calx

{{Short description|Residual substance, sometimes in the form of a fine powder}}

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Calx is a substance formed from an ore or mineral that has been heated.{{Cite web|url=https://en.oxforddictionaries.com/definition/calx|archive-url=https://web.archive.org/web/20171228232607/https://en.oxforddictionaries.com/definition/calx|url-status=dead|archive-date=December 28, 2017|title=calx {{!}} Definition of calx in English by Oxford Dictionaries|website=Oxford Dictionaries {{!}} English|access-date=2017-12-28}} Calx, especially of a metal, is now understood to be an oxide. The term is also sometimes used in older texts on artists' techniques to mean calcium oxide. {{Citation needed|date=December 2017}}

According to the obsolete phlogiston theory, the calx was the true elemental substance that was left after phlogiston was driven out of it in the process of combustion.{{cite book |editor-first=John |editor-last=Daintith |date=2008 |chapter=Phlogiston theory |title=A Dictionary of Chemistry |edition=6th |publisher=Oxford University Press |via=Oxford Reference |isbn=9780191726569 |doi=10.1093/acref/9780199204632.001.0001 |chapter-url=https://www.oxfordreference.com/display/10.1093/oi/authority.20110803100323514;jsessionid=F3EE9D03E3314E78021F1D253B297995 |quote=In the early 18th century Georg Stahl renamed the substance phlogiston (from the Greek for 'burned') and extended the theory to include the calcination (and corrosion) of metals. Thus, metals were thought to be composed of calx (a powdery residue) and phlogiston; when a metal was heated, phlogiston was set free and the calx remained. The process could be reversed by heating the metal over charcoal (a substance believed to be rich in phlogiston, because combustion almost totally consumed it). The calx would absorb the phlogiston released by the burning charcoal and become metallic again.}}

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