Lead and tin are malleable, flexible, ductile, and inelastic, whilst cold, but when their temperatures much exceed about half way towards their melting heats, they are exceedingly brittle and tender, owing to their reduced cohesion.
The alloys of lead and tin partake of the general nature of these two metals; they are flexible when cold, even with certain additions of the brittle metals, antimony and bismuth, or of the fluid metal mercury; but they crumble with a small elevation of temprature, as these alloys melt at a lower degree than either of their components, to which circumstance we are indebted for the tin solders.
Zinc, when cast in thin cakes, is somewhat brittle when cold, but its toughness is so far increased when it is raised to about 300° F. that its manufacture into sheets by means of rollers is then admissible; it becomes the malleable zinc, and retains the malleable and ductile character, in a moderate degree, even when cold, but in bending rather thick plates it is advisable to warm them to avoid fracture; when zinc is remelted, it resumes its original crystalline condition.*
Zinc and lead will not combine without the assistance of arsenic, unless the lead is in very small quantity; the arsenic makes this and other alloys very brittle, and it is besides dangerous to use. Zinc and tin make, as may be supposed, somewhat hard and brittle alloys, but none of the zinc alloys, except that with copper to constitute brass, are much used.
Gold, silver, and copper, which are greatly superior in strength to the fusible metals above named, may be forged either when red-hot or cold, as soon as they have been purified from their earthy matters, and fused into ingots; and the alloys of gold, silver, and copper, are also malleable, either red-hot or cold.†
Fine, or pure gold and silver, are but little used alone; the alloy is in many cases introduced less with the view of depreciating their value, than of adding to their hardness, tenacity, and ductility; the processes which the most severely test these qualities, namely, drawing the finest wires, and beating gold and silver leaf, are not performed with the pure metals, but gold is alloyed with copper for the red tint, with silver for the green, and with both for intermediate shades. Silver is alloyed with copper only, and when the quantity is small its colour suffers but slightly from the addition, although all its working qualities are greatly improved, pure silver being little used.
The alloys of similar metals having been considered, it only remains to observe that when dissimilar metals are combined, as those of the two opposite groups; namely, the fusible lead, tin, or zinc, with the less fusible copper, gold, and silver: the malleability of the alloys when cold, is less than that of the superior metal; and when heated barely to redness, they fly in pieces under the hammer; and therefore, brass, gun-metal, etc, when red-hot, must be treated with precaution and tenderness. Muntz's patent metal, which is a species of brass and is rolled red-hot, appears rather a contradiction to this; but in all probability this alloy, like the ingots of cast-steel, requires at first a very nice attention to the force applied. It will be also remembered the action of rollers is more regular than that of the hammer; and soon gives rise to the fibrous character, which so far as it exists in metals, is the very element of strength, when it is uniformly distributed throughout their substance. This will be seen by the inspection of the relative degrees of cohesion possessed by the same metal when in the conditions of the casting, sheet, or wire, shown by the table, page 288, and to which quality, or the tenacity of alloys, we shall now devote a few lines.
* It is considered that most of the sheet zinc contains a very little lead. + Gold alloyed with copper alone is not very malleable when hot.