No general rules can be given for alloying metals. Alloys differing greatly in fusibility are commonly made by adding the more fusible ones, either in the melted state or in small portions at a time, to the other melted or heated to the lowest possible temperature at which a perfect union will take place between them. The mixture is usually effected under a flux, or some material that will promote liquefaction and prevent volatilization and unnecessary exposure to the air. Thus, in melting lead and tin together for solder, rosin or tallow is thrown upon the surface is rubbed with sal ammoniac; and in combining some metals, powdered charcoal is used for the same purpose. Mercury or quicksilver combines with many metals in the cold, forming amalgams, or easily fusible alloys (q. v.).

Alloys generally possess characteristics unshared by their component metals. Thus, copper and zinc form brass, which has a different density, hardness, and color from either of its constituents. Whether the metals tend to unite in atomic proportions or in any definite ratio is still undetermined. The evidence afforded by the natural alloys of gold and silver, and by the phenomena accompanying the cooling of several alloys from the state of fusion, goes far to prove that such is the case (Rudberg). The subject is, however, one of considerable difficulty, as metals and metallic compounds are generally soluble in each other, and unite by simple fusion and contact. That they do not combine indifferently with each other, but exercise a species of elective affinity not dissimilar to other bodies, is clearly shown by the homogeneity and superior quality of many alloys in which the constituent metals are in atomic proportion. The variation of the specific gravity and melting points of alloys from the mean of those of their component metals also affords strong evidence of a chemical change having taken place. Thus, alloys generally melt at lower temperatures than their separate metals. They also usually possess more tenacity and hardness than the mean of their constituents.

Matthiessen found that when weights are suspended to spirals of hard-drawn wire made of copper, gold, or platinum, they become nearly straightened when stretched by a moderate weight; but wires of equal dimensions composed of copper-tin (12 per cent of tin), silver-platinum (36 per cent of platinum), and gold-copper (84 per cent of copper) scarcely undergo any permanent change in form when subjected to tension by the same weight.

The same chemist gives the following approximate results upon the tenacity of certain metals and wires hard-drawn through the same gauge (No. 23):

Pounds

Copper, breaking strain.........   25-30

Tin, breaking strain.......under          7

Lead, breaking strain......under          7

Tin-lead (20% lead).......about          7

Tin-copper (12% copper). . .about          7

Copper-tin (12% tin)......about  80-90

Gold (12% tin)..........,.....   20-25

Gold-copper (8.4% copper)......   70-75

Silver (8.4% copper)...........   45-50

Platinum (8.4% copper)........   45-50

Silver-platinum (30% platinum).   75-80

On the other hand, the malleability, ductility, and power of resisting oxygen of alloys is generally diminished. The alloy formed of two brittle metals is always brittle; that of a brittle and a ductile metal, generally so; and even two ductile metals sometimes unite to form a brittle compound. The alloys formed of metals having different fusing points are usually malleable while cold and brittle while hot. The action of the air on alloys is generally less than on their simple metals, unless the former are heated. A mixture of 1 part of tin and 3 parts of lead is scarcely acted on at common temperatures; but at a red heat it readily takes fire, and continues to burn for some time like a piece of bad turf. In like manner, a mixture of tin and zinc, when strongly heated, decomposes both moist air and steam with rapidity.

The specific gravity of alloys is rarely the arithmetical mean of that of their constituents, as commonly taught; and in many cases considerable condensation or expansion occurs. When there is a strong affinity between two metals, the density of their alloy is generally greater than the calculated mean; and vice versa, as may be seen in the following table:

Alloys having a density Greater than the Mean of their Constituents: Copper and bismuth, Copper and palladium, Copper and tin, Copper and zinc, Gold and antimony, Gold and bismuth, Gold and cobalt, Gold and tin, Gold and zinc, Lead and antimony, Palladium and bismuth, Silver and antimony, Silver and bismuth, Silver and lead, Silver and tin, Silver and zinc.

Less than the Mean of their Constituents:

Gold and copper, Gold and iridium, Gold and iron, Gold and lead, Gold and nickel, Gold and silver, Iron and antimony, Iron and bismuth, Iron and lead, Nickel and arsenic, Silver and copper, Tin and antimony, Tin and lead, Tin and palladium, Zinc and antimony.