Brass

The alloys of copper and zinc may generally be accepted under the term of brass. They are made in great variety of physical properties and appearances, varying from the colour of the latter metal to that of gold. The content of copper may be from 60 to 90 per cent., but generally the composition of English brass is about 70 per cent, copper and 30 per cent, zinc, with about .5 per cent, lead, which will make it work better in the machine shop, and is generally found in locomotive brass. The beat proportion for yellow brass is two of copper and one of zinc. When mixing, an allowance of about 2 per cent, must always be made for the elimination of zinc, caused by its point of volatilisation being very low, and for a similar reason - its low melting point; - brass cannot be forged. Zinc produces the hardness, which gives the alloy a greater resistance to wear, being at the same time malleable and ductile. If slowly cooled it is more hard and brittle than if plunged whilst at a red heat into cold water.

White Metals

White and antifriction metals are almost without number, and until recently their base was tin - in some cases to the extent of 78 to 86 per cent. Now, generally speaking, lead has taken the place of tin as a base, with from 10 to 20 per cent, of antimony as a hardener, and sometimes arsenic. Copper is in most cases added to the extent of from 2 to 10 per cent. It will be found, as in the case of brass and bronze, that every shop has its own mixing, which is quite as good as any floated upon the market, and mostly having the same ingredients, and nearly equal percentages. The chief object in a good metal is to have a low melting point, especially where there is a lot of work, so that it can be used over a pot fire, that is, without the aid of crucibles. It should be thin when melted, but slow to set, having a lengthy plastic stage, so that the workman can follow it up in a similar manner to wiping a plumbing joint when soldering two lead pipes.

Other Alloys

After these two distinct alloys of copper, brass and bronze, the rest may be disposed of as entirely of an experimental nature, although many, such as ferro up to 3.5 per cent, silicon, manganese, and aluminium bronzes, are being used extensively. In the case of manganese bronze, ferro-manganese is added to the molten copper, the principal use of the manganese being to free the copper from incorporated oxides, which rise to the surface as slag. If more manganese is used than is sufficient to remove the oxides, the alloy is of greater tenacity and more ductile, the maximum being attained with 10 per cent., beyond this amount the alloy becomes very hard and brittle, until at 15 per cent, it is of a grey colour. Aluminium bronze may he obtained by using good copper and re-melting; 90 per cent. copper and 10 per cent, aluminium having a definite chemical composition, Cu9 Al. There is a difficulty in getting the aluminium incorporated with the copper, also in pouring, as the alloy sets very rapidly. The best method, as found by many experiments carried out by the author for Mr. Aspinall. is to make the above rich alloy, cast into ingots, and use in an ordinary manner with the usual mixture. A 3 per cent, aluminium bronze gives very good results - double the tenacity of good brass - and at very low temperatures it will forge well, aluminium itself becoming red-short at 400° F. This 3 per cent, bronze would be very useful for castings, although it cannot compete with steel as far as first cost of material is concerned; but it has this advantage, that only working parts need be machined, as it makes a very smooth surface. Aluminium passes through a granular stage, and becomes fluid at about 1300° F., copper melting at 1980° F. It is advisable to use the best plumbago crucibles, because from silicious ones it becomes seriously contaminated with silicon. A flux is not necessary, and might become detrimental, owing to the extreme lightness of the metal. At the time of casting a skimmer should be used, in order to remove the thin film of oxide which always forms; and then it should be poured quickly. Larger gates are required, and the best castings are obtained from dry sand moulds, although very good ones may be obtained by skin drying only. Coruscation is characteristic of the introduction of aluminium into the crucible, and the castings sometimes present locally an iridescent appearance.