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Thermal paper (often supplied in roll form, and sometimes referred to as an audit roll) is a special fine paper that is coated with a material formulated to change color locally when exposed to heat. It is used in thermal printers, particularly in inexpensive devices such as adding machines, cash registers, and credit card terminals and small, lightweight portable printers.
The surface of the paper is coated with a substance which changes color when heated above a certain temperature. The printer essentially consists of a transport mechanism which drags the paper across a thermal dot matrix print head. The (very small) dots of the head heat up very quickly to imprint a dot, then cool equally quickly.
Most thermal papers' coatings turn black when heated, but coatings that turn blue or red, and multicolor coatings, are sometimes used. An unintended heat source, such as a coffee cup, can discolour the paper and obscure any printing. A fingernail rubbed quickly across the paper may generate enough heat from friction to produce a mark.