Removing calcium carbonate deposits from archaeological ceramics. Traditional methods under review

Although insoluble salts are one of the most common alteration forms that archaeological ceramics can present, the methods for their removal have not really changed within the past forty years. Furthermore, not many studies on the topic have been recently published. In this paper, a methodology to study the suitability of different traditional treatments for the removal of these salts, has been established. For this aim, ceramic mock-ups made out of a marketed red clay were fired up to 700ºC in an electric kiln. Their composition and physical properties were studied. Several tests were conducted in laboratory for the growth of calcium carbonate deposits similar to the ones that can appear naturally on archaeological ceramics. The method that provided the more alike deposits, was followed and applied onto the ceramic mock-ups. Different cleaning treatments based on three chemical products and three application methods were put into practice for the removal of such deposits. The products included two acids and a chelating agent and the application methods were direct and indirect, by immersion and using pulps and gels. The specimens’ characteristics were studied again to determine their changes due to the cleaning treatments. The analytical techniques of XRD and SEM, for mineralogical and elemental composition, microroughness, colour measurements, stereoscopic microscopy were used, and results showed efficacy differences linked to the product and application method. In general, the products applied by immersion and cellulose pulp poultice were more effective than gel treatments, which were less effective with any of the three products tested.