Shore A scale is for testing the following materials:
Rubber: Soft vulcanized (ie tyre), natural nitrile
Elastomeretric materials (rubber & rubber like): GR-S, GR-1, neopene, thiokol, flexible polyacrylic esters neoprene, thiokol, flexible polyacrilic esters.
Other: Wax, felt, leather etc. (materials that normally yield under fingernail pressure, such as the heel on your shoe).