The verisimilitude of verdigris: a review of the copper carboxylates
| Main Authors: | , , |
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| Format: | Article |
| Language: | English Slovak |
| Online Access: | http://www.viks.sk/chk/reviews_2_01_73_91.doc |
| Abstract: | Conclusions. The review of the verdigris pigment group has shown the considerable complexity of the basic acetate salts, and has highlighted the need for further chemical research into their structure, properties and synthesis. The information derived from the review and from the laboratory work undertaken has helped to update and correct the powder x-ray diffraction information and has shown, in passing, that FTIR can be used to differentiate between the neutral and basic verdigris group. Revised data is also presented here for the copper (II) formates, which should be of use in future examination of formate salts or corrosion products of copper resulting from exposure to formic acid pollutants within the museum. The same argument applies to the copper (II) acetates, although here the primary interest in the verdigris compounds is as pigments rather than corrosion products. The review and replication of historic recipes for making verdigris has shown than many of them produce a variety of basic verdigris, or a mixture of salts. Several recipes do not make copper acetates at all, and instead produce one of the copper trihydroxychlorides, such as atacamite or clinoatacamite [58, 59]. Despite the chloride ion content of these pigments, there is some evidence to suggest that they are far less responsible for the degradation of parchment or paper supports than distilled verdigris. The use of plant juices, such as rue or gladiolus, or plant extracts, such as saffron, may not only alter the colour of the applied verdigris pigment, but may provide enhanced stability for the copper and acetate ions in preventing chemical deterioration of cellulosic or proteinaceous material. As alchemical knowledge became more sophisticated during the period from the eighth to sixteenth century AD, so the recipes for preparing verdigris compounds becomes more complex, resulting in considerable difficulties in characterization of the products involved. As a result of these difficulties, the complete characterization of some copper green and blue pigments in works of art will therefore continue to be a problem for some time to come. Summaries. Copper formates and copper acetates occur both as corrosion products on works of art and as pigments. Some of the characteristics of these compounds are reviewed, particularly in relation to their occurrence in works of art, the deterioration of associated materials, and historical pigment recipes for the preparation of verdigris [copper (II) hydroxyacetate hydrates] and related compounds. X-ray diffraction analysis is discussed at length and new sets of powder diffraction measurements are provided, for both the basic copper formates and the basic copper acetates, which represent reference data for the verdigris salts [copper (II) hydroxyacetate hydrates], not previously available as a comprehensive set of data, as no entries for them at present exist in the International Centre for Diffraction (Joint Committee on Powder Diffraction Standards) (ICDD (JCPDS)) files. The utility of this new data is reviewed and the optical characteristics of the different verdigris preparations examined. Many of the historical recipes were replicated in the laboratory, providing further insight into the complexity of these verdigris salts and their chemical constitution, some of which include the basic copper chlorides, citrates, carbonates and other copper compounds which are often difficult to identify. The chemical literature relating to the copper acetates, and some of the classical literature describing pigment preparations based on verdigris, is reviewed. |
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| ISSN: | ISSN 1605-8410 |
