What is "thermofax"

Thermo-Fax (very often Thermo fax) is 3M's trademarked name for a photocopying technology which was introduced in 1950. It was a form of thermographic printing and an example of a dry silver process. It was a significant advance as no chemicals were required, other than those contained in the copy paper itself. A thin sheet of heat sensitive copy paper was placed on the original document to be copied, and exposed to infrared energy. Wherever the image on the original paper contained carbon, the image absorbed the infrared energy when heated. The heated image then transferred the heat to the heat sensitive paper producing a blackened copy image of the original.