Chelating agents for the removal of calcareous deposits from archaeological ceramic materials. Compositional evaluation after immersion and physical gel application methods

Abstract

The removal of calcareous deposits from archaeological ceramics is a very normal conservation-restoration treatment. Among the products used, chelating agents are quite common, including ethylenediaminetetraacetic acid salts (EDTA) (Berducou in La Conservation en archéologie: méthodes et pratique de la conservation-restauration des vestiges archéologiques, Masson, Paris, 1990; Buys and Oakley The conservation and restoration of ceramics, Butterworth-Heinemann, Oxford, 1993; Crisci et al. in ApplPhys A Mater Sci Process, 2010. https://doi.org/10.1155/2009/893528). Nevertheless, some studies have proved that they can cause damages on the ceramic pieces, regarding changes in their composition, such as dissolution of calcareous components and metallic oxides leaching (Gibson in Stud Conserv, 1971. https://doi.org/10.1155/2009/893528; Fernández and Seva Sautuola in Rev del Inst Prehist y Arqueol 9:471–982, 2003). As a consequence, their artistic values might also change. In spite of that, these products are nowadays still in use, meaning that the treatments might be changing the information that archaeological ceramics carry. However, from the 80 s onwards a more secure alternative to direct application methods based on thickening agents was developed. With the aim of analysing the degradation mechanisms that may take place after the cleaning treatments’ application, ceramic specimens with artificial calcareous deposits (Sáenz-Martínez et al. in Eur Phys J Plus 136:798, 2021) were treated with a low-concentrated solution of EDTA tetrasodium salt applied by immersion and thickened with xanthan gum powder (Vanzan® NF-C). Finally, the products from the cleaning treatments were neutralised, respectively, by immersion and by rinsing with deionized water. The composition of the ceramic samples was established before the growth of calcareous deposits and after the treatments, in order to determine their effectiveness and safety. According to the results, EDTA salt treatments, both by immersion and thickened, were effective regarding the removal of the calcareous deposits and did not modify the elemental and mineralogical original composition of the specimens (XRF, XRPD, TG-DSC). In addition, no gel residues were detected by FTIR-ATR.