Copper and tin mix well in almost all proportions, forming a class of alloys generally spoken of as bronzes, sometimes having incorporated with them lead, antimony, manganese and phosphorus, with iron and silicon as impurities. A small content of tin renders the alloy both hard and tenacious, a maximum hardness for all shop purposes being attained by the addition of about 15 to 18 percent. Table II. fairly represents the various mixtures of these alloys.

These alloys have a lower melting point than copper, a greater density than the mean density of the chief constituents, less liable to oxidation, and they are proportionately harder than either of the principal constituting metals, especially as the content of tin increases. The fact of their being more fusible than copper and less than tin, renders it very difficult to obtain a perfectly homogeneous alloy, portions richer in tin being interposed through the mass; therefore it should be cooled as rapidly as possible, to obviate any tendency to liquation during the cooling. Oxidation must be avoided as far as possible, which points to the rapid melting of the copper and keeping the tin immersed during mixing, because of the rapid formation of the peroxide. This formation is very disadvantageous, becoming a great nuisance to the finisher, rendering all machine work imperfect, because of the hard dirty spots, which destroy the cutting edge of the tools. Especially is this the case if the metal has been produced by the open hearth Siemens regenerative furnace, when that furnace has been working slowly; the metal will be found to contain a greater quantity of these hard dirty spots, also there will be more wasters in castings.

Table II

The small quantity of zinc used, say 2 per cent., plays the beneficial part of deoxidant, by reducing the incorporated oxides, rendering the product purer, consequently of greater tenacity, but at the same time it has a tendency to lower the elastic limit and soften the alloy. Phosphorus behaves in a similar manner to zinc, but with greater energy, and in increased proportions it sensibly hardens the alloy. Phosphor bronze is generally manufactured either by the addition of copper or tin phosphide, and after having eliminated the oxides, a further inclement to produce from .25 per cent, to 2.5 per cent, in the finished product will change the colour to a greater degree of evenness, the fracture will be finer, and its physical properties and fluidity at the time of casting will be greatly increased. In fact, the use of phosphorus in the brass foundry may be compared to carbon in the steel foundry.

The chief uses of phosphor bronze may be taken as slide valve and bearing metals, the former a most important item on a railroad. To obtain a standard slide valve mixture, valves of an experimental mixing should be made with varying contents of phosphorus, noting at the time of manufacture all the points connected therewith, as much depends upon the facilities of the shop and the skill brought to bear upon their production. Some of the points worth noticing are the condition of the furnace, length of time in melting the copper, and approximate temperature at the time of casting. The fracture of a small ingot should be reserved, and where possible mechanical and analytical tests should be made. The behaviour of these valves should be watched and compared with the wear of the ordinary mixture. In this class of work good clean copper shearings and new metals are resorted to, but for general work the accumulation of old valves has to be dealt with, or it may be that all the old scrap is melted in an open-hearth furnace and cast into ingots. In any case the scrap has to be disposed of, and in only one obvious manner. The quantity of scrap used in the mixings will of course regulate itself to the quantity on stock, say up to 50 per cent, of the charge. The use of this scrap is a very easy matter if its nature is thoroughly known, and consequently reduces the quantity of new metal required; but if this is not known, then the mixing must be based upon, say, an average analysis of half-a-dozen samples of the scrap.

The addition of the phosphide must be the last operation, after lead and tin, the copper being melted as rapidly as possible for reasons already stated, that is, to lessen the work of the phosphides, the whole being afterwards well stirred and poured. The knowledge necessary for pouring at the right moment is very soon acquired by the operator, by watching the working of the metal and the rising of the scum, the latter having to be removed. In the case of bearings, for durability alone, they should be as hard as the axles they support; but considering the wear of the latter, the former should be softer, so that the wear, say, per one thousand miles is about three to one. Owing to the difficulty in obtaining homogeneous alloys, it often happens that in liquation the harder alloys separate out, and form the interior metal of the bearing instead of the outside casing, resulting in having for the actual bearing a soft alloy winch rapidly wears, then the axle coming upon the hard places, causes, in the absence of ample lubrication, its destruction. The soft alloy cools first and forms the shell, the harder filling the interior spaces, which Will probably contain double the content of tin to the former. Everything points to the use of a rich alloy of phosphide just before the time of casting, well stirring, and rapid cooling. A fluid pressure caused by a large gate and head, will enhance its resistance to compression.

It has also been observed that those alloys which wear the longest have a low tenacity and a good elongation; but by retaining the elongation with an increased tanacity, the metal would have double reasons for increased durability. The granular structure is also an important factor in the wear of metals, because the finer the structure the finer will be the flaky abrasion, and consequently longer wear. Among the many conditions which will affect the wear may be enumerated lubrication, resistance to abrasion, pressure, speed, and in the case of slide valves, temperature; hut supposing these conditions to remain constant, then the resistance to wear must be sought for and applied in the alloy itself, and it will always he found that with any ordinary shop mixture, if precaution he taken to produce it with an increased amount of purity, it will resist wear to a greater extent, this probably being done by the small addition of zinc or phosphorus to remove the oxides, and beyond, by the addition of a greater amount of phosphorus, its wearing properties will be enhanced. It has also been found that the increased quantity of lead, diminishing tin in proportion, has a very beneficial effect upon the wear of this class of alloys, and as a matter of fact, 10 per cent, of lead and only 6 per cent, of tin is regularly used for the slide valves of the engines in question, with about .25 per cent, to .3 per cent, phosphorus.