Using Pollutant Absorbers Inside an Exhibit Case

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Incorporating pollutant absorbents in exhibit cases limits damage to collection objects by pollutants

What pollutants are of concern in museum exhibit cases?

Airborne pollutants include sulfur dioxide, nitrogen dioxide, ozone, acetic and formic acids, formaldehyde and hydrogen sulfide. These pollutants may be drawn into the museum along with outside air, or may outgas from construction materials, paints, and cleaning products. Concentrations build up in the confined air space of the exhibit case.

Because different materials react differently to pollutants, it is difficult to provide concrete guidance on acceptable verses harmful levels. In general, the air concentration for any pollutant should fall below 1 part per billion (ppb).

How can the concentration of pollutants be reduced inside an exhibit case?

Three methods have been successful in reducing pollution concentrations.

• substituting a more inert material for a high out-gassing material
• applying a physical barrier around the polluting material
• removing a contaminate after it has become airborne

Which pollutant absorbers can be used inside museum exhibit cases?

Most pollution control systems are based on activated charcoal or potassium permanganate. Activated charcoal has proven 100% effective for active filtration and highly effective for passive filtration of nitrogen dioxide, sulfur dioxide, ozone, formaldehyde, and hydrogen sulfide. Potassium permanganate is less effective for removal of ozone. Silica gel has been shown to remove formaldehyde but is not effective for nitrogen dioxide, sulfur dioxide or ozone.1

• Activated charcoal, made from coconut wood, has a large surface area (1100-1400 m²/g) that absorbs but does not chemically react with pollutants. To guard against secondary out-gassing, replace activated charcoal at least once a year. Activated charcoal pellets can be contained in mesh bags, custom made panels, or other containers (similar to those described in TechNote 2:8). The fabric and paper products can be incorporated into sealed frame designs. Commercial products incorporating activated charcoal used in exhibit cases include the following:

1. Activated charcoal filters for HVAC systems or room units, cut down to size.
2. Respirator cartridges inserted into port holes of ventilated cases or used as part of an active system that forces air intact through the filter.
3. 3M Anti-Tarnish Strips are 48 lb. weight paper (5 mil. thick) with 20% content of activated coconut carbon impregnated throughout the paper thickness. Available in pre-cut sizes and in 2' rolls. Manufacturer recommends 1 inch of paper for every 30 cubic inches of air space. Although the manufacturer does not have data on use within an exhibit case, the paper remains effective in a corrugated box for 6 months with effective time increasing to 2 years when used in a pressure seal plastic bag.
4. MicroChamber combines activated carbon with alkaline buffers in an alkaline, sulfur and lignin-free paper or board. Boards with a metallicized polyester barrier layer on one side prevent migration of contaminates into the board from a wooden surface. Both oxidizing contaminates and acids are trapped. Unlike other charcoal products, it does not become a secondary source of contaminates, perhaps due to the alkaline reserve.

• Potassium permanganate is sold commercially as Purafil which is 4% potassium permanganate on a neutral activated alumina. Potassium permanganate reacts with pollutants, and for this reason, will not become a secondary source of pollution. One test showed Purafil less effective for ozone removal (PURAKOL should be used to reduce levels of chlorine in the environment and is more effective at trapping heavy hydrocarbons.) Purafil can be purchased as:

1. loose pellets which can be held in mesh bags, custom made panels or containers
2. prepackaged disposable modules made of corrugated paper board coated with 2 mil of LDPE (low density polyethylene)
3. filters for HVAC systems.

• Pacific SilverCloth, a pollutant specific product, is 100% cotton cloth impregnated with silver nitrate which interacts with hydrogen sulfide in the ambient air. This brown, felt-like material can be used to line exhibit furniture, and can be covered with a loose weave decorative fabric. Pacific Silver Cloth is commonly used when displaying silver alloy objects and silver-based photographs.

How are pollutant absorbers used inside sealed exhibit cases?

