A comprehensive database of iconography from the Middle Ages that has been cataloged and indexed to allow users to browse and search images based on subject, location, medium, and other facets

A searchable database of various images, using an iconographic search tool.

A lexicon of Art and Science featuring paintings, pigments and scientific methods for investigation of works of art. Includes the stories behind the paintings, history of use of individual pigments, pigment properties and their identification in paintings, and more.

Graphics Atlas is a sophisticated resource that presents a unique, object-based approach for the identification and characterization of prints and photographs.

An e-book on historical, mostly European, painting techniques, from the Getty Conservation Institute, Los Angeles.

An online, open-access platform devoted to the technical study of art objects.

Tons of great links on technique and materials.