CHEMICAL REACTIONS BETWEEN COPPER PIGMENTS AND OLEORESINOUS MEDIA
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English Slovak |
| ISSN: | ISSN 0039-3630 |
| Online Access: | http://www.viks.sk/chk/studcon_1_02_12_23.doc |
| Abstract: | SUMARRIES. This study demonstrates that resin and fatty acids are able to extract copper (II) ions from verdigris (copper acetate) and verditer (basic copper carbonate). Ligand exchange reactions of basic copper carbonate with fatty acids and resin acids are much slower than is the case with copper acetate. The browning of paint layers is closely correlated with the relative ease of copper extraction; the copper diffuses in the form of fatty acid or resin carboxylic acid complexes. These complexes are formed in the painting layer during grinding of the pigment with binding media containing oleoresin acids, as well as being formed at the interface of this layer and organic upper layers, such as varnishes. CONCLUSION. This work has demonstrated the capacity of fatty and resin acids, exemplified by linoleic and abietic acids, either in the free acid or carboxylate forms, to extract copper(II) ions from verdigris (copper acetate). A kinetic study based on infrared and UV-vis spectroscopy showed that these two acids have similar reactivities and that extraction of copper ions begins immediately the starting materials are mixed. Verdigris ground with oil or with a mixture of oil and resin is thus partly transformed by exchange of the acetato ligands with the carboxylic groups of the acids present in the system. Such an extraction is also observed when an oleoresinous layer is placed on a paint layer containing verdigris. When the same reaction is carried out with verditer a basic copper carbonate with carbonate and hydroxo ligands (as in malachite and azurite), copper extraction is not immediately visible. This relative lack of reactivity could explain the fact that whilst browning is observed with verdigris, to the best of our knowledge no important examples exist of a similar change in painting layers containing malachite. The browning of the paint layers is thus closely correlated with the relative ease of copper extraction. From our results, we predict that copper extraction from pigments by fatty and resinic acids will be a slow process, since they are present in oils and resins both as the free acids and as esters. The fatty acids are mainly present as triesters of glycerol and hydrolysis is required to liberate the free acids: this hydrolysis could be induced by copper ions, as has been shown previously [13]. We believe that copper extraction could occur not only with resins or oily media but in the presence of other substances that are capable of coordinating copper ions, such as waxes and proteins. The question of the brown oleoresinous layers must also be carefully considered, since these alterations may be due to changes in the original paint layer or may equally be a result of changes in layers added during restoration. The chemical reactions involved in copper extraction are balanced equilibria, so that the removal of these brown layers carries with it the risk of disturbing a balance reached over centuries. The application of a new protective layer containing compounds liable to form complexes with copper ions could begin again the process of copper extraction, and thereby further degrade the painting. |
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| ISSN: | ISSN 0039-3630 |
