. For example, if a silver coupon rapidly tarnishes after being placed in a new showcase, then perhaps that case is most likely not a good storage environment for silver. Evaluation of this kind of testing should be done with some reservations, as the test is not very sophisticated. For example, a difference in alloys between the coupon and the artifact can create a false reading. A refinement of the coupon method is to accelerate the corrosion rate on different metal coupons by raising the relative humidity and temperature and comparing them to a control coupon (this is often referred to as an Oddy test and can be used to test the effects of individual exhibit materials).
Material sampling requires removing a small sample of material and subjecting it to different tests that are usually also contaminant specific. pH papers and pens can be used to test the acidity of materials. Other tests include: The Iodide-Azide Test for the presence of sulphides, the Beilstein Test for the presence of chlorides in plastics, the Phloroglucinol Hydrochloric Acid Test for the presence of lignin in paper products, the Chromotropic Acid Test for the presence of aldehydes and formaldehyde, and the Iodide-Iodate Test for the presence of volatile acids.
Hatchfield, P. (2002) Pollutants in the Museum Environment: Practical Strategies for Problem Solving in Design, Exhibit and Storage. London: Archetype books
Tetreault, J (2003) Airborne Pollutants in Museums, Galleries and Archives: Risk Assessment, Control Strategies and Preservation Management. Ottawa: Canadian Conservation Institute.
For more information on who to contact for PSD and material testing, see the Directory of Analytical and Materials Testing Services for Historic Preservation, published in the WAAC Newsletter, May 1998, vol. 20, number 2.
As a rule, the less an artifact is handled, the better. Even artifacts that are on exhibit and are not being handled regularly have a higher risk of damage because they are more likely to be exposed to light and other environmental risks than materials in storage. Nonetheless, there is very little reason to do archaeology if the artifacts recovered cannot be studied, analyzed, and available to researchers and the public as much as possible. The following tips will help you minimize damage to artifacts caused by handling and public display.
Handling: When handling archaeological materials for analysis and research purposes, be as gentle as possible at all times. When removing storage boxes from shelves and setting them on tables, be sure to place them down gently since any jarring of the box will increase damage to artifacts inside, especially glass and corroded metals that may be bagged together and might hit and break each other. Also take care when pulling these bags out of boxes.
Cover work surfaces with some sort of padding, such as polyethylene foam. This will prevent shock to artifacts when they are placed on otherwise rigid tables and desks. If possible, remove artifacts from bags one or two at a time, rather than dumping out multiple artifacts. This can prevent friction and damage to items bagged in bulk such as glass, iron, and fragile ceramics. If you have ziplock-closure bags and the bag proves to be too small to remove a fragile item without its scraping the sides, cut the bag away and replace it with a bigger one. It is better to sacrifice the bag than damage the artifact.
Wear gloves when handling the artifacts, especially metals, which will corrode after contact with oils from the skin. Choose gloves based on what you’re handling. Nitrile or latex gloves are usually a good choice since the fibers in cotton gloves can sometimes catch on and damage fragile items with uneven surfaces.
If possible, try not to handle the artifacts to inspect them. Instead, place them in a padded box or box lid, such as acid-free boxes with metal edges lined with polyethylene foam or tissue. Pick up the box to get a closer look instead of handling the artifact directly. This is an especially effective way of protecting artifacts while showing them to visitors from the public; let them get a close look without handling the artifact itself.
Public Displays: There are countless ways to exhibit artifacts to the public, ranging from permanent exhibition to impromptu spreads of dirty artifacts in the palm of the hand as visitors pass by open excavations. Although each situation is unique, as a rule you should keep in mind the effects of handling and the environment on the artifacts. If the artifacts have been conserved, you must also consider the effects that display might have on the materials used to conserve the items. For example, if Acryloid B-72 is used to mend ceramic pieces, high temperatures from intense exhibit lighting may cause the Acryloid B-72 to soften and the mends may fail or warp. Know how the materials you are displaying will interact with the environment and monitor them for changes.
When you let the public interact with archaeological materials, the best thing to do is to be there to monitor the situation and teach people how to look at items without handling them, or, if they must be handled, how to do this with minimum risk to objects. You should also take into consideration the risk of theft.
If you wish to make semi-permanent displays of artifacts to use for educational purposes, use archival materials such as acid-free boxes with clear mylar lids for viewing, and be sure to pad and protect the boxes. Also, choose the artifacts according to their condition and the risk that is posed to them by handling. For example, you may want to use a lot of durable ceramics and avoid iron items that are extremely sensitive to humidity and will be likely to corrode if removed from a controlled environment.
