Konrad Witz (around 1400, c. 1445/46), Joachim and Anna at the Golden Gate, 15th c. Varnished tempera on canvas, on oak wood 156x120.5. A present from Louise Bachofen-Burckhardt 1894, Inv. No. 647, to the Art Museum Basel. Photographed by Martin Bühler, by the courtesy of the Art Museum Basel

In the course of time, paintings were often altered. This occurred intentionally as well as unintentionally. Restoration and retouching fit into the first mentioned category and all kinds of damages into the second category.

If, after many years, new and also very thin layers of paint were applied in such cases, they could react differently to the ultraviolet light than the older layers that are underneath (due to the changed chemical composition).
This becomes more apparent when picture varnish (also protective coatings) is applied over the paint after the processing. Both the varnish and the paint colours alter in the course of time. Depending on the artist, this protective coating was applied large-area, locally or not at all. Particularly because this varnish (according to the composition and the way it is applied) reacts differently to ultraviolet light, many conclusions can be drawn about the past of the painting when it is photographed in UV light.
This analysis can be executed in a simple way using a broncolor UV attachment in combination with a broncolor lamp and the corresponding power pack. Please note that the protecting glass of the lamp, coated with a UV blocking filter, will have to be removed during shooting. However, longer wavelengths in the infrared area are used particularly for the visualisation of so-called preliminary drawings and pictures, which have been painted over.
With normal shooting conditions (i.e. sufficient distance to the subject, without long flash series with full power) and using the UV- attachment, UV radiation is no problem; analogue to flash tubes with a UV protecting glass.

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