PLATINUM is very difficult to file and polish, but these processes are not often required, as the great use of platinum is for chemical apparatus, which are wrought almost exclusively with the hammer and soldered with pure gold. Platinum is sometimes inlaid in the limbs of mathematical instruments to receive the graduations, and then in filing this peculiar metal, the file is generally moistened with oil to prevent it from tearing up; and in polishing platinum the mathematical instrument makers use 1st, water of Ayr stone; 2ndly, blue stone; 3dly, charcoal, - all with water; and 4thly, they lay the grain with charcoal and abundance of oil, in order that the metallic particles may be floated away. It is necessary to use two pieces of charcoal, and these are rubbed together at short intervals, in order to remove from the one, those minute particles of metal which become embedded in the other, and that if allowed to remain would scratch the work.
PORCELLANOUS SHELLS. - See Shells.
PORPHYRY is not much used in this country, but is successfully worked in Sweden, - first with the pick and chisels, and afterwards by grinding it into form with emery and water applied through the medium of heavy rubbers, also of porphyry. As in other cases it is needful to employ a gradual succession of emery as to coarseness. It is probable the final polish is obtained by rubbers of wood with flour emery, and wood covered with buff or felt and fed with crocus, much the same as in the treatment known to be applied to granite. From the homogeneity of porphyry it is less difficult to manage than granite, but they each demand great time and patience.
The Elvans of Cornwall require similar treatment to porphyry and granite, between which they are systematically placed.
By the lapidary porphyry is treated like Agate or Carnelian.
POTSTONE, a magnesian mineral, allied to Serpentine and Steatite, is very soft when first raised, and then admits of being very easily turned with chisels of various forms. See vol. 1, page 166. The common practice in Germany for polishing the Potstone, is to use first sand and water, and afterwards tripoli and water, occasionally also rottenstone and oil for the highest gloss, the whole are mostly applied on woollen cloths.
When the lapidary polishes the Potstone, is it usually by the process recommended for Alabaster, unless from long exposure it has become hardened, and then it is worked as Carnelian.
PUMICE-STONE is a volcanic product, and is obtained principally from the Campo Bianco, one of the Lipari islands, which is entirely composed of this substance. It is extensively employed in various branches of the arts, and particularly in the state of powder, for polishing the various articles of cut glass; it is also extensively used in dressing leather, and in grinding and polishing the surface of metallic plates, etc."
Pumice-stone is ground or crushed under a runner, and sifted, and in this state it is used for brass and other metal works, and also for japanned, varnished, and painted goods, for which latter purposes it is generally applied on woollen cloths with water.