Alloys of Tin

Tin forms many important alloys, and these may be divided into two classes - those which contain copper and those which do not.

The properties of the copper-tin alloys - the bronzes - bear no definite relation to those of their constituent metals. Whilst tin itself is soft and fusible, its progressive addition to copper produces the following changes. Five per cent, of tin yields a nearly copper-coloured alloy which is tough and strong and much harder than copper, so that it may be employed for coinage and for engraving work. Ten per cent, of tin gives a rich yellow alloy which is still harder, so that it must be cast; and with the addition of more tin up to 20 per cent, the alloy becomes sonorous, though somewhat brittle, but suitable for making small bells. With further addition of tin up to 24 per cent, an alloy is obtained out of which the largest bells are cast; it is decidedly brittle and has a grey fracture. When 33 per cent, of tin is reached.the alloy is white and constitutes speculum metal, and is so brittle that it may be powdered in a mortar. The brittleness continues till about 65 per cent, of tin is present, after which the hardness diminishes as pure tin is approached.

The tin-copper alloys are here summarised:

Percentage of Tin.	Nature of Alloy.
5	Bronze for coinage and medals (a little tin often replaced by zinc), Roman nails, engineers' soft gun-metal or " brasses."
7	Bronze suitable for mathematical instruments.
8.5	Bronze suitable for wheels to be cut with teeth.
8-12	Soft bronze of the ancients: 9-10 per cent, for gun-metal; 10-12 per cent, for medium engineers' " brasses."
12-14.5	Hard bronze of the ancients, used for weapons and tools. Now used for engineers' hard bearings.
16	Soft musical bells.
18-20	Chinese gongs and cymbals.
20	Small house bells; Indian gongs.
22	Large house bells.
24	Limit for large church bells.
33	Often also a little arsenic. Speculum metal.
66	" Temper," an alloy used to harden pewter.

Bronze is hardened by a process opposite to that employed with steel. Instead of being suddenly cooled by plunging into water, it is slowly cooled to induce hardness; if suddenly cooled it becomes soft and malleable. This difference is due to the different kinds of crystals that are formed at different temperatures. Repeated hammering and cold working of bronze greatly increases its hardness.

Two compounds of copper and tin are known: Cu₃Sn and Cu₄Sn. The existence of these compounds, however, is probably insufficient to account for the remarkable differences in properties between the various alloys and their constituent metals. These alloys are probably to be regarded as mixtures of one or two definite compounds with excess of a solid solution of tin in copper. The electrical properties of copper-tin alloys have been studied by Ledoux.

Bronze bearing metal, employed for the bearings of locomotives, is an alloy composed of copper 77, tin 8, lead 15 parts. The presence of lead diminishes loss by wear and reduces local heating. The function of the tin is to provide a solvent medium for copper and lead, which do not mix well.

Phosphor-bronze is bronze containing 5 to 15 per cent, of tin and from a trace to 1.75 per cent, of phosphorus, added in the form of phosphor-tin. The phosphorus confers greater hardness, elasticity, and toughness upon the alloy. Manganese and silicon bronzes are also in vogue, but they contain little, if any, tin.

Tin enters into the composition of the following alloys, which contain little or no copper: common pewter, soft solder, type-metal, Britannia metal, plate-pewter, white bearing metal, and the various fusible metals.

Tin and lead will mix in all proportions; the alloys produced are more fusible than either separate metal, and are likewise harder and tougher. There is no evidence, however, of the existence of any compound of the two metals. The lead-tin eutectic point lies at 181° C. at 24.4 atomic per cent. Pb ( = 36.0 per cent. Pb by weight).

Soft solders are alloys of tin and lead in varying proportions; they occasionally contain bismuth, which lowers their melting-point. The table on the previous page (Tomlinson), showing the composition of soft solders, with their melting-points, illustrates the influence of admixture upon the melting-points of metals.

Ordinary soft plumbers' and tinmen's solder is made of equal parts of tin and lead; fine tinmen's solder contains 2 parts of tin to 1 of lead.

The toughest and hardest variety of pewter contains 3 parts of tin to 1 of lead; the proportion may, however, rise as high as 4 of tin to 1 of lead.