Ultraviolet light photographs are used to better understand the components of the painting and to evaluate its state of conservation.
They allow:
- To characterize the nature, the thickness and the application of the different layers
- To identify certain pigments
- To identify and locate repaints
They are also a tool for certain restoration treatments
Ultraviolet light photographs are taken in the dark with “black light” neon lights that illuminate the artwork. These neon lights emit UV rays of varying lengths (from 350 to 380nm) which are partially absorbed and reflected by the surface of the painting. A filter is placed on the camera lens which allows only the UV light to pass through.
This technique helps to characterize the varnish thanks to the fluorescence of some natural resins. It also allows to notice and to differentiate the possible restoration campaigns, in particular by the presence of repaints or reintegrations which appear in a dark purplish tint. It also helps to identify certain pigments such as zinc white which fluoresces in a particular way. Ultraviolet light can also be useful to check the efficiency of a solvent or to verify the uniformity of a cleaning/lightening of varnish.