Aniline
the organic base or parent substance for many dyes and drugs. In its pure form, aniline is a highly poisonous, oily, colorless liquid with a distinctive odor.
It was first obtained (1826) from indigo but now produced synthetically. It is used to make chemicals used in producing rubber, dyes and intermediates, photographic chemicals, urethane foams, pharmaceuticals, explosives, herbicides, and fungicides as well as to make chemicals used in petroleum refining.
Stamps printed in aniline inks usually saturate the paper and bleed through to the reverse.
