by Catherine Nicholson and Elissa O'Loughlin

Introduction
Acid detection (A-D) indicator strips allow fast and easy detection of acidic fumes created by a large number of proposed storage or display materials. Our research indicates that A-D strips can be used as a quick, preliminary screening method before undertaking more time-consuming tests. Further research is needed to correlate the A-D strip results with other tests.

Using A-D strip tests requires no special equipment. Results generally are available within a week.

A-D Strips
A-D strips are paper-based indicators that use bromcresol green to show the presence of acids. Although developed by the Image Permanence Institute for monitoring the release of acetic acid in acetate film collections, A-D strips can be used to detect other acids such as those released by adhesives, paper, textiles, wood products and plastic. When placed in a closed container with the sample material, the strips change from blue to green at the slightly acidic pH 5.4 and then to yellow at pH 3.8, if acid gasses are released. The color changes within a few days. The strips come with a color keyed pencil to assist in identifying results.

Test Method
To test materials, place an A-D test strip with a sample in a tightly closed container. Prevent direct contact between strip and sample. This limits the test to acidic fumes produced by the sample rather than contact acidity.

Store test containers in the dark, so the strips won't fade. Read the directions provided with the A-D strips for further information. Prior to use store A-D strips in the resealable bag in which they came. Strips should not be reused.

Ideally, test materials should be conditioned to 50 percent relative humidity and the test container should be a glass stoppered jar. However, differences between tested materials are still obtained using materials conditioned to the normal building environment and tested in resealable polyethylene bags. Use new bags because polyethylene absorbs and retains acidity. Different plastic film thicknesses (known as gauges) vary in their permeability to gasses. We recommend heavyweight freezer bags. These appear to retain acidic gas longer and give more accurate results.

Controls
Every test group should have a control, ie. an empty container with an A-D strip in it. This backup ensures that the test container is not the source of acidic fumes. When using resealable bags as a test chamber, run two controls, one with an indicator strip in a glass stoppered jar and another with a strip in a resealable bag.

Sample Selection & Preparation
This test indicates the presence of acidic gases inside the test chamber. A-D strips are sensitive to the quantity of sample present. Use similar sized samples prepared in the same manner to compare related materials.

When considering materials for a display case or storage container, test each component separately and then test them in combination, exactly as they would be used. For example, if all raw edges will be sealed, then do the same with the sample.

To test adhesives, paints or coatings, dip a glass rod in the liquid and pull a smooth coating across inert polyester film (Mylar D). Thoroughly air these films for 96 hours to reduce the amount of acidic fumes. After airing, equivalent sized pieces of coated polyester films can be cut as samples.

Supplies & Procedure
· A-D strips, reference pencil
· tweezers for handling strips to avoid contamination
· glass stoppered jar for control strip
· resealable freezer bags or glass stoppered jars
· samples
· glass rods
· polyester film (Mylar D)

1. Select and prepare samples, choosing a standard weight, size or area similar to the actual use.

2. Insert sample into jar or resealable bag. Use tweezers to insert indicator strip. Close jar with stopper. When using resealable bags, insert hand (careful not to touch test strip or sample) into bag to introduce a volume of air, remove hand and seal to retain air.

3. Store sample bags or glass jars in dark.

4. Examine strips daily for a week and note changes in color, comparing to reference pencil supplied with A-D strips.

General Results
We tested more than 100 sample materials. These included materials generally considered to be stable, old materials considered unacceptable, and materials of unknown stability.

Related materials were tested in groups to give a relative indication of which might be better. For example a variety of cardboards, films and adhesives were tested as groups. Structures that combine materials, such as storage and book boxes, were replicated so that the interrelationship of materials could be tested.

In the first test of adhesives coated on polyester, adhesives were dried 48 hours before samples were cut and inserted into resealable bags. The test was later repeated, allowing the films to air for 96 hours after coating, before samples were cut and inserted. This change in the length of airing had a dramatic effect on the amount of acid released by the adhesives.

In another group of samples, a variety of cardboards and adhesives, and the same book cloth and alkaline endsheet, were combined and aired for 96 hours to replicate a hand-made book box. Composites made with poorer quality cardboards released more gases, as did those made with older dispersion adhesive. Composites made with alkaline buffered matboard released less acidic gas. Layers of the same components, but without adhesive, also were tested. Comparing results showed that the dispersion adhesives are a significant source of acidic gases within such composite structures. Older adhesives were worse sources of acidic gases, a new pH neutral adhesive released less.

An indicator strip inserted in a 10-year old book that smelled acidic also changed color. This shows that some sources of acidity can continue for long periods. Samples of poor quality older materials were tested to see if acidic gases are released after considerable natural aging. Gases were observed with old samples of pressed wood, pressure sensitive adhesive tapes and groundwood core matboard.

Limitations
The A-D indicator strips are sensitive to acidic pH between 5.4 and 3.8. They will not indicate an environment that is above pH 5.4 or below pH 3.8. The strips will interact with carbon dioxide in the atmosphere and must be used in a closed container and over a short period. Because the dye is water soluble, indicator strips should not be used in close proximity to original materials to avoid staining.

Catherine Nicholson and Elissa O'Loughlin are conservators at the National Archives and Records Administration, Washington, D.C. This article has been reprinted, with editorial changes, from an earlier publication: "The Use of A-D Strips for Screening Conservation and Exhibit Materials"