The deterioration of cadmium sulphide yellow artists’ pigments
| Main Authors: | , , , , , |
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| Format: | Book Chapter |
| Language: | English Slovak |
| ISBN: | ISBN 1-84407-253-3 |
| Online Access: | http://www.viks.sk/chk/14tmh_46.doc |
| Abstract: | SUMMARIES. This paper investigates the deterioration of cadmium sulphide yellow pigments, through analysis of artificially aged pigments and samples from 12 paintings. The deterioration occurs mainly in the lighter zinccontaining yellows and leads to discolouration and loss of adhesion within the paint, resulting in a chalking and crumbly surface. Samples were characterized using light/ultraviolet and electron microscopy/energy dispersive X-ray (EDX) spectroscopy and X-raydiffraction. Darkening of yellow paint was characterized by the formation of brown cadmium oxide at the surface of some samples, while the formation of colourless/white products including cadmium sulphate and zinc hydroxide, and the degradation of the oil bindingmedium, is associated with chalking. A hypothesis is suggested involving the oxidation of cadmium zinc sulphide. The cause of instability may be linked with early manufacturing methods, where the product is largely amorphous. CONCLUSIONS. Evidence from the present study suggests that the deterioration of cadmium yellow paints includes deterioration of the binding medium and alteration of the pigment, resulting in the formation of brown, white and colourless compounds. A hypothesis about the oxidation of amorphous cadmium zinc sulphide is proposed, yielding cadmium oxide and sulphate, zinc oxide and sulphur dioxide gas. Further reactions of the degradation products may include acid hydrolysis of the binding medium and the formation of zinc salts. The presence of amorphous cadmium sulphide in artists’ pigments is related to the early manufacture of the pigment where calcination, which results in the formation of a crystalline and thus more stable product, does not always seem to have occurred. Samples containing a high proportion of amorphous cadmium yellow pigment may be most reactive, and pigments made using manufacturing methods where the amorphous intermediate stage is not properly converted to the crystalline product may explain why some pigments are more prone to deteriorate. The implications for conservation include an awareness of the physical and chemical changes in degraded paint films containing cadmium yellow pigments, and the consequent risks involved in cleaning and the consolidation of loose pigment at the surface. Change in paint solubility due to the formation of soluble degradation products is an issue in the choice of solvents used for cleaning, consolidation or coatings. This study points towards the possibility that moisture treatments or even environments of high humidity may trigger the deterioration. |
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| ISBN: | ISBN 1-84407-253-3 |
