Thermal paper is a special fine paper that is impregnated with a chemical that changes color when exposed to heat. It is used in thermal printers and particularly in inexpensive or lightweight devices such as adding machines, cash registers, and credit card terminals.
The surface of the paper is impregnated with a solid-state mixture of a dye and a suitable matrix. When the matrix is heated above its melting point, the dye reacts with the acid, shifts to its colored form, and the changed form is then conserved in metastable state when the matrix solidifies back quickly enough.
Typically the coating will turn black when heated. But coatings that turn blue or red are sometimes used. While an open heat source such as a flame can discolor the paper, a fingernail swiped quickly across the paper will also generate enough heat from friction to produce a mark on such paper.
