Pyrognomic materials are said to become visibly

incandescent Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. The term derives from the Latin verb ''incandescere,'' to glow white. A common use of incandescence is ...

at relatively low

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

s. 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 A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

takes place that produces

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

as a result of an exothermic process. This limit is called the

Draper point The Draper point is the approximate temperature above which almost all solid materials visibly glow as a result of blackbody radiation. It was established at 977 °F (525 °C, 798 K) by John William Draper in 1847. Bodies at tempera ...

. The incandescence does not vanish below that

, but it is too weak in the visible spectrum to be perceivable. Pyrognomic materials are thought to visibly incandesce at much lower

s than the

but a material with this property has never been proven to exist.

Allanite Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rock ...

and gadolinite are examples of minerals which have been claimed to exhibit true pyrognomic properties but have since been shown to exhibit

thermoluminescence Thermoluminescence is a form of luminescence that is exhibited by certain crystalline materials, such as some minerals, when previously absorbed energy from electromagnetic radiation or other ionizing radiation is re-emitted as light upon he ...

. 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 William Hyde Wollaston (; 6 August 1766 – 22 December 1828) was an English chemist and physicist who is famous for discovering the chemical elements palladium and rhodium. He also developed a way to process platinum ore into malleable ingot ...

and Jöns Jacob Berzelius. The term is still used today to describe the

exhibited by various

metamict Metamictisation (sometimes called metamictization or metamiction) is a natural process resulting in the gradual and ultimately complete destruction of a mineral's crystal structure, leaving the mineral amorphous. The affected material is therefore ...

minerals.

References

{{Reflist
Weisstein Encyclopedia
* Theodor Scheerer, ''Erörterung der plutonischen Natur des Granits und der damit verbundenen krystallinischen Silikate (nach einer Übersetzung von Frapolli)'' / ''Discussion sur la nature plutonique du granite et des silicates qui s′y rallient (traduit de l′allemand par L. Frapolli)'', Bulletin de la Société géologique de France, 2e série, IV, p. 468-498, 1847 Mineralogy