Heat Sensitive Paper

Thermal paper is a special fine paper that is impregnated 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, ECG recorders, Defibrillators, etc.

The surface of the paper is impregnated with a solid-state mixture of a two layers. As an example  combination of a fluoran leuco dye, octadecylphosphonic acids..
Schematic of thermal paper and thermal sensitizing process


The coating undergoes a color change in the areas where it is heated, producing an image. This temperature-induced color change is termed as thermochromism. 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.