Rensselaer Republican, Volume 17, Number 15, Rensselaer, Jasper County, 18 December 1884 — Grinding Materials. [ARTICLE]

Grinding Materials.

' The finest of emery cuts and leaves minute scores in the metal, particularly if the metal be soft; it is impossible to produce a gopd, polishable surface on silver with flour of emery; burnishing wonld be necessary to make a surface, and even then it would present a striated appearance under a reflected light. Other grinding substances are required for some- fine surfacing work. Molding sand that has been used in the foundry for some time makes an excellent material for surfacing light brass—brass that oontains a large proportion of zinc. Some excellent results are gained by the levigation of tbe sand—rubbing it under a muller on a marble sVib, as paints are ground for the artist By this means the foundry sand may be reduced to an impalpable powder, which, however, retains much of its abrading quality. There is a turer of fine tools in an eastern city who uses coal ashes togive the last sur-

face, before polishing, to his hardened steel tools. He takes the asherf of Lehigh coal, pours them into a tub of 'water, stirs them up violently, and tv'hen the v/ater is turbid with the lino ashes held iu suspension, he draws it off into; a shallow tank and allows it, to feettle. The sediment is the polishing powder. 1f a higher degree of fineness is required, the operation of stirring, and washing, and settling is repealed. i material thus obtained mazes an excellent surfacing material. In the manufacture of silverware, the surfacing before burnishing is done by a blue clav, technically called “grit.” It is found in several localities, particularly in the Connecticut river valley rip to fifty miles from its mouth. This clay appears to be the substance of which blue slate iB formed, but it is usually obtained in a semi-liquid form, and is dried for use. It is not .surface clay, being found below the alluvium and sometimes below gravel, its depth or thickness of bed having been discovered, by boring for artesian wells, to be in some places more than sixty feet. Its identity with slate substance appears to be suggested by its behavior under heat, it assuming a stratified, porous form. It does not scratch pure silver, nor copper, nor mar. coin gold, but it will not give a polish. It grinds without leaving a shining surface; this is produced by burnishing, by®rubbing with whiting, chalk, or even with the bare hand.