Theory of Polishing
We know that grinding, as performed by the mirror maker, is a fragmentation of the surface of the glass. The older theory of polishing was that it consisted of a sort of continuation of this process on a fine scale; that is. that the rouge particles,
measurable in diameter in units of a wave length of light, were partially embedded in the pitch, the protruding edges having a planning action on the pitted surface of the glass. (This is what occurs when the surface of a pitch lap is charged with a fine grade of carborundum.
A fairly good semi-polish can be obtained by-charging the lap with fine emery.) Later, the theory was advanced that a molecular flow occurred on the surface glass, induced by a high surface temperature resulting from friction. Substantiating this theory, there is evidence that minute particles of glass become detached from the surface, are adsorbed into the liquid, and either are
redistributed or flow away.
In experiments conducted by Dr. E. D. Tillyer,1 of the American Optical Company, a lens surface polished with rouge was tested and found to contain no trace of iron, but after etching away the surface with hydrofluoric acid, a positive test for iron was obtained, indicating that glass had actually been deposited on top of minute particles of rouge. With further etching away of the lens surface, the actual marks of grinding were revealed, which certainly could not have been there if the polishing action were an abrading one.
In commercial work where close tolerances of figure need not be maintained, the polishing matrix, or rouge carrier, is usually of felt or even of paper. Spectacle lenses, simple magnifiers, and the like are thus polished. In the polishing of lenses, prisms, or mirrors, where high precision of figure is needed, a viscous substance such as pitch must be used.
The surface of the pitch is charged with the polishing compound which acts upon the surface of the glass. The pitch can be made to conform with the optical surface, contacting it at every point and, in working, will flow so as to permit maintenance of contact with the ever-changing surface of the glass. On large work, channels are cut into the pitch, to facilitate its flow and reduce friction. These channels likewise serve as reservoirs for the liquid mixture of water and polishing
agent.
In polishing with either cerium oxide or Barnesite, a higher frictional heat is generated than with rouge, and thereby a more rapid surface flow of glass is probably induced; at any rate, these agents are two or three times faster than rouge. The author can attest to numerous instances in which all trace of scratches that might ordinarily defy rouge was removed in upwards of an hour's polishing with the aforementioned agents.
Although there is little choice among the three products in the quality of polish obtained, Barnesite is probably superior in this respect. Some authorities are of the opinion that the fine figuring of optical surfaces cannot be successfully accomplished with the fast polishers, and adhere
to the belief that the slower-working rouge should be used for this delicate task.
But the drastic action of cerium or Barnesite can be materially subdued by applying it well diluted with water or by the addition of water only, since the charged lap will polish, with lessening effectiveness, indefinitely. So with a dilute mixture, and frequent interruptions of the polishing for long intervals of pressing, figuring can be expeditiously concluded with cerium oxide or Barnesite.
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