Providing collection care, preservation and conservation treatment services to collectors and collecting institutions.
by Cecily Grzywacz and Helen Alten
Does your silver need to be polished frequently? Are the lead bullets in your military exhibit covered with white powder? Are the seashells in your collection covered with white puffs? Indoor-generated pollutants may be attacking your collection.
This unseen enemy lurks in exhibit cases and storage cabinets. It causes irreversible damage, particularly to metals and shells. Left unchecked, air pollution can cause objects to disintegrate completely.
So what is indoor-generated pollution? In short, it is gases or fumes released from wood, acidic paper, fire-retardant fabrics and other products used in construction. Surface contact with wood causes yellowing and brittleness in paper and textiles. Corrosive vapors released by certain glues, paints, fabrics and urea-formaldehyde in plywood corrode metals.
1. Organic acids may be the most damaging indoor-generated pollutants, specifically formic acid and acetic acid. Major formic acid sources are glues and woods, especially oak. Acetic acid, the chemical name for vinegar, is found in many cleaning products. Both damage metals and calcareous objects such as limestone, shells and low-fired ceramics.
2. Aldehydes, in particular formaldehyde, are common in wood products, especially plywood. Although a known carcinogen, formaldehyde is found in nail polish and household cleaners. It is also a by-product of combustion, released during heating, cooking and smoking tobacco.
3. Hydrogen sulfide and carbonyl sulfide tarnish silver. Sulfides are released from fabric dyes, rubber, wool, silk and felt. The human nose is sensitive to hydrogen sulfide. A rotten egg smell warns us of deadly exposure. Objects are 100 times more sensitive than humans are. Once a molecule of hydrogen sulfide reacts with a molecule of silver or a silver salt in a photographic gelatin layer, that silver molecule is forever changed.
4. Ozone is a reactive molecule of oxygen atoms. Many older photocopiers and other electrical equipment generate high indoor levels of ozone. Collections should not be stored or displayed near this electrical equipment.
5. Insecticides and other chemicals.
What does pollution do?
Determining potential damage to a collection is difficult. Many factors impact the rate of damage.
· Type of pollutant,
· concentration, and
· time of exposure.
Human health risks to toxins are determined by dose, which is chemical concentration and exposure time. But a particular object's sensitivity is complex. Risk factors for museum collections include:
· temperature, light and humidity,
· the specific materials of which objects are made,
· the history of an object, particularly former storage or display conditions,
· and previous conservation treatments.
The combination of these factors can have a wide range of effects on objects. An entire collection could be destroyed. Or no damage could occur. For example, often only one object in a drawer full of like artifacts shows damage.
How can you tell if you have a problem with indoor-generated pollutants? The early signs are usually small white puffs, tufts or spots of corrosion. These usually occur in crevices or areas with little air circulation. These "spots" can be the size of a pinhead. As corrosion continues, they grow and eventually may merge into patches or develop into tufts or sprigs.
A good place to start looking for damage, besides surveying susceptible objects, is the lock on a display case. Steel locks are coated with zinc or cadmium. Both react to air pollution, making dull or spotted locks good indicators that you may have problems.
Zinc doesn't rust under normal environmental conditions, but it does react with formic acid, acetic acid and high levels of formaldehyde. These gases can make a once-shiny zinc coating dull and powdery.
Cadmium coated locks develop a toxic yellow coating when exposed to sulfur (usually generated from collection objects such as sulfur treated animal skins). This yellow coating is cadmium sulfate, which can poison you. Do not touch it.
Identifying the problem
Before attempting to improve displays and storage units you need to determine whether there is a problem. This means developing a program to determine a baseline, then testing periodically to determine if you have fixed or improved the situation.
Also, when constructing new exhibits or storage units, it is best to test all materials before using them. There are long lists of problem materials, but in reality, each museum and new material batch is unique. Test cleaning and pest control products, too.
To determine which pollutants are in your museum, start by asking how much information you really need. Is it enough to know that there are corrosive fumes present? Or do you need to know the exact chemical and the amount present?
The more specific the question, the more expensive the answer. Once you determine how much you need to know the next step is a monitoring survey.
For most museums, the starting point is to determine if corrosive pollutants are present without seeking specific pollutants. Small polished pieces of silver, lead and copper; or the newly developed A-D strips (see article pg. 4) give a simple yes/no answer. These tests are inexpensive and easy. However, they are only moderately sensitive and do not determine the exact pollutant.
Direct Reading Passive Monitors, or diffusion tubes, provide more information. They are low to moderate in cost and easy to use. Some produce non-specific results, but have acceptable accuracy.
The simplest and least expensive passive samplers are the Draeger diffusion tubes (NOT the detector tubes!). They detect the presence of acetic acid or sulfur dioxide. Available from the Lab Safety Catalog at $50 for 10, they provide a good initial screening. Color change after a day in an enclosed area indicates if a pollutant is present. However, the test is subject to interference, so a number of tubes should be used to test the environment.
To determine the concentration and type of pollutant as accurately as possible requires active sampling or lab analyzed passive monitors.
