Chalky paper is a philatelic term

Philately

Philately is the study of stamps and postal history and other related items. Philately involves more than just stamp collecting, which does not necessarily involve the study of stamps. It is possible to be a philatelist without owning any stamps...

for a type of paper coated with a chalky solution for security purposes. The postmark cannot be removed without damaging the surface of the stamp 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.