Corrosive Media: Verdigris Studies into Deterioration Induced by Copper-containing Pigments and Methods for Treatment
Background: Many historic blue and green pigments are copper-based, including malachite, azurite, and verdigris. While the former pigments are mined ores, verdigris is man-made; it has been manufactured according to many different recipes since antiquity from copper metal, vinegar (acetic acid) and other possible ingredients. Technically, true verdigris is the copper acetate salt, either neutral or basic, and is a bright green-blue color. It has often been mixed with other pigments, such as yellow saffron, to make more true green colors, but some kinds of verdigris undergo natural color change from blue to green in the first month after manufacture.
Verdigris was used as a colorant for book illustrations and maps by European painters from the Middle Ages through the 17th century. However, it was known early on that some formulations turn brown unless used in rich oil media. The pigment may also be “corrosive” to documents and manuscripts on paper and parchment, causing browning, embrittlement and / or cracking of these substrates. Since any copper-based colorant can potentially cause deterioration of paper and parchment under certain conditions, this is not a definitive way to identify verdigris.
Substrates damaged by corrosive copper-based colorants have been treated by washing, deacidification and reinforcement by mending or lining. It is hoped that accurate identification of these colorants can lead to improved preventive and interventive actions.
Project Description: The Library has initiated a project to
- characterize and categorize so-called verdigris corrosion by complementary Raman spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy, and spectral imaging;
- study the mechanisms of this type of deterioration as they pertain to paper- and parchment-based collections through artificial aging and characterization of model systems that are based on the findings of step 1, in combination with collaborative determination of the molecular weight and degree of oxidation present in the substrates adjacent to these pigments;
- evaluate methods for treatments of artifacts displaying these symptoms through a series of experiments on the same model systems, to be determined.
Initial work includes the preparation of reference model samples, in which a range of copper-containing colorants based on historic recipes have been applied to two types of paper substrates and characterized, as above, before and after different periods of aging.
Contributing Studies: The Library has recently initiated and hosted a working dialogue with various colleagues interested in “verdigris corrosion,” including the Institute for Conservation of the Austrian National Library, and the Christian Doppler Laboratory for Advanced Cellulose Chemistry and Analytics at the University of Natural Resources and Applied Life Sciences in Vienna, with whom we are coordinating cooperative research objectives.
Support: Library of Congress Library Services
Acknowledgements: Library of Congress and the Staff of the Preservation Directorate
Updates and Images
Image of reference sample of verdigris pigment being made on Whatman paper substrate.
Image of XRF analysis of map displaying verdigris corrosion in the Rosenwald Collection 1513 Ptolemy Geographia.
