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Pyrognomic
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<p>Pyrognomic materials are said to become <a href="/facts/Visible_light/jeofpMn4">visibly</a> <a href="/facts/Incandescent/8Tng7IuE">incandescent</a> at relatively low <a href="/facts/Temperature/QyGe4gmj">temperatures</a>. 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 <a href="/facts/Chemical_reaction/GpqWlKrz">chemical reaction</a> takes place that produces <a href="/facts/Light/jeofpMn4">light</a> as a result of an <a href="/facts/Exothermic/gCuSvpa4">exothermic</a> process. This limit is called the <a href="/facts/Draper_point/3elHQgqt">Draper point</a>. The incandescence does not vanish below that <a href="/facts/Temperature/QyGe4gmj">temperature</a>, but it is too weak in the visible spectrum to be perceivable. Pyrognomic materials are thought to visibly incandesce at much lower <a href="/facts/Temperature/QyGe4gmj">temperatures</a> than the <a href="/facts/Draper_point/3elHQgqt">Draper point</a> but a material with this property has never been proven to exist. <a href="/facts/Allanite/s82K0jvE">Allanite</a> and <a href="/facts/Gadolinite/kahQI57c">gadolinite</a> are examples of minerals which have been claimed to exhibit true pyrognomic properties but have since been shown to exhibit <a href="/facts/Thermoluminescence/v8qxKI1n">thermoluminescence</a>. 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 <a href="/facts/William_Hyde_Wollaston/U0MXYKA3">William Hyde Wollaston</a> and <a href="/facts/J%25C3%25B6ns_Jacob_Berzelius/HakSv3sP">Jöns Jacob Berzelius</a>. The term is still used today to describe the <a href="/facts/Thermoluminescence/v8qxKI1n">thermoluminescence</a> exhibited by various <a href="/facts/Metamict/60dEABID">metamict</a> minerals. </p> <ul><li><a href="http://scienceworld.wolfram.com/chemistry/Pyrognomic.html">Weisstein Encyclopedia</a></li> <li>Theodor Scheerer, <i>Erörterung der plutonischen Natur des Granits und der damit verbundenen krystallinischen Silikate (nach einer Übersetzung von Frapolli)</i> / <i>Discussion sur la nature plutonique du granite et des silicates qui s′y rallient (traduit de l′allemand par L. Frapolli)</i>, Bulletin de la Société géologique de France, 2e série, IV, p. 468-498, 1847</li></ul>