Chalky paper

In philately, chalky paper is a type of paper coated with a chalky solution for security purposes.[1] The postmark cannot be removed without damaging the surface of the stamp[2] thus discouraging erasure of cancellations and fraudulent reuse of stamps. The paper was first coated with a chalk-like powder, and the ink for the stamp was then impressed upon the paper. Collectors are cautioned not to attempt to remove a stamp printed on chalky paper from an envelope or paper backer by soaking it in fluid, as this may destroy the stamp's design. A simple test for chalky paper is to rub a silver coin over it, resulting in a "pencil" type mark.[2]

Example of a stamp (from an Austrian series printed 1908 to 1916), printed on ordinary paper (on left) and on chalky paper (on right)