930° C. It shrinks about twice as much as brass, and due allowance must be made for this in the pattern and mould. As the metal solidifies rapidly it is necessary to pour it quickly and to make the "gates" amply large, so that there will be no "freezing" in the "gates" before the casting is properly fed. To prevent shrinking as much as possible, the metal is allowed to enter the mould at a temperature no higher than will admit of its running freely. Where there is a heavy mass of metal in the shape of an envelope surrounding a core, the contraction upon solidification will cause the metal to split, unless the core is made to yield equally with the contraction. Baked sand moulds are preferable to green sand, except for small castings. Instead of ordinary green sand cores, it is recommended to use resin in coarse sand, and plenty of cinders to make the core yielding. Cores may be made on hay-bands, if large, and as yielding as possible, and the mould must be cut away on the metal setting, to prevent breaking of casting, if the shape requires it. One of the chief difficulties met with in the casting of aluminium-bronze, is the oxidation in transferring the metal from the crucible or ladle to the mould. If any The of the film of oxide which floats on the surface of the metal should get into the casting during the pouring it will appear there like so much dirt, and is apt to cause trouble. ordinary skim-gate will prevent this in the case of small castings, but with large masses the metal is first poured into a receiver, which is connected with, and is part of, the pouring "gate," but is prevented from entering the mould by means of a plug, which closes up the mouth of the gate." To illustrate this more clearly, imagine the pouring 'gate," shaped like a funnel, into which the metal is first poured. It is prevented from running into the mould by the plug already mentioned. As soon as the dirt has risen to the top the plug is withdrawn, and consequently nothing but the clear metal from the bottom enters the mould. For castings over fifty pounds, the metal is poured from a large IV ALUMINIUM-BRONZE 217 ladle through a hole in the bottom. Ample facilities should be made for the escape of gases. Both aluminium and copper volatilise only at very high temperatures, so that alloys of these metals can be remelted without any appreciable change in the strength or quality of the metal. In the case of casting in iron moulds for wire strips, etc., the mould may be blackened with a mixture of plumbago, pipe-clay, and lard oil. When aluminium-bronze is made by simply mixing the ingredients, it is brittle, and does not acquire its best qualities until after it has been remelted three or four times. It then reaches a maximum, and can afterwards be remelted without sensible change. Thin castings come out very sharp, but if a casting is thin and suddenly thickens, small offshoots must be made at the thick place, into which the metal can run and then soak back into the casting as it cools and shrinks, thus avoiding cavities by shrinkage at the thick part. § 79. Cowles's Electrical Method. Aluminium alloys are prepared by the Cowles Electrical Smelting Company in an electrical furnace represented in vertical section in Fig. 29. 1"It consists of an oblong receptacle, the internal dimensions being 5 feet long, 3 feet 3 inches deep, and 20 inches wide. It is built below the floor line and lined with fire-bricks. Into the ends are built cast-iron tubes, inclined as shown, for convenience in working the electrodes. On the ends of the tubes are covers having bosses upon them, through which pass the electrodes. On the covers are screwed blocks of wood surrounding the bosses. Near the outer ends of the electrode rods are easily removable clamps, each of which holds a nut, through which passes a screw having a T handle at its outer end, whilst its inner end rests against the wooden block. The rotation of this screw moves the electrode inwards or outwards. The top of the brickwork is covered with iron plates, and an iron cover, as shown in Fig. 29, is placed over the mouth of the furnace. In 1 Paper by Mr. W. Robey, Cleveland Soc. of Engineers, December 1888. the cover is a hole through which the gases issue as they are driven off by the heat in the furnace. The electrodes consist of carbon rods 24 inches in diameter, and nine of them are attached to each electrode rod. The attachment, as shown in the figure, consists of metal, which is cast around the ends of the carbons and of the electrode rod, so as to form a conductor, which is in perfect contact with the whole of the parts connected. The metal from which this attachment is cast is selected so as to be suitable for the particular alloy to be treated in the furnace, as, although in comparison with the actual charge, it is not subjected to a very intense heat, the metal of which it is formed is found to exercise a slight influence upon the furnace product. 66 Here, then, we have a covered furnace provided at its ends with two electrodes, which are capable of being easily moved apart. The application of the furnace is as follows: The two electrode rods are connected to the two leads of the dynamo, the cable joined to the positive pole sending the current into the left-hand electrode, whence it travels through the charge, where it meets with the resistance which causes the heat, and passes by way of the right-hand electrode into the cable connected to the negative pole of the dynamo. Resistance is caused by the particles of the carbon which are mixed in the charge. Thus, the current acts independently of the structure of the furnace itself, acting simply on the charge which is in immediate contact with the carbon, by means of which the heat is produced. The heat IV ALUMINIUM-BRONZE 219 developed is enormous, and charcoal is found the most convenient form of non-conductor for a protective lining, but the intensity of the heat converts it in a short time into graphite, so that it loses its non-conducting properties. To renew the charcoal frequently is expensive, and the difficulty is overcome by dipping the partially graphitised charcoal into limed-water, the lime being found to render the charcoal again sufficiently non-conducting. "When charging a cold furnace, some charcoal is put into the bottom, and lightly beaten down till it forms a firm bottom of sufficient thickness. The electrodes are run inwards till their ends nearly meet in the furnace, and the charge is then put into the centre of the furnace, and surrounding the electrode ends. The charge for copper alloy consists of 300 lbs. of materials, about one-fourth being aluminium ore broken up small, a good deal of which is supplied by broken-up slag taken from a former heat, about 4 per cent of carbon, and the balance is granulated copper. "The ore employed is sometimes corundum, also called sapphire and ruby. The dull crystals are termed corundum, and the gray and black varieties emery. It is essentially alumina Al2O3. At the present time the ore employed in England is bauxite, the composition of a sample of which is as follows: "This was found to answer equally well, at a fraction of the cost of corundum. The charge while being inserted is prevented from spreading out laterally (in which case some portions of it might be too distant from the electrodes) by two pieces of sheet-iron connected by a distance piece, which are placed one on either side of the electrodes; the charge is put in between them, and the charcoal is then filled in between the sheet-iron and the sides of the furnace, after which the sheet-iron is withdrawn, the top of the charge covered, and the ends of the furnace filled with charcoal to the depth of a few inches above the charge and the electrodes. The iron cover is then luted on with fire-clay. A current of 2000 to 3000 amperes is applied at first, and, as the charge becomes fused, the resistance diminishes. The electrodes are then drawn farther apart, the speed of the engine increased, and a current of 5000 amperes flows through the charge, until the whole of the ore and metal are fused. One operation requires about one hour and three-quarters. It is assumed that the aluminium and copper unite when both are in the gaseous state, and the Company claim that by this means a completeness of union between the constituents of the alloy, and a homogeneity and strength are obtained superior to alloys formed in any other way." The rich alloys prepared in the electrical furnace just described are used for making aluminium-bronze, brass, and iron, by melting them with copper, tin, zinc, etc., in a crucible in an ordinary furnace, instead of using pure metallic aluminium, and it is found by this means that more perfect and uniform alloys are obtained. ALUMINIUM-BRASS $80. This alloy is prepared by adding spelter to aluminiumbronze, or by melting together various proportions of ordinary brass and aluminium-bronze. The greater the proportion of the bronze present in admixture, the harder and stronger the brass becomes. Alloys containing copper, zinc, and aluminium between the following limits— Copper 67 to 71 31 to 25 1 to 3 and combined in different proportions, have a tensile |