Varnishes fluoresce when a painting is lit with an ultraviolet source. Examination of a painting under ultraviolet light provides information about the varnish of a painting (its thickness, its way of application, its originality and sometimes its nature). Also ultraviolet light renders visible glazings done by the artist or retouchings, and cleaning tests that have been done during previous restorations.
Examination under ultraviolet ligth Icon, Madonna and Child, 19th century, Russian school.
The following illustration shows the Icon before cleaning under UV light. Significant differences in the intensity of fluorescence can be identified. Non-fluorescent areas (dark areas) are due to retouchings applied on the final varnish. Differences in fluorescence intensity are probably due to previous heterogeneous varnish removal or older retouchings that are below the varnish.
Examination under ultraviolet light Oil on canvas, Jesus Christ teaching in the temple, 18th century.
One observes the presence of a coating emitting a greenish fluorescence. This allows to conclude that the painting has probably been coated with a varnish based on a natural resin. The edges of the painting, protected by the frame, show less fluorescence. This could either be due to the protection of light and air pollution or indicate that the painting has been re-varnished being mounted in its frame. Non-fluorescent areas (dark areas) are due to retouchings applied on the final varnish.