http://pubs.acs.org/cen/news/89/i04/8904scene.html
Rembrandt starch, continued

"Working on a tiny sample from the painting's dark background, the investigators confirmed the finding by the other analyses that the starch molecules were in the second ground layer, the layer in which an artist sketches the image before painting. Because TOF-SIMS allowed the researchers to observe the starch grains' shape and size, they could tell that the grains were wheat starch."

cross section image here
http://pubs.acs.org/doi/abs/10.1021/ac1017748