New insights on the glaze technique through reconstruction of old glaze medium formulations

Abstract

The glaze technique, which appeared with the oil painting, has permitted a real improvement in the aspect of Renaissance paintings. A better understanding of this technique, which made the smile of Mona Lisa so famous, and especially of the composition and the properties of the various materials used is necessary to improve the conservation and restoration of art works. Historical sources have been investigated to list the possible glaze recipes. If raw oil can be used, in most cases, the oil is modified, thickened in the sun or treated with a lead compound. Additives can also improve the medium properties and many recipes indicate the common addition of varnish to the oil for glaze medium preparation. This paper describes the reconstitution of two formulation procedures: the mixture of mastic resin, turpentine and raw oil, and the heating of oil with lead oxide. The flow behaviour of these media is then investigated. These rheological data were used to discuss their brushing and levelling properties which are of great interest for glaze media. The ratio of the various ingredients in the mixture of resin, oil and turpentine can be adjusted by the painter to reach the required viscosity on the palette, but it does not change the general behaviour of these mixtures. On the contrary, the rheological behaviour of the glaze medium can be modified by the treatment of the oil with lead oxide. The saponification induced by this procedure leads to shear thinning mixtures, with higher viscosities. No difference is seen depending on the nature of the drying oil whereas the addition of water during the heating process has a real influence on the flow behaviour of the medium. This analysis provides first clues for the reconstruction of the glaze medium formulation.