Pyrognomic materials are said to become visibly incandescent at relatively low temperatures.
In practice, virtually all solid or liquid substances start to visibly incandesce around 798 K (525 °C; 977 °F), with a mildly dull red color, whether or not a chemical reaction takes place that produces light as a result of an exothermic process.
[1][2][3] The incandescence does not vanish below that temperature, but it is too weak in the visible spectrum to be perceivable.
[5][6] The term was originally introduced by the German chemist and mineralogist Theodor Scheerer (1813-1873) in 1840, but the phenomenon had been previously observed by William Hyde Wollaston and Jöns Jacob Berzelius.
The term is still used today to describe the thermoluminescence exhibited by various metamict minerals.