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Alum and blue vitriol (sulphate of copper) are manufactured from decomposed schists at Khetri in Shaikhawati.
It is mentioned in the De inventione veritatis ascribed to Geber, wherein it is obtained by calcining a mixture of nitre, alum and blue vitriol.
In working, the muller is raised 2 in., the pan charged with water and then with ore; the muller is then lowered, salt and blue vitriol added, and the charge ground for 3-4 hours.