Pumice-stone is a volcanic product, and is obtained principally from the Carnpo 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 woolen cloths with water.
Putty Powder is the pulverized oxide of tin, or generally of tin and lead mixed in various, proportions. The process of manufacture is alike in all cases - the metal is oxidized in an iron muffle, or a rectangular box, close on all sides, except a square hole in the front side. The retort is surrounded by fire, and kept at a red heat, so that its contents are partially ignited, and they are continually stirred to expose fresh portions to the heated air; the process is complete when the fluid metal entirely disappears, and the upper part of the oxide then produced, sparkles somewhat like particles of incandescent charcoal. The oxide is then removed with ladles, and spread over the bottom of large iron cooling pans and allowed to cool. The lumps of oxide which are as hard as marble, are then selected from the mass and ground dry under the runner; the putty powder is afterwards carefully sifted through lawn.
As a criterion of quality it may be said that the whitest putty powder is the purest, provided it be heavy. Some of the eommon kinds are brown and yellow, while others, from the intentional admixture of a little ivory black, are known as grey putty. The pure white putty which is used by marble workers, opticians and some others, is the smoothest and most cutting; it should consist of the oxide of tin alone, but to lessen the difficulty of manufacture, a very little lead (the linings of tea chests), or else an alloy called shruff (prepared in ingots by the pewterers) is added to assist the oxidation.
The putty powder of commerce of good fair quality, is made of about equal parts of tin and lead, or tin and shruff; the common dark colored kinds are prepared of lead only, but these are much harsher to the touch, and altogether inferior.
Perhaps the most extensive use of putty powder, is in glass and marble works, but the best kind serves admirably as plate powder, and for the general purposes of polishing.
Putty powder for fine optical purposes is prepared by Mr. A. Ross by the following method, which is the result of many experiments. Metallic tin is dissolved in nitro-muriatic acid, and precipitated from the filtered solution by liquid ammonia, both fluids being largely diluted with water. The peroxide of tin is then washed in abundance of water, collected in a cloth filter, and squeezed as dry as possible in a piece of new clean linen; the mass is now subjected to pressure in a screw-press, or between lever boards, to make it as dry as possible. When the lump thus produced has been broken in pieces and dried in the air, it is finally levigated while dry on a plate of glass with an iron spatula, and afterwards exposed in a crucible to a low white heat.
Before the peroxide has been heated, or while it is in the levigated hydrous state, the putty powder possesses but little cutting quality, as under the microscope, the particles then appear to have no determined form, or to be amorphous, and, on being wetted, to resume the gelatinous condition of the hydrous precipitate, so as to be useless for polishing; whereas, when the powder is heated, to render it anhydrous, most of the particles take their natural form, that of lamellar crystals, and act with far more energy (yet without scratching) than any of the ordinary polishing powders. The whole mass requires to be washed or elutriated in the usual manner after having been heated, in order to separate the coarser particles.
Mr. Ross usually adds a little crocus to the putty powder by way of coloring matter, as it is then easier to learn the quantity of powder that remains on the polishing tool, and it may be added that this is the polishing powder employed by Mr. Boss in making his improved achromatic object-glasses for astronomical telescopes.
Vienna lime and alcohol give a beautiful polish to iron or steel. Select the soft pieces of lime, such as will be easily crushed by the thumb and finger, as they are the most free from gritty particles. Apply with a cork, piece of soft pine wood, leather, chamois, etc.