Testing acid products thickened with xanthan gum for the removal of calcium carbonate deposits on ceramics

Abstract

One of the most frequent alterations to archaeological objects is the presence of calcium carbonate deposits, which usually appear on stone and pottery. Their existence makes analysing these historical artefacts more difficult, so their removal becomes necessary. Nonetheless, the most popular methods for their elimination include applying a combination of chemicals followed by a mechanical action, both of which have the potential to harm the artefacts. Since the 1980s, thickened products have been widely employed as safer substitutes for conservation treatment procedures. They are not yet widespread in the archaeological community, though. This research arose from the need of determining the possible changes that pottery objects might suffer due to the use of thickened acid products to remove surface calcium carbonate deposits. Acetic and nitric solutions thickened with xanthan gum were used to remove the artificial deposits generated over ceramic mock-ups fired up to different temperatures. The results were satisfactory based on visual and colorimetric evaluation. Nevertheless, the confined acetic acid on mock-ups fired below 1100 °C showed surface stains or colour changes, that were connected to calcium acetate residues by TG-DSC and ATR-FTIR techniques. However, mock-ups treated with the confined nitric acid showed no signs of soluble salts remaining. Besides, no mineralogical changes were detected on the ceramic mock-ups treated, regardless of the acid used. Furthermore, tests were performed on low fired mock-ups without calcium carbonate deposits as a worst-case scenario to establish the safety of the designed treatments. The findings indicated that there were no compositional changes caused by the removal procedures. As a result, it may be inferred that the thickening acid-based treatments were safe and effective on the ceramic mock-ups and could be applied to other similar pottery findings.