Volume 41, Issue 4

This study explores the natural alteration of verdigris, both in the form of neutral verdigris (Cu(II) (CH 3 COO) 2 ⋅H 2 O) and basic verdigris (Cu(II) x (CH 3 COO) y (OH) z ⋅ n H 2 O), through combined Raman spectroscopy and X-ray diffraction investigation of samples created seven to eleven years prior to analysis. The naturally aged paint films of neutral or basic verdigris in gum arabic on paper and parchment provide insight into the pigment’s well-known instability relevant to historical works in aqueous media on maps, prints, books and manuscript materials. The latter historical application is an area that has received far less attention than alteration of verdigris in oil-based paint films. Findings shed new light on alternate pathways for conversion of neutral verdigris to basic verdigris, including the formation of a previously unknown form of verdigris and amorphous material on alkaline paper substrates. Additionally, we demonstrate for the first time that copper hydroxyl chlorides can form in situ from neutral verdigris, in this case on parchment that has a chlorine-rich surface. These results advance our understanding of neutral verdigris alteration, and complement results from our prior artificial ageing study. Both studies point to neutral verdigris as the historically more important form throughout its heyday. Improved understanding of neutral verdigris instability and its alteration pathways are critical for confident identification of the pigment in historical works, leading to better risk assessment of collections of verdigris-containing heritage, such as maps.

The aim of this research was an investigation into creating a rigid gel application of benzotriazole (BTA), a complexing agent, as a new potential way of treating verdigris-damaged paper. Various gel recipes were mixed and tested on historical samples. The gel recipes varied in gellan gel concentration, BTA/solvent solution concentration, and BTA concentration. The recipe effectiveness was assessed using Hulthe’s indicator paper and MQuant™ Test Cu indicator strips, two types of indicator papers which detect free copper ions. The results showed that rigid gel application of BTA is effective in complexing the copper ions which may inhibit further damage to the paper caused by free copper ions. Some of the other effects of the gel were the simultaneous removal of paper discolouration by the gel. Further research is needed to refine the gel recipes as well as the treatment process to prevent or reduce potential tidelines and other possible negative side-effects of gel treatment.

The focus of this research is the stabilisation of paper with Verdigris, a green copper pigment. Due to its corrosive effects on paper, many important documents, paintings and maps are in danger. The efficiency of several commercially available deacidification agents on paper samples with Verdigris was tested, including magnesium-based Bookkeeper® dispersion, nano calcium hydroxide containing Nanorestore® and CaLoSiL® dispersions as well as a recently developed dispersion of nano calcium carbonate. The antioxidant tetrabutylammonium bromide was tested either alone or in combination with nano calcium based deacidification agents. The effect of the treatments was evaluated using colour, tensile strength, degree of polymerization and pH measurements. The results indicate that acidic degradation does not play a major role during accelerated degradation of paper containing Verdigris with moderately acidic pH value and that oxidative decay could be the main culprit of the decay. The method described, which involves the use of antioxidant tetrabutylammonium bromide in combination with calcium carbonate based deacidification dispersion, was proved to have a superior effect against degradation of paper with Verdigris in comparison to the treatments which involve deacidification agents only. Nano calcium hydroxide based deacidification agents result in high pH values of the paper samples and therefore cannot be advised for use on paper documents.

The article describes a historic Japanese green pigment which was identified in a painting attributed to the Kano school, dating to the Edo period. According to literature, malachite – which is the common translation of the Japanese term rokushō – has been the most widely used green pigment in Japan over a long period of time. Its colour shade could be modified by the use of different degrees of grinding and by heating the pigment. The green paint layer found in the painting was examined using XRF, SEM-EDS and XRPD, and cross sections. Examinations revealed a heterogeneous paint layer which consists of a mixture of various natural copper-containing minerals, some of which also contain arsenic and other elements. A concluding discussion of pigment nomenclature in Japan raises the question if rokushō may in fact be equated with pure malachite.