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

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.

KEYWORDS:

• Cellulose acetate degradation
• VOCs
• A-D Strips®
• passive monitoring
• SPME-GC-MS

Acknowledgements

This work has been carried out within the context of Tate's contribution the COMPLEX project, which explores a systems dynamic approach to polymer degradation, with principal investigator Dr Katherine Curran at the Institute for Sustainable Heritage, University College London. The artworks and storage materials were analysed by COMPLEX team members in some cases, and in others by Tate conservation scientists Dr Bronwyn Ormsby, Dr Judith Lee, and Dr Angelica Bartoletti, using FTIR. Environmental monitoring was carried out by Helen Smith and the data was processed by Rhian Ward, then both preventive conservators at Tate. The work was first presented at Plastics Heritage 2019. Former Tate colleagues Valeria Duplat and Rachel Crome suggested the use of small neodymium magnets for holding the strips in position.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes

1 Image Permanence Institute, Rochester Institute of Technology, Rochester, New York, U.S.A., https://store.imagepermanenceinstitute.org/ad-strips. A batch of 250 A-D Strips® can be purchased for $60 (at July 2021). Third-party suppliers do not invariably provide or refer to the User’s Guide for A-D Strips®, which gives recommendations on use, exposure time, shelf life etc., and is available at https://s3.cad.rit.edu/ipi-assets/publications/adstrips_instructions.pdf (accessed 9 July 2021).

2 Sofer, Cane, and Townsend (2021).

3 The equivalent spectrophotometer today is the Konica Minolta CM-26d, sufficiently close in specification to be used for repeat measurements begun with the CM-2600d, according to the manufacturer’s information.

4 https://sensing.konicaminolta.us/products/cm-2600d-spectrophotometer/specifications (accessed March 2020 when still online).

5 https://dancan.dk/?page_id=1072 (accessed 9 July 2021).

6 Within the COMPLEX project which takes a systems dynamics approach to elucidating the factors affecting the degradation of CA and CN in objects, see https://www.ucl.ac.uk/bartlett/heritage/research/projects/current-projects/complex (accessed 9 July 2021). Tate is one of two museum partners in this project.

7 Hacke and Korenberg (2021).