The focus of this thesis is the discoloration of a common historical green pigment, copper resinate. In this research the discoloration was investigated in Jacopo Tintoretto's painting, Allegorical Figure of Spring , painted c. 1555. This painting is believed to have been painted with copper resinate which has discolored to brown over the centuries. The state of repair of the painting was determined using visual analysis under visible and UV light and the use of copper green pigments was confirmed using energy-dispersive X-ray fluorescence, including copper resinate based upon supplementary historical evidence. Substantial evidence was found from visual inspection and XRF determination of pigments that the painting had originally been lozenge-shaped and had been given its current rectangular shape only after the addition of supplementary canvas to its corners and top and bottom. Inspection of the painting under UV light highlighted areas of damage and repair, and the appearance of modern pigments such as titanium white and zinc white indicated that repairs and overpainting had been conducted as recently as the twentieth century.

I also conducted an investigation of copper resinate in the laboratory to determine its chemical composition, stability and possible modes of degradation. For my study of copper resinate, I used pigments synthesized by historic and modern recipes to determine its composition using mass spectrometry and FTIR. I determined that in either case copper resinate is a mixture of copper compounds containing acetate and abietate ligands in varying ratios including [Cu2Ab4], [Cu2Ab3Ac], and [Cu 2Ab2Ac2]. I investigated its chemical properties and stability by extraction, UV absorption, subjection to various solvents and environments. I found that the green color of copper resinate is a result of a combination of blue copper carboxylate compounds and free abietic acid, which is yellow in color. I also found that the drying process of copper resinate is light- and water-dependent and that both verdigris and copper resinate quickly discolor to brown upon the addition of a dilute basic solution. From this evidence, I hypothesize that the discoloration of copper resinate results from exposure to water in the environment and traditional treatments leading to the slow degradation of the pigment to brown copper oxide.