History of Chemistry in Ancient and Medieval India, Incorporating the History of Hindu Chemistry by Acharya Prafulla Chandra Ray c37.s1.24

c37.s1.24

In connection with the copper mines at Rajputana, Khetri and Singhana, considerable quantities of blue vitriol, copperas and alum were used to be manufactured till the end of the 19th century. It is stated that in 1864 A.D. there were twenty of these factories at Khetri and about double the number at Singhana. "The process of manufacture consisted in the lixiviation of the broken shales from the mine with water in a series of earthen *gharas* (vessels), arranged on ledges prepared for the purpose on the heaps of refuse. The shales were mixed with the crusts derived from the previous lixiviation (Fig. 39). Each charge of shale was subjected to the action of three changes of water; and the water itself was changed from one *ghara* to another, till it had taken up sulphates from seven different steepings. It was then of a thick dirty-bluish colour and taken to the boiling house, where it was boiled in earthen *gharas*; when sufficiently concentrated, it was left to cool; and thin sticks being introduced, the blue vitriol crystallized on them. The mother liquor was then poured off and again boiled, and on the addition of saltpetre, the alum crystallized at the bottom of the vessel. The residual sulphates still in solution were allowed to crystallize out by exposing the mixture to the sun."