We included infra-red, x-ray and colour information and overlayed them. Users could scroll around the surface of the paintings in great details looking at how the artist constructed their work. The tech we used was previously used to render 3D globes with overlayed information. I think you could only buy the CDROMs at the Washington Museum of Art.
At the time it was pretty ground breaking and I certainly enjoyed working on it. It's amazing to think that this is all free on the web now at massively high resolutions.