Conservators can look closely at works of art using an Ultra-violet lamp in a darkened room. Rays from an ultra-violet lamp cause some paints and varnishes to fluoresce (turn bright white). The substances that become fluorescent absorb the invisible U-V energy and re-emit it as visible light. The painting glows in the dark. This technique is useful to detect where the painting has been restored or re-touched. Since the varnish, rather than the pigment, is usually the most fluorescent material, any paint on top of the varnish appears darker. Sometimes older retouchings, repairs and damages are covered by a further layer of fluorescing varnish and are more difficult to see. But even then they will be a little darker and with experience the conservator can usually get a good idea of the condition of a painting.
By looking at a small sample of paint from the edge of Ophelia under U-V, we can understand the method by which the painting was made. This ‘cross-section’ of paint has been removed from an area of the painting that would normally be under the frame. The sample is viewed through a microscope so we can see the layers of paint and the thickness of the paint. The sample tells us that there is a layer of lead white underneath a layer of zinc white.