Evaluation of Treatments for Stabilization of Verdigris and Malachite Containing Paper Documents

Copper ions and acids lead to enhanced degradation of the paper carrier. Stabilization treatment of documents with pigments such as verdigris and malachite should therefore include addition of alkalis to combat acid hydrolysis of cellulose and antioxidants to retard oxidative degradation catalysed by copper ions. The paper reviews present approaches for stabilisation of green copper pigments. Furthermore, two recently proposed treatments containing tetraalkylammonium bromide antioxidants and alkali are evaluated. The first treatment is based on interleaving of paper samples containing malachite or verdigris pigments with papers, impregnated with alkaline buffer and antioxidant at elevated relative humidity and applied pressure. The second one is based on the use of non-aqueous suspensions of nano calcium carbonate and/or magnesium oxide particles and tetrabutylammonium or tetrapropylammonium bromide. Size exclusion chromatography is used to monitor the extent of degradation of samples during accelerated ageing, while colour changes of pigments are determined using colourimetry. Results demonstrate that effective stabilization of paper samples can be achieved with tetrabutylammonium bromide and calcium carbonate containing interleaves at 90% RH. When non-aqueous immersion treatment is applied, tetrabutylammonium bromide at a concentration 0.09 M in combination with calcium carbonate nano particles had the most beneficial effect on the paper as well as on the colour of the copper based pigments.