If you are looking for a compromise between accessibility and storage and you have a sufficient budget, you might want to purchase gasketed specimen cabinets with drawers that may allow you to keep commonly viewed artifacts in storage until you have visitors, and then open the cabinets and pull out the drawers for easy viewing. Cabinet drawers can be padded to support large objects or subdivided by using padded acid-free boxes to hold smaller objects. Additionally, the whole cabinet might be converted into a desiccated microenvironment by using silica gel. By using cabinets to house the types of artifacts that you might want access to most often, you are protecting the artifacts that remain in regular storage by limiting the frequency with which the boxes are handled.
If you cannot afford specimen cabinets, consider having a “special” box that holds the items you need to see most often. This, too, will limit damage to items that do not need to be as accessible for research and display.
Finally, if you notice that public display of objects is causing deterioration, it does not necessarily mean that you should remove them from display. Ideally, you should consult a conservator if objects need treatment after public display or handling has harmed them. In order to avoid the expense of conservation, however, some archaeologists might be inclined to replace a deteriorating example with another object from storage. Unfortunately, this technique increases the risk of deterioration of the collection as a whole and can create a damaging cycle. If you cannot afford conservation services, it may be better to continue to exhibit objects that have already experienced some damage than to remove them to storage and get out ‘new’ artifacts that will then also be exposed to risk.
If you are not sure whether a particular artifact will hold up well to handling and public display, consult a conservator. Conservators can help to advise you about different treatments that might help preserve objects under continuous handling or exhibit.
One of the greatest dangers to artifacts is handling. Archaeological artifacts are often more fragile than they appear. It is important to minimize damage by minimizing handling of artifacts throughout all stages of excavation, transport, conservation treatment and display. Objects should always be carefully examined for potentially weak areas prior to handling. An area of instability, such as a crack or flaking surface should never be assumed to be obvious, since many are masked by corrosion or repairs. Therefore, notes indicating areas of fragility should be kept with the artifact.
Never lift an object by a handle or other protrusion, as these may be weakly connected. When handling an object that is in multiple pieces, always handle each piece individually; never try to pick up all the fragments at once or to carry them on top of each other. When examining objects always work over a soft surface.
It is best to remove loose rings and other jewelry before handling artifacts as they can scratch soft or sensitive surfaces. Always wear gloves when handling metal objects (this is for the protection of the object and the handler). If gloves are not available it is best to handle the metal indirectly i.e. through a polyethylene bag or piece of cotton. Some treated wood can cause rashes because of degradation products in the wood or because of a past history of microbial growth, so it may be desirable to wear gloves. Handle all other objects with clean hands. Wash your hands after handling any objects for your own safety, as some materials used in treatments can be toxic.
Plan any object move carefully beforehand to reduce the risk of accidents or injuries. Never attempt to lift a heavy or oversized object without assistance, not only can you damage the object but you can also injure yourself. When moving artifacts from one location to another, carts should be used. Rubbermaid carts are very useful for moving most artifacts. It is important to ensure that an artifact is properly supported and will not shift or roll during transport. Artifacts may be stabilized on carts or tables using combinations of the following:
- Acid-free tissue paper. Tissue paper can be used to line supports or can be rolled into airy pillows or rolls to stabilize an object on a table or cart.
- Non-abrasive foam blocks or wedges.
- Ethafoam® blocks or wedges. Cut edges of Ethafoam may be smoothed with applied heat or lined with Volara, using hot-melt adhesive to provide a non-abrasive surface.
- Ethafoam® rings are useful for supporting ceramic pots. However, tissue should be placed between the ring and the object, so as not to abrade the ceramic surface.
- Weighted beanbags. Contrary to their name, these bags are most often made either with small pieces of lead shot or with polyethylene shot sewn into unbleached muslin or stockinet. The use of real beans should be avoided as they may attract bugs.
Whenever possible, artifacts should be handled indirectly by providing a stable surface such as an underlying support or by placing on a cart to move from one place to another. Flat objects, such as textiles may be placed on a rigid support or in trays lined with Ethafoam® or Volara. Blueboard and trays made from corrugated plastic, such as Coroplast®, work well in these instances.
When transporting or handling artifacts with an underlying support, it is important to ensure that the support is rigid enough to handle the weight of the artifact. All supports or linings must be non-abrasive to the artifact. One must ensure that both the lining and the artifact are stable.
