Analytical techniques used for characterising materials have progressed to the point where it is now possible to investigate the composition and internal structure of valuable pieces of art and cultural artefacts with only microscopically small sample removal or without even touching the object.
Key techniques that have revolutionised the analysis of art in recent years include Surface-Enhanced Resonance Raman Scattering (SERRS), where a spectacular signal enhancement is produced by nanoparticles, and the NMR-MOUSE (for MObile Universal Surface Explorer), a portable NMR device pioneered by Bernhard Blümich and colleagues.
I have summed up developments in this exciting field (am very jealous, in fact, of the scientists who get to play not just with fascinating instruments, but also with art objects!) in a feature article that should be out in Chemistry & Industry today, and should be showing up on their website soon (appears to be open access!).
I've also covered the SERRS work by Marco Leona at the Metropolitan Museum in New York in a recent Spektrum piece:
Spektrum der Wissenschaft 2009 Nr 11, 16-17
Madonna mit Schildlauslack