Active sampling is expensive and complex. Lab analyzed passive monitors are more moderately priced and easier to use. To test an environment, close it for a month, then quickly insert the sampler and close the case or cabinet
. Because most passive monitors are pollutant specific, they may not detect the combination of pollutants present in your museum. Therefore, you must test for as many different pollutants as possible. This adds to the cost.
Here, listed according to their cost and ease of use, are the available lab analyzed passive samplers:
· GMD Formaldehyde dosimeter badge. $40. Excellent for formaldehyde, questionable results for acetaldehyde and other aldehydes.
· Ogawa Inc. Passive Air Sampler. Reusable, Teflon, two-sided sampler. $85 new; $30 re-used after initial purchase. Takes eight hours to five days to collect samples. Tests available for sulfates, nitrates and ozone. Developing tests for aldehydes and organic acids.
· Purafil Museum Six-Pack: silver or metal strips. $500 for six pack or $100 per location; two-for-one deal available. Sampling takes 30 to 90 days, but expensive and provides limited information.
· Diffusion Tubes or Palms Tubes. Sampling takes two to three weeks. Tests available for aldehydes and organic acids. The only organic acid sampler that provides reliable quantitative information.
There are four methods of addressing indoor-generated pollutants:
(1) Use materials that do not create fumes.
(2) Block fumes from reaching collections.
(3) Absorb fumes that are produced.
(4) Increase airflow to keep fume concentrations low.
Ideally, you should use inert materials such as metal or Plexiglas instead of wood. Also, you should test any material before using it. This is extremely important. It may not always be possible, but do the best you can with your resources.
If you have wood cases that emit fumes, block them. Aged oak can release acetic acid for 800 years. Although a wood product's acid and aldehyde emissions are decreased with age, a fresh cut or hole exposes new wood, emitting more fumes.
Metal films are best for blocking wood fumes. These include Marvelseal (a laminated aluminum film) and industrial-grade aluminum foil. Special multi-laminate plastic films, such as Aclar, also work. Products such as Mylar D and plastic drop cloths (polyethylene film) do not provide a complete barrier to fumes. Neither do most paints or coatings. They lessen, but do not eliminate, the problem.
Only testing indicates how much each method of prevention works.
Pollutants also can be removed by forcing air through or over a bed of absorbent. Active filtration systems are easy to fit to specific cases in many circumstances.
Sometimes using the best materials is not enough because the collection itself generates fumes. In such cases, you may drastically decrease a pollutant problem by constructing an intentionally leaky cabinet or by installing a vent system that encourages air movement into and out of the case. For example, remove storage cabinet doors and replace them with washed, unbleached, not dyed cotton or linen fabric. If the cabinet is wood, line shelves with the same fabric or thin polyethylene foam to lift collections off of acidic wood.
Often the best approach is a combination of methods. For example, a cost-effective solution to improve old oak exhibit cases that are causing a problem to the collection is to line them with aluminum foil (attached with double stick encapsulating tape) and place activated charcoal filters in the air space. Be careful not to puncture the foil. Activated charcoal can be purchased as a cartridge filter in any store that sells kitchen vent fans. Monitor the air space with polished metal samples.
Any improvement to indoor air quality is valuable. There is no such thing as a threshold limit for objects. All damage is cumulative. Seeing the damage is just a matter of time.
Cecily Grzywacz is a Conservation Scientist at the Getty Conservation Institute. She is part of an international team of scientists studying indoor-generated pollutants in museums. She can be reached at The Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, CA 90049-1684. Tel: 310/440-6260. Fax: 310/440-7711. E-mail: firstname.lastname@example.org.
Helen Alten is an object conservator specializing in treatments for historical, archaeological and ethnographic materials.She can be reached at Northern States Conservation Center, P.O. Box 8081, St. Paul, MN 55108. Tel: 651/659-9420. Fax: 651/644-0633. E-mail: email@example.com.
Where do indoor pollutants come from?
* tobacco smoke
* electrostatic precipitator air cleaners
* carpentry shops
* acidic storage and display materials
* wood pulp paper
* wood products, particularly oak, plywood or chipboard
* artist's activities
* conservation labs
* products containing oils, ammonia, sulfur and formaldehyde (especially urea formaldehyde)
* air conditioning and refrigeration coolants
* Polyvinyl chloride plastic (known as PVC or vinyl)
* Polyurethane foams and solvent-based coatings
* fabric, especially dyed, fire retardant and crease resistant (formaldehyde additive)
* cleaning products such as ammonia in glass cleaner and scents in cleansers
* cooking by-products
Signs that you may have a problem with indoor generated pollutants:
* Rapid blackening of silver
* White powder forming on lead
* White, fluffy crystals forming on sea shells, limestone, ceramics
* Yellow-brown stains on textiles and paper
* Strong odors
* Discoloration, weakening or other unusual deterioration
* Significant metal corrosion despite proper relative humidity
Improperly cured baked enamel cabinets can produce formaldehyde. Test for complete curing with xylene on a swab. If paint comes off, it is improperly cured. Read the Material Safety Data Sheet (MSDS) for xylene, use appropriate safety precautions and wear appropriate clothing.