thermal [fax] paper

Somehow it's not that easy to find the temperature threshold of thermal pape. Here is one semi-quantitative description.

Slight darkening was noticed at 70° c, then up to complete darkening at 100°c. The darkening did not transfer to blotter when the paper was heated from the back.

[http://cool.conservation-us.org/byorg/abbey/an/an13/an13-8/an13-802.html]

Available locally at Statples.