Investigation of the stabilization of verdigris-containing rag paper by wet chemical treatments

Copper pigments promote the deterioration of paper objects; hence, it has been problematic to paper conservation since many valuable historical manuscripts contain copper green pigments. In particular, verdigris yields relatively mobile copper ions that can cause a higher risk of degradation depending on the relative humidity of the storage conditions. Although several research studies have demonstrated potential chemicals to slow down the degradation of the paper with copper ions or copper pigments, passive treatments such as mechanical reinforcement or environmental controls are still preferred since wet chemical treatments need to be further investigated. In the present study, various wet chemical treatments of rag paper with verdigris are tested with consideration for the practical situations of application as well as the sample conditions. GPC-Fluorescence-MALLS system after fluorescence labeling of carbonyl groups of cellulose was employed to evaluate the inhibition of both, hydrolysis and oxidation of cellulose after treatments. Samples were prepared to simulate a partially soluble verdigris pigment bound in gum arabic on gelatin-sized rag paper. With the given sample conditions, beneficial effects from deacidification was rather limited despite the use of a mixture of water and ethanol as a co-solvent. In contrast, treatments with tetrabutylammonium bromide in ethanol or 1H-benzotriazol in ethanol showed significant stabilization of the samples. For both cases, hydrolytic degradation and oxidation of cellulose were retarded significantly when brushing application on verso was employed. Testing various conditions of the solutions and two different application methods led to the conclusion that the outcome of the solution treatment depends on not only the chemical, but also how it is applied.