+ CO2(g) Today, this reaction largely occurs in a cement kiln. The standard Gibbs free energy of reaction in [J/mol] is approximated as ΔG°r ≈ 177,100 J/mol − 158 J/(mol*K) *T. The standard free energy of reaction is 0 in this case when the temperature, T, is equal to 1121K, or 848 °C. Oxidation In some cases, calcination of a metal results in oxidation of the metal to produce a metal oxide. In his essay "Formal response to the question, why Tin and Lead increase in weight when they are calcined" (1630), Jean Rey notes that "having placed two pounds six ounces of fine English tin in an iron vessel and heated it strongly on an open furnace for the space of six hours with continual agitation and without adding anything to it, he recovered two pounds thirteen ounces of a white calx". He claimed "That this increase in weight comes from the air, which in the vessel has been rendered denser, heavier, and in some measure adhesive, by the vehement and long-continued heat of the furnace: which air mixes with the calx (frequent agitation aiding) and becomes attached to its most minute particles: not otherwise than water makes heavier sand which you throw into it and agitate, by moistening it and adhering to the smallest of its grains", presumably the metal gained weight as it was being oxidized. At room temper­a­ture, tin is quite resis­tant to the impact of air or wa­ter, as a thin ox­ide film forms on the surface of the met­al. In air, tin starts to ox­i­dize at a temper­a­ture of over 150 °C: Sn + O2 → SnO2. Antoine Lavoisier explored this experiment with similar results time later. Alchemy In alchemy, calcination was believed to be one of the 12 vital processes required for the transformation of a substance. Alchemists distinguished two kinds of calcination, actual and potential. Actual calcination is that brought about by actual fire, from wood, coals, or other fuel, raised to a certain temperature. Potential calcination is that brought about by potential fire, such as corrosive chemicals; for example, gold was calcined in a reverberatory furnace with mercury and salammoniac; silver with common salt and alkali salt; copper with salt and sulfur; iron with sal ammoniac and vinegar; tin with antimony; lead with sulfur; and mercury with nitric acid. There was also philosophical calcination, which was said to occur when horns,