Alloys are compounds of two or more metals. Every alloy may be regarded as a new metal, since it generally possesses properties different from those of the metals of which it is composed; bat as the properties resulting from the combination of two metals rarely represent the mean of those metals, it is impossible to foretell the nature of a new alloy. All true alloys consist of compounds of metals in their definite chemical proportions; it is, however, a matter of some difficulty to obtain them in a separate state, owing to the readiness with which they dissolve in the excess of that metal which may happen to preponderate. There are some few alloys also in which the constituent metals seem to be merely mechanically mixed. Alloys possess the properties which are characteristic of metals, such as metallic lustre, conductivity of heat and electricity, and, in a greater or less degree, malleability, ductility, and tenacity. The specific gravity of an alloy appears to depend upon the amount of cohesion or attraction exerted by the constituent metals for one another, and to bear no reference whatever to the high or low specific gravity of those constituents in their free state. The specific heat of alloys was found by Regnault to be very nearly the mean of the specific heats of the constituents.
The following rule for obtaining the specific heat of alloys gives a very close approximation to the figures obtained by actual experiment:-Multiply the specific heat of each constituent into the percentage amount of it contained in the alloy, and divide the sum of the products by 100. Alloys are not, as a rule, such good conductors of heat and electricity as the metals of which they are made. Their fusibility does not at all depend upon that of their constituents, but is generally greater; thus the melting-point of tin is 455° F. (235° C), and that of lead 626° F. (330° C), whereas a compound of 5 parts of tin and 1 part of lead melts at 381 1/4° F. (194° C), and a compound of equal parts of both metals melts at 465 3/4° F. (241° C). The ductility of alloys is usually slightly less than that of their most ductile constituent, and their hardness is greater than the mean hardness of both or all the metals. The tenacity of an alloy is often much greater than that of either of the metals alone.
Alloys of gold, silver, and copper, are generally superior in strength to any of the more fusible metals, and may be forged either when red-hot or cold. These 3 metals seem to unite in any proportions, and always form au alloy that is malleable when either hot or cold. Pure gold is but little used in the arts: it is too soft. It is generally alloyed with silver and copper, both to harden it and depreciate its value. Alloyed with copper, it forms gold of a red tint; with silver, it forms gold of a green tint; and alloyed with both copper and silver, it gives intermediate tints. Pure silver is but little used alone; it is generally alloyed with a small amount of copper, which does not change its colour, and greatly improves its malleability and working qualities. When gold, silver, or copper is alloyed with the more fusible metals-lead, tin, and zinc - the alloy is less malleable and ductile than alloys of gold, silver, and copper. They are " extreme red-short," and when heated to redness they will fly to pieces under the hammer; and alloys of brass, bell-metal, etc, must be treated with caution, and should never be taken out of the mould while red-hot. Alloys of 2 parts copper and 1 zinc are very soft and malleable, and may be drawn by hammering or easily cut with a file, but an alloy of 1 copper and 2 zinc is as hard and brittle as glass, and may be easily pulverized.
An alloy of 2 copper and 1 lead makes a soft, malleable metal, but is inferior to an alloy of copper and zinc. In alloys of 1 copper and 1 lead, the lead will ooze out in cooling. In alloys of 1 copper and 2 lead, the lend will not unite, but will sink to the bottom when cooling. Alloys of 6 copper and 1 tin make a very hard alloy, which gets harder and whiter the more tin is added. Alloys of tin and copper should not be too rapidly exposed to the air, for if a large percentage of tin is used it will strike to the surface and ooze out, or make hard spots in the casting. Alloys of zinc and lead cannot be made without the addition of arsenic, unless the lead is in very small quantity. Alloys of zinc and tin are very hard and brittle, and are but little used alone. By the addition of copper to alloys of these 2 metals, the alloy is rendered more malleable and soft. Arsenic makes all alloys hard and brittle, and is very dangerous to use. It is seldom used except to impart' fluidity to the very infusible metals. Alloys of lead and tin are. very malleable and ductile when cold, but at a temperature of about 200° F. (93 1/2° C), they lose the power of cohesion, and are exceedingly brittle. The alloys of tin and lead partake of the general nature of these 2 metals.
They are soft and malleable when cold, even when a small amount of brittle antimony has been added. An alloy of 6 lead and 1 antimony is very soft and malleable, but an, alloy of 3 lead and 1 antimony is very hard and brittle; and an alloy of 1 lead and 1 antimony is harder and more brittle than antimony. (E. Kirk.)
Alloys are made by melting metals together. This operation may be carried on in an earthenware crucible, when small quantities are being operated upon; but when large masses of metal have to be dwelt with, as in the case of statues, etc, a rever-beratory furnace must be employed to effect the melting. As a rule, the least fusible metal is placed in the crucible first, unless it be in very small quantity, and will dissolve readily in the other metal, in which case it goes in last; and if, as in the case of zinc, the volatilization of the metal be extremely rapid, it is introduced only the moment before the fused mass is ready to be poured into the mould or other receptacle. The order in which the metals are melted has a material effect upon the nature of the resulting alloy, for it has been proved by experiment that the latter often possesses different properties when the mixing has taken place in a' different order. The fused metals should be kept thoroughly well stirred up until the mixture is complete; otherwise the heaviest metal will sink to the bottom of the mass, and the alloy will not be of uniform composition. This contingency is sometimes avoided by melting the mass a second time.