Thermal paper (sometimes referred to as an audit roll) is a special fine paper that is coated with a material formulated to change color when exposed to heat. It is used in thermal printers, particularly in inexpensive or lightweight devices such as adding machines, cash registers, and credit card terminals.
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The surface of the paper is coated with a solid-state mixture of dye and a suitable matrix; a combination of a fluoran leuco dye for example. 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 a metastable state when the matrix solidifies back quickly enough. The reactant acid in thermal paper is often bisphenol A (BPA).
Usually, 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.
Multicolor thermal paper first became available in 1993 with the introduction of the Fuji Thermo-Autochrome (TA) system. This was followed in 2007 by Polaroid's development of the Zink ("zero-ink") system. Both of these methods rely on multi-layer coatings with three separate colorizing layers, with different methods used for independent activation of each layer.