In 1991, McCrone visited Yale to take new microsamples from the map, partly to check his earlier results, and partly to apply new techniques. Photomicrographs taken at 1 micrometer intervals through the thickness of ink samples demonstrated that the manufactured anatase particles were not just sticking to the surface as Cahill's criticisms had implied, and Fourier transform spectroscopy identified the ink's binder as gelatin, probably made from animal skin. In July 2002, using Raman spectroscopy, the presence of significant quantities of anatase in the map ink was confirmed by British researchers Katherine Brown and Robin Clark, and the remaining traces of black pigment in the ink were found to consist essentially of soot-type carbon.