Large artifacts that are too heavy for carts should be moved using dollies or specially designed supports with wheels. Gantry cranes or forklifts may be necessary for lifting unusually large artifacts, such as cannons.
More often than not, gloves are used to prevent damage to artifacts. However, wearing gloves when handling archaeological objects is often as important for the safety of the person handling the materials. Gloves should be worn at all times if there is a danger of chemicals, residues, byproducts or off-gassing that may harm an individual who is performing cleaning or conservation of an object.
A number of different types of gloves, disposable and non-disposable, are available. Different tasks may require different gloves. General handling of non-dirty or conserved archaeological materials should be done with disposable vinyl, latex or nitrile gloves.
Although cotton gloves are often recommended for use when handling decorative arts or social history objects, they have disadvantages when used to handle archaeological artifacts. They do not provide a good grip when used on smooth surfaces (such as large ceramics or glass with smooth surfaces) and they can become snagged on sharp and uneven surface, such as most metal artifacts and ceramics with loose or flaking glaze, causing damage to the artifact. Cotton gloves also have a tendency to trap dirt and can transfer dirt or harmful products from one artifact to another, contaminating other surfaces. Additionally they may wick oils and sweat from the skin of the person handling the object to the surface of the piece, causing damage. Do not use cotton gloves to handle wet artifacts, or artifacts with chemicals or other residues on the surfaces, which may be wicked into contact with your skin.
Vinyl, latex and nitrile gloves are smoother and less likely to snag on a fragile artifact and they often provide a better grip when handling smooth surfaces. Some work places no longer use latex gloves, as there is a large percentage of the population that is allergic to latex. If you do decide to use latex make sure that alternatives are available and that the gloves are clearly labeled. Some people prefer to wear white cotton gloves underneath a more protective disposable glove, such as latex or vinyl, for comfort or to protect their skin. These gloves are available through most preservation suppliers, and are often referred to as glove liners. They should be changed and washed after each use. Most disposable latex and vinyl gloves can be purchased lined with powder or powder-free. The powder is used primarily for ease in removal and in reuse; it is best to purchase powder-free gloves for use around artifacts. The powder can easily contaminate objects, work surfaces and the skin of the person wearing the gloves.
When wearing gloves, particularly those that have been used for chemical work or to handle toxic materials such as lead, it is important to remember not to contaminate other surfaces such as doors or phones by opening them or answering them with the gloves on. Additionally, one should be mindful of how one might be contaminating an artifact or what one may be transferring from one artifact to another.
The suitability of a building for collections storage depends largely on the nature of the collections that are being kept within it. The building serves as the first line of defense against many sources of deterioration. A survey can establish how well the building cushions the indoor environment from that outdoors, and isolates the conditions within. If the building has poor features, yet is capable of satisfactory modification, its physical improvement should come before consideration of new internal control equipment that will be expensive to install and energy-inefficient to operate.
The environment within the building should be surveyed using recording devices placed in a number of locations throughout the building. The recording devices should be deployed for a full calendar year in order to best understand the effects of seasonal change on the structure. An external sensor, to provide contrasting readings, is essential to judge the efficiency of the building fabric. Such a survey may quickly identify the existence of differing environmental conditions within the building. As an immediate response, the collection may be reorganized, and the most sensitive items placed in the most stable and easily controlled zones – for example, in rooms near the center of the building that are usually furthest from the effects of the weather. Any additional control measures that are needed can then be economically and effectively targeted in specific zones, rather than applied indiscriminately throughout the building. It may be that no area within the building can easily be brought to an adequate standard. In such cases, consideration should be given to relocating the storage.
TABLE 5c – Surveying a collections storage facility: good & bad features (adapted from a table created by Fergus Read of the North West Museums Service, Lancashire, England)
|Site||Well-drained. Sheltered (but not shaded).||Low-lying. Poorly drained. Exposed. Shaded|
|Building Materials||. Damp-proof course
. Watertight. Good ventilation
. Poor insulation
|Building Design (external)||. Pitched roof. Controllable ventilation
. Few windows.
. External rain water disposal
|. Flat roof. Large skylights
. Large number of windows
. Internal pipes & gutters over collections
|Building Design (internal)||. Easy physical access. Fire divisions
. Planned environmental zoning
. Separation of collection (storage & display) and non-collection areas. Secure areas
|. Poor access. Open plan.
