Diamond polishing
Posted March 16th, 2008 by adminFor the final polishing, the stone is soldered firmly into a holder, a hollow cup called a “dop,” and then held against a fast rotating mill or lap of soft iron. The dop is usually a hemispherical bronze cup on a copper stem, and the stone is held in the dop and almost embedded in it by melting lead and tin in the proportions of three to one respectively. Mechanical dops Which hold the stone by screw adjustments are now usually used. Lines across the surfaces and edges of cleavage planes are eradicated by the disc moving across the grain; otherwise polishing follows the grain.
The dop is placed in a flexible copper arm reaching over the polishing lap, and additional lead weights are placed on the arm to increase the pressure when so required.
Since the laps are rotated at high speeds, they quickly become hot and the oil in the paste soon dries, leaving powdered diamond on the metal. The dop is occasionally immersed in water, and inclined at different angles for the different facets. The stone must be re-set in the dop for each new facet, which is quite a lengthy process requiring some skill. Adjustments are made by the eye only. If the stone contains flaws, a dry disc is used for polishing to obviate the risk of the dirty paste creeping into the cracks and thus discoloring the stone. The polished cut stone is finally cleaned by boiling in a solution of strong nitric or sulphuric acid to free it from oil and grease. In its process of being fashioned from the rough stone to the finished gem, it has lost from one-half to two-thirds of its original weight.
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