One procedure we use to analyse the condition of an oil painting is examination under ultraviolet light, which can yield much information.
Museum Shop, Ltd. has found hidden signatures which were not visible under normal light. We have found second, presumably original, signatures. Details not visible to the naked eye can be apparent under ultraviolet light.
This is a close-up of part of a painting from the 1700s brought to us for restoration. When the owner purchased the painting, she was assured that it had never been restored. Our ultraviolet examination, however, clearly showed the tell-tale spots of previous retouching, (the dark spots) which was extensive. The owner was shocked!
Today's standard of practice in art restoration are to retouch a painting as little as possbile. The more a conservator adds to a painting, the less it becomes the work of the original artist.
Additionally, all work done by a conservator should be fully reversible.