The Bakerian Lecture. On a method of rendering platina malleable
William Hyde Wollaston · Abstracts of the Papers Printed in the Philosophical Transactions of the Royal Society of London · 1833
Abstract In this paper the author details the processes which, from long experience in the treatment of platina, he regards as the most effectual for rendering that metal perfectly malleable. When it is purified by solution in aqua regia, and precipitation with sal-ammoniac, sufficient care is seldom taken to avoid dissolving the iridium contained in the ore by due dilution of the solvent. The author states the exact degree of dilution requisite for this purpose, and the exact proportions in which the two acids are to be used. The digestion should be continued for three or four days, with a heat which ought gradually to be raised; and the fine pulverulent ore of iridium allowed to subside completely before the sal-ammoniac is added. The yellow precipitate thus obtained, after being well washed and pressed, must be heated with the utmost caution, so as to expel the sal-ammoniac, but at the same time to produce as little cohesion as possible among the particles of platina. It is then to be reduced to powder, first by rubbing between the hands, and next by grinding the coarser parts in a wooden mortar with a wooden pestle; because the friction of any harder substance would, by producing burnished surfaces, render them incapable of being welded together by heat. The whole is then to be well washed in clean water. In this process the mechanical diffusion through water is made to answer the same purposes as liquefaction by heat in the case of the other metals; the earthy impurities being carried to the surface by their superior lightness, and the effect of fluxes being accomplished by the solvent powers of water.