Verdigris is one of the most important and frequently used green or blue-green pigments from antiquity until the 19th century. Its colour changes with time and depending on atmospheric conditions. Despite its poor durability, it was extremely popular among artists. Because it is semi-transparent and does not obscure the printed ink, it can be found on illuminations, book illustrations and maps. There are two types of Verdigris, namely neutral and basic Verdigris. Verdigris does not only change its colour, but also causes the degradation of the substrate on which it is applied. Since many historical works of art are also at risk, studies of pigment stabilization and the development of conservation-restoration procedures are of utmost importance.
In my master’s thesis I investigated the degradation and possibilities for stabilization of various model papers with applied pigment Verdigris. I studied three different model papers, namely Whatman No. 1 filter paper as a reference paper and two different handmade papers that resemble historical papers. Paper samples with pigment Verdigris were exposed to conditions of thermal accelerated degradation at 50 °C and 65 % relative humidity, which are comparable to the degradation of paper with pigment Verdigris at ambient conditions. For paper stabilization studies, two antioxidants that have proven to be the most effective so far, 1(H)-benzotriazole (BTA) and tetrabutylammonium bromide (TBABr), were used.
The copper content in model paper samples was determined using atomic absorption spectroscopy. The results show that an even application of the pigment on paper samples can be obtained using a spray gun. Monitoring the pH value of paper samples showed that the alteration of neutral to basic Verdigris occurs in the first month of thermal accelerated degradation. The pH values of paper indicate that for the stabilization of paper the use of an appropriate antioxidant, without the addition of alkali or deacidification agent, is sufficient. Measurements of the colour change of paper samples also indicate the fact that the neutral Verdigris eventually transforms into various forms of basic Verdigris. From the point of view of colour persistence immediately after application, TBABr is the most effective of the two antioxidants, as colour changes were minimal, but after three months of thermal accelerated degradation, the degradation was more pronounced than in samples treated with BTA. In addition, black spots appeared on some samples, which is not desirable. By monitoring the degree of polymerisation (DP) of cellulose, I found that the most suitable antioxidant for inhibiting cellulose degradation is TBABr, as the degradation of paper treated with TBABr was on average 33 % lower after three months of thermal accelerated degradation compared to untreated papers. In contrast, BTA did not show significant stabilization. Comparison of DP values, determined by size exclusion chromatography and by viscometry, indicates that viscometry may be suitable for determining DP values for Verdigris paper. Measurements of the mechanical properties showed that the untreated paper samples did not show as pronounced loss of mechanical properties after three months of thermal accelerated degradation as might be inferred from the determined values of the degree of polymerisation of cellulose. Using Raman spectroscopy, we confirmed that neutral Verdigris was used to prepare the colour, which changes over time into different forms of basic Verdigris, regardless of whether the paper is treated with an antioxidant or not.
Based on the obtained results, the antioxidant TBABr is the most suitable for stabilizing paper with applied pigment Verdigris, as it was the most effective in inhibiting paper degradation. In addition, it had no negative effect on the pH value of paper and did not cause significant colour changes immediately after application. The negative effect became apparent only after three months of thermal accelerated degradation, as black dots appeared on some samples. The cause of the appearance of these black dots must be carefully studied before the treatment is used on original documents.
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