now fixed on the top of the other, and the inside of the bell pattern dusted. Sand is then added and rammed in as before, a board placed on the top, and the double frame inverted. It is then beaten with a mallet to loosen the pattern, and the board taken off, thus exposing the patterns, which are now carefully taken off the cores, as they are termed, which are then dusted with a mixture of charcoal and flour. The frame is then screwed together, leaned against the spilling hearth, and the metal poured in. The caster usually makes about five moulds before pouring, this being termed a heat. Small bells are now also cast in iron moulds.

It has often been observed that bells cast from metal which has been repeatedly remelted acquire a disagreeable tone, and this has been attributed to the formation of metallic oxides and the solution of these oxides by the molten metal. If such metal be treated with a substance capable of exerting a deoxidising influence, such as phosphorus, silicon, manganese, magnesium, etc., a complete reduction of the metallic oxides takes place, the liberated oxygen uniting with the reducing agent added and passing into the slag. Deoxidising agents must be used very sparingly, otherwise the excess will enter into combination with the alloy and may be a greater evil than the one it is employed to remedy.

Chinese tam-tams and gongs are characterised by a strong penetrating sound, which is conferred by the peculiar mechanical treatment they are made to undergo. As soon as the plates are well solidified they are taken from the mould, raised to a cherry-red heat in a furnace, then inserted between iron discs to prevent warping, and the whole plunged into cold water and allowed to cool. After this treatment they are found sufficiently malleable to be worked under the hammer.

For large bells the metal is melted in a reverberatory furnace, and the molten alloy ought to be exposed to the heat for several hours, which produces a more homogeneous texture and less crystallisation. If any zinc is to be added to such an alloy, it is advisable to add it in the form of brass, calculating of course the quantity of copper it contains. The

relative quantity of the metals forming the alloy can be calculated and mixed to this arrangement; but the melting operation has an influence upon the strength of the metal. The more volatile constituents are volatilised to a greater extent than the copper, so that the founder takes proofs before casting and adds the constituent which is deficient. The trial sample is taken in a small iron ladle, the metal broken when cold, and the quality determined by the character of the fractured surface, tenacity, etc.

The following description of the casting of a large bell is taken from Overman's Founders' Guide: "In Fig. 28 a mould is represented

FIG. 28.

as it is sunk in the pit for casting. The core is built in brick upon an iron platform, which is to have 'snugs' in case the mould is made above ground. This brick core is covered with three-fourths of an inch or one inch thick of hairloam, and the last surface-washing is given by a finelyground composition

consisting of clay and brick-dust. This latter is mixed with an extract of horse-dung, to which is added a little salammoniac. Upon the core the 'thickness' is laid in loam sand, but the 'thickness' is again washed with fine clay to give it a smooth surface. Ornaments which have been previously moulded, either in wax, wood, or metal, are now pasted on by means of wax, glue, or any other cement. If the ornaments are of such a nature as to prevent the lifting of the cope without them-for the cope cannot

be divided - the ornaments are fastened to the 'thickness' by tallow, or a mixture of tallow and wax. A little heat given to the mould will melt the tallow, after which the ornaments adhere to the cope, from which they may be removed when the cope is lifted off the core. The 'thickness' is to be well polished, and as no coal can be used for parting, the whole is slightly dusted over with wood-ashes. The parting between the core and the 'thickness' is also made with ashes. The cope is laid on at first by means of a paint-brush, the paint consisting of clay and ground bricks made thin by horse-dung water. This coating is to be thin and fine; upon it hair-loam, and finally strawloam is laid. The crown of the bell is moulded over a wood pattern after the spindle is removed. The iron staple for the hammer is set in the core, into the hollow left by the spindle. It projects into the thickness so as to be cast into the metal. The facing of the mould ought to be finished when the cope is lifted off. Small defects may occur, and are, if not very large, left as they are; the excess of metal in those places is chiselled off after the bell is cast. All that can be done in polishing the facing of the mould is to give it a uniform dusting of ashes. When the mould is perfectly dry it is put together for casting. The core may be filled with sand if preferred, but there is no harm done if it is left open, for bell-metal does not generate much gas, and there is no danger of an explosion. The cope is in some measure secured by iron, but its chief security is in the strong, well-rammed sand of the pit. The cast-gate is on the top of the bell, either on the crown or, if the latter is ornamented, on one side of it. Flow-gates are of no use here, the metal is to be cleaned before it enters the mould; there is no danger of sullage."

SPECULUM METAL

§ 70. This is a perfectly white alloy which admits of a beautiful polish, used formerly for mirrors but now only used

for such purposes as the construction of mirrors for optical instruments, and even here they are being gradually displaced by glass mirrors. Its typical composition is represented by the formula Cu,Sn, containing 66-6 per cent of copper and 33-4 per cent of tin. The speculum metal of Lord Ross's large telescope is composed of 68.21 copper and 3179 tin. This alloy is of a brilliant white lustre and has a specific gravity of 8.811; it is nearly as hard as steel, and brittle. The speculum is cast 6 feet in diameter and 51 inches thick, and weighs upwards of three tons. The casting of this mirror was only effected after repeated failures. A mould was made whose bottom consisted of a wrought-iron ring, packed full of hoop-iron laid edgeways, so close that air but no metal could escape through the crevices; this bottom was turned convex on a lathe, true to the concavity of the speculum; it was then placed upon a level floor and enclosed by a sand dam left open from above. The metal was melted in cast-iron crucibles, because wrought-iron or clay would have injured the alloy. The cast was carried while hot into the annealing oven, which was previously heated to a red heat, and left there sixteen weeks to cool.

Good speculum metal should be pure white, of a finegrained structure, perfectly sound and homogeneous when cast, and sufficiently tenacious to stand grinding and polishing without rupture. It should contain 65 to 68 per cent of copper to comply with all these requisites. The following table exhibits different varieties :

Copper. Tin. Zinc. Arsenic. Lead. Other metals.

CHAPTER IV

MACHINE BRONZES OR BRASSES

§ 71. Under this general term will be included various alloys employed for different parts of machinery, such as bearings, and parts subjected to great friction. These alloys very frequently contain ingredients other than copper and tin. They must be sufficiently hard to resist wear, capable of being easily cast into various shapes, worked with the file and turning tools, and otherwise prepared by mechanical treatment for the uses for which they are designed.

For the bearings of large axles and shafting, especially those which revolve with great rapidity, alloys containing 80 to 90 per cent of copper are used. These alloys are capable of being forged at a red heat, a property often required for their manufacture into different shapes. Some alloys are required to possess great strength, so as to resist sudden shocks without yielding; others are required to offer little frictional resistance under a heavy load when in contact with other metals.