Pollutant absorbers are effective only when used within an enclosure, such as a sealed exhibit case. If not well-sealed, the absorber is quickly exhausted because it is, in effect, purifying the air of the entire room. A practical approach is to locate the sorbent in a maintenance chamber below the case deck. Volatile contaminates are removed as air in the case passes over the sorbent bed. Another method is to line the case deck, walls, or pedestals with a paper or fabric containing the sorbent. In either approach, the case design must incorporate the following:

• a well-sealed exhibit case, or one that supports a slight positive air pressure
• adequate air flow over the sorbent using natural convection, and good air-mixing within the case—limiting the air flow over the sorbent bed decreases the rate of pollutant removal
• an adequate quantity of sorbent; base this on the volume of air to be purified and adjust for the anticipated pollutant concentration, the sensitivity of the objects, the degree of seal in the exhibit case, and the rate of air mix inside the case;
• maximum surface area of the sorbent - the rate of pollutant removal increases with the surface area of sorbent. It is better to spread the sorbent over a larger area than to increase its depth
• isolation of the sorbent from any direct contact with the objects
• a way to monitor the pollutant concentrations to determine when to replace the sorbent

Additional considerations for each approach are listed in the following chart:

Absorbent Beds in an Exhibit Case

Cloth and Paper-based Products as Case Liners

Provide a holding container of metal or plastic; or use a pre-packaged module or filter that allows maximum exposure of sorbent. Locate the sorbent bed in a maintenance chamber; provide access to the maintenance chamber for replenishment. Ensure air flow between the sorbent bed and the display chamber (in large cases a fan may be required). Make sure that any activated charcoal or potassium permanganate is separated from any silica gel which has a separate maintenance.

Use a non-adhesive method to attach the material to the case. Do not allow the objects to touch the cloth or paper. Cloth or paper can be used to line cases or frames. Papers can be bent into a V or corrugated (to increase surface area) and placed in a discreet location within the case display chamber. Cloth or paper can be covered with a loose-weave fabric; incorporate additional sorbent.

What is the difference between passive verses active control techniques?

A passive system relies on the natural air flow in a case to expose volatile pollutants to a sorbent bed. Passive systems are satisfactory in most applications; to increase effectiveness in oversized cases, use a small fan that circulates the air over the sorbent.

In an active system, air inside the case is forced through a cartridge containing a filtering material or sorbent; a plastic tube filled with 200 grams of activated charcoal and outfitted with a small sampling pump has been recommended (Removal of Air Pollutants from Museum Display Cases, Sucha S. Parmar and Daniel Grosjean, Getty Conservation Institute. Final Report 1989 page iii). Active control is not required for most applications; it is recommended for:

• initial purge of a highly contaminated case;
• use when there are important emissive case materials or emissive objects;
• a ventilated case having a high air-exchange rate with a polluted area;
• a case holding highly susceptible objects.

What are the maintenance requirements when using sorbents inside a case?

Over time, any of the sorbents will become ineffective and must be replenished. Purafil should be changed according to the color indicator. As a general rule, activated charcoal products should be changed at least once a year. It is particularly important that activated charcoal does not remain in a case once it has lost its sorbent capacity; monitor the case interior to identify premature exhaustion. Many construction products out-gas more when they are new, so that the time between required replenishment of the sorbent may lengthen over the life of the exhibit.

1. Activated Charcoal and Respirator Cartridges

Available at chemical supply companies such as Fisher Scientific

1. Anti-Tarnish Fabric

Pacific Silvercloth, Eureka Company, Norton, MA, 02766

1. Potassium Permanganate

Purafil, Purafil Inc., Doraville, GA, 30340

Suggested Further Reading

Cass, Glen, William Nazaroff, Christine Tiller, and Paul Whitmore (1991). "Protection of Works of Art from Damage Due to Atmospheric Ozone" Atmospheric Environment, 25A(2) pp. 441-451.