An Investigation of color Changes in Cuprite Layer due to Application of Chemical Reagents in the Cleaning of Archaeological Copper-Based Artifacts

Chemical cleaning reagents used on archaeological bronze artifacts not only attack the corrosion products but also may cause some visual effects in the patina. There are few investigations on the visual effects of cleaning reagents and studies have mostly focused on dissolution corrosion products. Regarding archaeological objects as historical documents and aesthetic entities, it is essential to assess the level of treatment damages to both of these aspects. This paper surveys the visual effects (color changes) of cuprous oxide, Cu2O, treated with traditional cleaning reagents. Artificial cuprite patina was formed electrochemically on almost pure copper. Samples were immersed in various solutions separately. Color measurement of cuprite patina was carried out using a colorimeter before and after 1-hour immersion. Solutions used were Calgon®, disodium ethylene diamine tetra-acetic acid (Na2-EDTA), tetrasodium ethylene diamine tetra-acetic acid (Na4-EDTA), sodium tripolyphosphate (STPP), citric acid, formic acid, alkaline glycerol, alkaline Rochelle slat and distilled water. Na2-EDTA, citric acid and formic acid were found to cause the most visual impact on cuprite patina as it is possible for human eye to discriminate between the two colors. Alkaline Rochelle slat, alkaline glycerol, Na4-EDTA and distilled water result in less color changes. Calgon® and STPP caused the least color changes which could be disregarded.