What is thermal paper?
Thermal paper is a copy paper that uses heat to produce its image. Thermal papers have been in use in various applications since the mid-1950s. Early thermal papers were used for office copying and were early precursors to modern xerox copies (the chemistry was quite different). More recently, thermal papers have been used as the print-out paper for fax machines as well as for such things as automatic ticketing machines and chart recorders (referred to here as 'modern' thermal papers).
The National Archives is concerned that thermal papers have found their way into archival collections, as many show rapid deterioration such as overall darkening, embrittlement and fading of the image.