Copper Green Pigments in 17th Century Atlases

In a conservation project at the Austrian National Library, the condition of green colouration with copper-based pigments on printed maps was assessed in 163 atlases from the 17th century. The pigments of the atlases were characterised by copper indicator papers, XRF, and FORS. The condition of the cellulose fibres was studied with optical microscopy, SEM, and cellulose analysis with carbonyl group labelling. Based on the study of painters’ manuals for the green colouration of maps, test specimens were prepared, treated and submitted to accelerated ageing. The test specimens were evaluated visually, with optical microscopy, colour measurements, and cellulose analysis. The addition of alum and vinegar to solutions of verdigris produced degradation on flax-hemp paper comparable to the degradation observed on copper-green colouration in 17th century atlases. Nine atlases with severe degradation caused by copper-green pigments were selected for treatment with the complexing agent benzotriazole and for stabilisation with precoated Japanese tissue papers that contained the antioxidant tetrabutyl-ammonium bromide.