. Lack of internal physical & environmental barriers
. Mixed use of spaces, causing/requiring compromises
|Building Environment||. Stable and moderate RH & temperature, suggesting:
– Good buffering effect by shell of building from prevailing weather conditions, and/or
– Environmental services control systems that work
|Fluctuating or extreme RH & temperature, suggesting:
– Poor buffering effect of building fabric, and/or
– Environmental services control systems that do not work in desired way
|Building Maintenance||Good condition. Preventive as well as repair maintenance
. Disaster plan in place. Regular checks and rapid repairs of roofs, gutters, and other elements
. No regular checks or reporting procedure
. No procedures governing contractor work. No disaster plan
The creation of microclimates, or smaller zones in which the climate varies from those areas surrounding it, is an important concept for any form of cultural heritage storage. Microclimates can be created within existing storage facilities in order to meet a number of preservation needs ranging from the necessity for ultra-dry storage (i.e. less than 20%RH) to wet storage or oxygen free storage. In an archaeological context, the most commonly used microclimate is the desiccated one. This is usually created for the storage of archaeological metals and the goal is generally to create an environment with a relative humidity lower than 20% RH.
The simplest way to create a desiccated micro-environment is to use several polyethylene ziplock bags sealed one inside the other. First make a perforated pouch of silica gel by pricking a polyethylene ziplock bag over its entire surface with a bamboo skewer, paperclip, or similar tool, and then filling it with silica gel (make sure that the holes are large enough to allow airflow but not large enough to allow silica gel to escape). Place this pouch of silica gel into a larger, un-perforated 4mil polyethylene ziplock bag with the artifact(s). Add a humidity-indicating strip that can be read through the bag, in order to monitor the RH at the artifact. Place this double-bag inside a third un-perforated 4mil polyethylene ziplock bag, and seal the entire set of bags. Multiple artifacts in individual bags may be grouped together with the silica gel, as long as each artifact bag is also perforated for the free passage of moisture into the silica gel. Bags that are thinner than 4mil will not have strong enough seams to restrict the entrance of external moisture, and may not bear the weight of the artifacts within. If the artifacts do not fit inside standard ziplock bags, custom bags can be made with 4mil polyethylene sheet and a heated seam-sealer. How do I know my plastic bags are archival quality? Since the polyethylene bags and sheeting are somewhat permeable to moisture vapor, this system will be more effective (remain desiccated longer) if the sealed bags are placed inside another sealed container.
Another way of creating a microclimate is to use food-quality air-tight freezer boxes made from polyethylene/polypropylene, such as those made by Rubbermaid® or Tupperware®. Place the perforated pouch of silica gel into the box together with the artifacts and a humidity-indicating strip, seal the lid tightly (you can ‘burp’ out excess air to improve the seal), and placed into a standard storage box for handling. Two large Rubbermaid® Model No. 3863-87 boxes (approximately 36x26x13cm, 2.2 gallon/8.3 liter capacity) will fit neatly into a standard Coroplast box (40x30x26cm) with room for other bagged materials. The tighter the polyethylene lid seals on the container, the less air exchange will occur, and the longer the gel will stay desiccated.
In order to calculate the amount of silica gel needed to desiccate the environment of a container, calculate the volume of the container (multiply the length by the width by the depth of the container). For every 5 liters (5000 cm3) of volume within the container add 400 grams of silica gel.
Because there is still some exchange with the exterior environment no matter how tight a seal is, microclimate containers should be monitored semi-annually. The silica gel will also require regeneration periodically to remove the moisture it has absorbed. To regenerate the silica gel, spread the gel on a baking sheet or Pyrex (oven-resistant glass) tray, place it in an oven set at 110° C (230°F) and bake it for four hours.
For detailed descriptions of silica gel’s properties and uses, see Silica Gel, Canadian Conservation Institute Technical Bulletin 10, Silica Gel, 1984, ISBN 0-662-53370-4.
For information on the disposal of silica gel, see Cobalt Indicating Silica Gel Health and Safety Update, National Park Service Conserve-O-Gram number 2/15 (June 2001). It can be accessed from: http://www.cr.nps.gov/museum/publications/conserveogram/02-15.pdf
Not all plastics are safe for artifact storage. Some contain Polyvinylchloride (PVC), which becomes acidic as it breaks down and can cause damage to archaeological collections. The test demonstrated in the following PowerPoint provides a quick and inexpensive way to check storage materials for PVC.
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.