Under UV light, this painting produces strong, pink-red luminescence. This luminescence was first observed in the company of Alexander Marshack, who came to Gouy with a battery-powered Wood lamp (Marshack 1969). Using similar material, following this memorable visit, it was possible to differentiate the two red paintings:
• Red no. 4 was used over a bigger area than red no. 5.
• Red no. 4 was exclusively used for outlines of shapes, lines, flatwash applications.
• Superficial flaking of surfaces only affects the surfaces painted with red no. 4.
• Certain parts painted with this red could be simple washes.
The luminescent materials that are commonly encountered beneath the earth are generally restricted to calcite and aragonite (Aujoulat 1987). They appear as a dazzling white which cannot be confused with the colour emitted by red no. 4. However, 'red and orangey red' are mentioned in regard to the fluorescence colour of certain forms of calcite in Arizona, California, and Franklin, New Jersey (Eastman Kodak 1972), when using short-wave UV, in contrast to Gouy. No publication mentions a phenomenon similar to that observed at Gouy. With this lighting, painted motifs usually appear dark without any light emission, but at Gouy the pink-red photoluminescence was recorded on colour photographic film of'daylight' type (Figure 9.23). There may be many reasons for this light emission. It could be the incorporation of particular substances in the paint (purposeful or not) or the presence of micro-organisms (certain lichens?), but they would need to have been exclusively present on the painted surfaces, and to never have prospered outside those areas. Pigment analyses are indispensable (Martin 1993b). A request for authorization to carry out a programme of research has been made towards that objective. A project for studying the pigments of Gouy has been submitted, and a first series of discussions with Bernard Guineau, a research engineer with the CNRS 'Study of pigments, history and archaeology' (Centre Ernest Babelon, Orleans) has been undertaken.
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