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Paint used by Dutch masters turns into rat poison
May 17, 1999The painters Willem Kalf, Jan Davidsz. de Heem and Balthasar van der Ast are particularly famed for their bright yellow orpiment ("royal yellow"). The researchers have shown that light causes a chemical reaction in this yellow which separates the sulphur and the arsenic. The sulphur is released from the canvas as sulphur dioxide and hydrogen sulphide. It is mainly the sulphur dioxide which causes the paint to turn brittle and which breaks down chemical bonds in the linseed oil used as a painting medium. These processes ultimately lead to the paint turning to powder and breaking away from the canvas. The process of conversion of the paint into rat poison also turns it whiter. Because the optical refractive index of the poisonous arsenic trioxide is smaller, the originally yellow areas of the canvas where the chemical breakdown has taken place appear white and transparent.
The Dutch scientists also discovered the reason for the fading of the blue which Van der Velde and Van der Ast used to paint the shadows thrown by Chinese pottery. A fatty substance in the lamp black which they used causes the pigment to weather more quickly and become lighter. Surviving recipes from paint dealers and painters themselves show that this phenomenon was already known in the seventeenth century and that attempts were made to prevent fading by heating the lamp black several times. The researchers managed to find a large number of early painters' handbooks and recipes in archives and libraries, enabling them to reconstruct the pigments and paints. They then aged these artificially in all sorts of different ways, giving them a better understanding of the way paintings deteriorate. The paints used on the seventeenth century canvasses were also measured using special techniques, such as chromatography, microscopy, mass spectrometry and X-ray diffraction. These techniques gave the researchers more information about the way the linseed oil had aged, the varnish had yellowed and the pigments had discoloured.
NWO (Netherlands Organization for Scientific Research)
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