Page:The New International Encyclopædia 1st ed. v. 01.djvu/74

ABRASIVES. now used are corundum, emery, garnet, quartz, carborundum, diatomaceous earth, tripoli, pumice, rouge, crushed steel, abrasive stones, and sand. Corundum is a crystalline mineral substance, large deposits of which are mined in North Carolina. The process of manufacturing corundum ore into an abrasive powder consists in crushing and grinding it to a powder, which is mixed with water and fed onto sieves or screens; the properly ground material passes through the screens and the coarser powder remains on top and is reground. The remainder of the process consists in refining and sizing the powder into eight or ten grades for the market. Emery is an impure grade of corundum, and is prepared for the market by crushing, screening, and sizing, like corundum proper. Emery is used in the form of powder for polishing plate glass and stones, as emery paper and as emery wheels. Emery paper or emery cloth is paper or cloth covered with hot glue and dusted with powdered emery. Emery wheels are sometimes solid emery stone, and sometimes wheels the faces of which are coated with emery. Garnet occurs in segregated masses scattered through other rocks. Formerly the process of production was to separate the garnet masses from the barren rock by hand after the rock had been broken down by picks or by blasting. This method of separation resulted in the loss of a considerable portion of the garnet in the rock, and a process has recently been perfected by which the rock is crushed by machinery and the garnet separated from the barren rock by water. Garnet is harder than quartz, and, unlike quartz, does not wear smooth, but by its cleavage presents new cutting edges. It is used chiefly in the form of garnet paper or as a facing for cylinders, disks, belts, etc., for smoothing and finishing wagons, cars, carriages, wooden parts of bicycles, furniture, etc., and in boot and shoe manufacture for smoothing and polishing the heels and soles. Carborundum is an artificial product manufactured by a single American company whose works are at Niagara Falls, N. Y. The raw material for carborundum manufacture consists of 34.2 parts coke, 54.2 parts sand, 9.9 parts sawdust, and 1.7 parts salt. This mixture is smelted by electricity in special furnaces of fire-brick 16 feet long, 5 feet high, and 5 feet wide. In the centre of the end walls are the terminals or electrodes, each of which consists of 60 carbon rods 30 inches long and 3 inches in diameter, into the outer ends of which small pieces of ⅜ inch copper rods are fixed. A square copper plate bored with 60 holes holds the carbon electrodes in place. The carbons having been put in place from the inside of the furnace, the spaces between them are tightly packed with graphite, which prevents the oxidation of the carbons and adds materially to their durability. The charge is next thrown into the furnace until it is a little more than half full, when a semi-circular trench about 21 inches in diameter is made the full length of the furnace. Into this trench the core of coke is placed and built up to form a cylinder 21 inches in diameter. Around this core more material is packed to the full height of the side walls, and heaped above their tops, the furnace then being ready for operation. This consists of passing an electric current through the charge between the two terminals, which is maintained for thirty-six hours, after which the furnace is allowed to cool slowly for twenty-four hours, when the side walls are torn down and the charge removed. The carborundum forms a layer about 10 or 12 inches thick around the coke core. This is crushed and treated with dilute sulphuric acid for three days at a temperature of 100° C. to remove the iron and alumina. The clean material is then washed with water, dried, and graded according to fineness. Carborundum is used like emery and garnet in the manufacture of abrading cloth, cylinders, wheels, etc., and in the form of powder for polishing stones, steel balls, etc. Diatomaceous or infusorial earth is a natural product consisting of the siliceous framework of diatoms, which is ground and used principally in polishing metals and finishing wood. Tripoli is distinguished from infusorial earth by the mode of origin, it being the porous silica left from a siliceous limestone from which the lime has been leached, leaving the silica. The natural product is ground in a mill and sifted for use in polishing metals, horn, shell, etc., and is also cut out into the form of disks and used in household filters for filtering water. Rouge as usually sold is made by dissolving iron in sulphuric acid so as to form iron sulphate; this salt is heated and the sulphur driven off, leaving a residue of sesquioxide of iron, which after washing is known as rouge. Rouge is used for polishing plate glass. Crushed steel and steel emery are manufactured preferably from pieces of high grade crucible steel heated to a temperature of about 2500° F. and then quenched in a bath of cold water or other suitable hardening solution which gives the steel a granular structure. The pieces are then reduced to powder by powerful hammers or crushing machines, after which the steel particles are tempered in the following manner: They are placed in a steel pan or cylinder and heated to a temperature of 450° F., and then cooled by being subjected to cold air in various ways. The final process is the grinding and sizing of the powder. Steel emery is made exactly like crushed steel but is given an intensely hard temper. Crushed steel ranks close to the diamond in hardness. Crushed steel and. steel emery are extensively used in stone sawing and polishing, in lens grinding, glass beveling, brick grinding, and by lithographers, engineers, and plate glass manufacturers. Grindstones are cut from a hard sandstone of a peculiar quality, and whetstones, scythestones and oilstones are quarried and cut from similar natural rocks. Millstones or buhrstones are cut down or built up from various kinds of rock; the American buhrstone is a quartz conglomerate which is known under various local names: the German buhrstone is a basaltic lava, and that which comes from France and Belgium is a hard, porous material consisting of small particles of silica in a calcareous cement. The foreign stone is brought into the United States in small pieces, which are cut and built up into wheels with cement, but the domestic stone is worked down from quarry, blocks into a solid wheel of the required size. Millstones are used for grinding grains, cement, pigments, etc. Sand is extensively used as an abrasive in the form of sandpaper and in the sandblast for cleaning castings, structural iron-work, etc. Pumice is a volcanic ash or tufa which may be ground into powder for scouring and polishing or sold in lumps for similar purposes. See ;.