A Critical Assessment of A-D Strips® Used in Degradation Studies of Artworks Made from Cellulose Acetate

Abstract

ABSTRACT Acid-detecting strips (A-D Strips®) available from the Image Permanence Institute (IPI) and third-party suppliers outside the USA were originally developed to detect volatile acids from deteriorating film stock made from cellulose acetate (CA) by using a five-step colour scale to represent acetic acid concentration. The strips are shown here to be sufficiently sensitive for the detection of acetic acid as a volatile organic compound (VOC) generated by degrading CA in objects in storage. In confined spaces, the local air concentration of VOCs can deviate significantly from background levels due to low air circulation and interaction with solid materials and can be explored by this means. The usefulness of placing a number of A-D Strips® with CA-based artworks stored in closed containers, instead of or during the deployment of SPME fibres to monitor the degradation of these plastics, is assessed critically. The useful level of indicative and semi-quantitative information they can offer in such practical situations is presented. It is proposed that an interpretation of acetic acid concentration on an eight-step colour scale of measurement of b* in CIELAB1976 colour space with a hand-held spectrophotometer, or visual comparison with the IPI-provided colour scale is more useful than a direct calculation of concentration, because placement around stored objects introduces significant measurement errors. This paper gives guidance on the limitations and benefits of using A-D Strips® as a low-cost passive monitoring tool for CA-based objects, whether visibly degraded or not. The guidance is also relevant for monitoring VOCs derived from wood-based crating materials used in museum storage areas, which also generate acetic acid.