A practical treatise on mechanical dentistry / by Joseph Richardson.

  • Richardson, Joseph, D.D.S.
Date:
1868
Catalogue details

Licence: Public Domain Mark

Credit: A practical treatise on mechanical dentistry / by Joseph Richardson. Source: Wellcome Collection.

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    to coo] gradually. The crucible may then be broken, and the button of gold at the bottom removed and separated from the slag that covers it with a hammer. The button is then put into a fresh crucible and re-melted. If there is any known base metal present likely to render the gold brittle, the particular re-agent which will most readily attack it may now be used. If, however, as is generally the case, the alloy is of uncertain composition, or contains various metals having distinct affinities, the process becomes, to some extent, ex- perimental, and it may become necessary to use first one refining agent and then another, until sufficient separation is effected. Generally, it will be sufficient to use the nitrate of potassa alone, as most metals are oxydable. After roasting with nitre for half or three-fourths of an hour, adding small portions at a time, the melted metals may be poured into ingot moulds previously warmed and oiled. If, after ham- mering, annealing, and rolling^ the ingot, it should still be found brittle, it must be remelted, and chloride of mercury used as the refining agent. This will remove any traces of tin which may be present. If the alloy, however, is greatly impoverished, it may be more advantageously treated with sulphuret of antimony; in which case the metals should be melted in a large crucible with about twice or three times their weight of the native sulphuret, which should be added in small quantities at a time. The heat decomposes the sulphuret of antimony; the sulphur uniting with the base metals forming sulphurets, and the antimony uniting with the gold forming a leaden-colored alloy. The antimony may be parted from the gold alloy in the following manner: Place the mixture in a clean crucible, and when melted, force a current of air with a pair of bellows upon its surface; this oxydizes the antimony, which passes off in the form of vapor. The current should be mild at first, as too great a draught is apt to carry off portions of gold by a too hurried volatilization of the antimony. A current strong enough to produce visible fumes will be sufficient.
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