Aiding the cleaning of four 19th-century Tsimshian house posts: investigation of museum-applied surface coatings and original polychromy

Abstract The collection of the American Museum of Natural History, New York, includes a set of four elaborately painted 19th-century Tsimshian house posts that have been on continuous open display for more than a century. Over time, these objects have accumulated surface grime, and several coating layers applied in undocumented past restoration campaigns, some of which have yellowed with age. The combination of heavy soiling and multiple coatings has obscured the underlying polychromy, shifting the colors of the wood and paints, and yielding improper surface appearances. Further, some of the coatings appeared to have sunk into the media-lean paints, complicating removal. Questions regarding the number, types, and timing of coating applications, as well as the nature of some of the paint below, set the stage for an in-depth technical study aiming to inform the conservation treatment of these poles being undertaken in connection with the renovation of the Northwest Coast Hall at the museum. Microscopic examination of numerous cross sections removed from the four house posts under polarized and ultraviolet (UV) illumination shed light on the number of coating layers present in selected areas and their location in relation to the overall stratigraphy. Micro-invasive analysis of samples relied on a combination of spectroscopic and chromatographic techniques, including transmission Fourier-transform infrared (FTIR) spectroscopy, attenuated total reflection (ATR)-FTIR spectroscopy, and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), aiming to determine the exact chemical composition of the coatings. Portable X-ray fluorescence (XRF), FTIR, Raman, and scanning electron microscopy with energy-dispersive X-ray spectroscopies (SEM/EDS) provided insight into the color palette used, with a focus on green paints. Investigation of the coatings revealed the application of multiple layers, including cellulose nitrate and localized areas of polyvinyl acetal. Analysis of the paint identified a limited range of inorganic pigments and, additionally, assisted in tackling issues that arose during the cleaning of specific colored areas. Results from this technical study helped address outstanding questions regarding this group of historical Tsimshian house posts, informing the development of a suitable treatment tailored to the safe removal or reduction of surface coatings and grime, ultimately improving the legibility of the design and colors.