Page:The American Cyclopædia (1879) Volume IV.djvu/83

 CASTING 75 FIG. 4. Perpendicular Section of Cupola. of holes in the inner wall. In the npper part of the back of the cupola is the door for receiving the charges. Fig. 5 shows the exterior of the lower part of a cupola. A cupola is charged by placing a sufficient quantity of kindling wood upon the floor, and above this a layer of the best anthracite coal in large lumps, and in sufficient quantity to fill the cupola to the height of FIG. 5. Lower Part of Cupola, a, a. Tuyeres. 6, b. Small isinglass windows for showing the state of combustion and position of the layer of coal. o. Pot for receiving the melt- ed metal, d, d. Columns of support. (The smaller up- right rods support the movable floor, and stand in the pit below the cupola.) several inches above the entrance of the tuy- eres after it has well settled and the wood has burned away. This precaution must be carefully observed, because if the charge of iron above the coal should come down to a level with the entrance of the blast, combus- tion would be checked, the metal become chilled, the process stopped, and the dumping of the charge necessitated. Upon the layer of coal thus carefully deposited, one of* pig iron is placed, varying in quantity from 1,000 to 5,000 Ibs., according to the size of the cupola and to the rapidity with which it is proposed to effect the melting; and upon this another layer of coal is deposited, and afterward suc- ceeding layers of iron and coal. Fluxes are added where occasion requires, according to the judgment of the founder, pounded marble or limestone being most frequently employed. The wood is usually ignited when the first layer of coal is deposited, and in from an hour to an hour and a half the furnace may be tapped. It usually requires about 1 Ib. of coal to melt 6 Ibs. of iron. There is a wide differ- ence in the processes of making heavy and light castings. A description of the latter, which is the simpler, will be given first. The first mat- ter to receive attention is the selection and proper treatment of the sand, which is the ma- terial used for making the moulds of light cast- ings ; and it is one of the utmost importance, for it is only by the use of sand possessing certain properties that the formation and retention of a smooth and well defined cavity can be pro- duced, having at the same time sufficient po- rosity to allow of the escape of air and 'gases which are generated during the pouring of the metal. If the sand is too dry, it will not ad- mit of the formation of a defined cavity within it. It must possess in a certain degree the na- ture of a plastic or adhesive substance. Pure sand cannot therefore be used for a mould, and the best material with which it can be mixed is clay, but not to an extent to form what might be called a loam. Enough moisture must also be present to produce a proper de- gree of adhesion, but the quantity must be as small as possible, for too much would pro- duce an amount of vapor wb/en the molten metal is poured in that would injure or destroy the mould, or cause the surface of the sand to adhere to it on i$s removal. A fine sand which is slightly loamy is therefore selected, and this is not found in every locality where common sand exists, but has often to be transported considerable distances. The moulding sand which is used in New York city is principally obtained in New Jersey, in the vicinity of Troy and Albany, and from some parts of Long Island ; but the cost of sand is not an item of much consequence after the first supply is ob- tained, as it is used over and over many times. That which is used in some parts of the mould is mixed with finely pulverized bituminous coal, coke, or plumbago, as the circumstances may demand; and these substances are frequently spread upon the surface of the mould, to effect various purposes, as will be explained further on. An apparatus called a flask contains the sand in which the mould is made. It is com- posed of three or more parts, a bottom board, a drag, a cope, and upon occasion one or more