AIC's 45th Annual Meeting

This study presents the intervention criteria chosen for a chemical cleaning in a brass dial clock from the XIX century. This item belongs to the Abílio Barreto's Historical Museum settled in the state of Minas Gerais (Brazil). Usually, clocks are complex objects composed of many devices and different materials. However, in this case, all the mechanism were lost, but the dial clock was preserved. In the past, the clock had a decorative and functional value and was used to measure the time. Nowadays, the dial clock holds a historical value and is important as a document. It provides historical evidence of Brazilian techniques in clocks and metals during the XIX century (forging, chasing and engraving). This clock also improves our knowledge about people and objects of the past century. This single piece holds many singularities which suggest it was handmade. It has a break arch type, a cartouche with personal inscriptions, a chapter ring with carved roman numerals and other unique decorative marks.
In the brass surface, a passive and stable cuprite layer was formed. However, low relief marks were covered by white cleaning residues. Probably a result of a past intervention, this material was reducing the contrast between low relief areas and the metal. The white residues were identified using EDXRF and FTIR. The analyses suggest a calcium carbonate composition, similar to those used in commercial mechanical cleaning products. Local cleaning tests were performed in some areas with sodium citrate in order to know how the surface was below the white residues. As a result, green corrosion products were found. They were magnified with a microscope and visually characterized as active corrosion products. The support has losses and dents. In the back of the plate, there is a thick surface made of grease and corrosion products with many scratches. These abrasions suggest past mechanical structures which once were in contact with the back of the plate.
Our intervention plan consisted of removing white cleaning residues as well as green corrosion products formed beneath them. This decision was made once the clock is now understood as a historical document and for that reason, it should gather a better visibility and legibility. The white residues were removed with a solution of sodium citrate in water (1%), first using a swab and then from the irregular surfaces with an adapted toothbrush. After that, the green products were removed with a sodium diethyldithiocarbamate solution (0.1 mol.L-1). Finally, the dial clock face was covered with a microcrystalline wax surface (paste) in an attempt to avoid moisture attacks. Structural damage, losses on the support, and scratches in the back of the plate were preserved because they are associated with the history of the clock.