Reference Library - PIXE (Proton Induced X-Ray Emission)
Glazer, M. "A Novel Approach to Looking at Stamps", The American Philatelist, Vol. 104, No. 8, August 1990, pp. 722-726.

Liston, E.M. "The 'China Clay' Variety of the 1908-1910 Washington-Franklin Issues?", The Collectors Club Philatelist, Vol. 84, No. 2, March-April 2005, pp. 105-112 and Vol. 84, No. 3, May-June 2005, pp. 141-146.