Closely related to the structure of metals is their degree of
“plasticity” (susceptibility of being constrained into new forms
without breach of continuity). This term of course includes
as special cases the qualities of “malleability” (capability of
being flattened out under the hammer) and “ductility” (capability
of being drawn into wire); but these two special qualities
do not always go parallel to each other, for this reason amongst
others—that ductility in a higher degree than malleability is
determined by the tenacity of a metal. Hence tin and lead,
though very malleable, are little ductile. The quality of plasticity
is developed to very different degrees in different metals, and
even in the same species it depends on temperature, and may
be modified by mechanical or physical operations.
A bar of zinc, for instance, as obtained by casting, is very brittle; but when heated to 100° or 150° C. it becomes sufficiently plastic to be rolled into the thinnest sheet or to be drawn into wire. Such sheet or wire then remains flexible after cooling, the originally only loosely cohering crystals having got intertwisted and forced into absolute contact with one another—an explanation supported by the fact that rolled zinc has a somewhat higher specific gravity (7·2) than the original ingot (6·9). The same metal, when heated to 205° C., becomes so brittle that it can be powdered in a mortar. Pure iron, copper, silver and other metals are easily drawn in to wire, or rolled into sheet, or flattened under the hammer. But all these operations render the metals harder, and detract from their plasticity. Their original softness can be restored to them by “annealing,” i.e. by heating them to redness and then quenching them in cold water. In the case of iron, however, this applies only if the metal is perfectly pure. If it contains a few parts of carbon per thousand, the annealing process, instead of softening the metal, gives it a “temper,” meaning a higher degree of hardness and elasticity (see below).
What we have called plasticity must not be confused with the notion of “softness,” which means the degree of facility with which the plasticity of a metal can be discounted. Thus lead is far softer than silver, and yet the latter is by far the more plastic of the two. The famous experiments of H. E. Tresca show that the plasticity of certain metals at least goes considerably farther than had before been supposed.
He operated with lead, copper, silver, iron and some other metals. Round disks made of these substances were placed in a closely fitting cylindrical cavity drilled in a block of steel, the cavity having a circular aperture of two or four centimetres below. By an hydraulic press a pressure of 100,000 kilos was made to act upon the disks, when the metal was seen to “flow” out of the hole like a viscid liquid. In spite of the immense rearrangement of parts there was no breach of continuity. What came out below was a compact cylinder with a rounded bottom, consisting of so many layers superimposed upon one another. Parallel experiments with layers of dough or sand plus some connecting material proved that the particles in all cases moved along the same tracks as would be followed by a flowing cylinder of liquid. Of the better known metals potassium and sodium are the softest; they can be kneaded between the fingers like wax. After these follow first thallium and then lead, the latter being the softest of the metals used in the arts. Among these the softness decreases in about the following order: lead, pure silver, pure gold, tin, copper, aluminium, platinum, pure iron. As liquidity might be looked upon as the ne plus ultra of softness, this is the right place for stating that, while most metals, when heated up to their melting points, pass pretty abruptly from the solid to the liquid state, platinum and iron first assume, and throughout a long range of temperatures retain, a condition of viscous semi-solidity which enables two pieces of them to be “welded” together by pressure into one continuous mass.
According to Prechtl, the ordinary metals, in regard to the degree of facility or perfection with which they can be hammered flat on the anvil, rolled out into sheet, or drawn into wire, form the following descending series:—
| Hammering. | Rolling into Sheet. | Drawing into Wire. |
| Lead. | Gold. | Platinum. |
| Tin. | Silver. | Silver. |
| Gold. | Copper. | Iron. |
| Zinc. | Tin. | Copper. |
| Silver. | Lead. | Gold. |
| Copper. | Zinc. | Zinc. |
| Platinum. | Platinum. | Tin. |
| Iron. | Iron. | Lead. |
To give an idea of what can be done in this way, it may be stated that gold can be beaten out to leaf of the thickness of 13800 mm.; and that platinum, by judicious work, can be drawn into wire 120000 mm. thick.
By the “hardness” of a metal we mean the resistance which it offers to the file or engraver's tool Taking it in this sense, it does not necessarily measure, e.g. the resistance of a metal to abrasion by friction. Thus, for instance, 10% aluminium bronze is scratched by an ordinary steel knife-blade, yet the sets of needles used for perforating postage stamps last longer if made of aluminium bronze than if made of steel.
Elasticity.—All metals are elastic to this extent that a change of form, brought about by stresses not exceeding certain limit values, will disappear on the stress being removed. Strains exceeding the “limit of elasticity” result in permanent deformation or (if, sufficiently great) in rupture. Referring the reader to the article Elasticity for the theoretical and to the Strength of Materials for the practical aspects of this subject, we give here a table of the “modulus of elasticity,” E (column 2), for millimetre and kilogramme. Hence 1000/E is the elongation in millimetres per metre length per kilo. Column 3 shows the charge causing a permanent elongation of 0·05 mm. per metre, which, for practical purposes, Wertheim takes as giving the limit of elasticity; column 4 gives the breaking strain. These values may vary within certain limits for different specimens.
| Name of Metal. | E. | For Wire of 1 sq. mm. Section, Weight (in Kilos) causing | |
| Permanent Elongation of 120000. |
Breakage. | ||
| Lead, drawn | 1,803 | 0·25 | 2·1 |
| Lead,„ annealed | 1,727 | 0·20 | 1·8 |
| Tin, drawn | 4,148 | 0·45 | 2·45 |
| Tin,„ annealed | 1,700 | 0·20 | |
| Cadmium | 7,070 | 2·24 | |
| Gold, drawn | 8,131 | 13·5 | 27 |
| Gold,„ annealed | 5,585 | 3·0 | 10 |
| Silver, drawn | 7,357 | 11·3 | 29 |
| Silver,„ annealed | 7,140 | 2·6 | 16 |
| Zinc, pure, cast in mould | 9,021 | ||
| Zinc,„ ordinary, drawn | 8,735 | 0·75 | 13 |
| Palladium, drawn | 11,759 | 18 | |
| Palladium,„ annealed | 9,709 | under 5 | 27 |
| Copper, drawn | 12,449 | 12 | 40 |
| Copper,„ annealed | 10,519 | under 3 | 30 |
Platinum wire, medium
thickness, drawn |
17,004 | 26 | 34 |
| Platinum, annealed | 15,518 | 14 | 23 |
| Iron, drawn | 20,869 | 32 | 61 |
| Iron,„ annealed | 20,794 | under 5 | 47 |
| Nickel, drawn | 23,950 | 32×61 | |
| Aluminium | 7,200 | ||
| Nickel,„ bronze | 10,700 | ||
| Brass (ZnCu2) | 8,543 | ||
| German silver (Zn4Cu13Ni4) | 10,788 | ||
Specific Gravity.—This varies in metals from ·594 (lithium) to 22·48 (osmium), and in one and the same species is a function of temperature and of previous physical and mechanical treatment. It has in general one value for the powdery metal as obtained by reduction of the oxide in hydrogen below the melting point of the metal, another for the metal in the state which it assumes spontaneously on freezing, and this latter value, in general, is modified by hammering, rolling, drawing, &c These mechanical operations do not necessarily add to the density; stamping, it is true, does so necessarily, but rolling or drawing occasionally causes a diminution of the density. Thus, for instance, chemically pure iron in the ingot has the specific gravity 7·844; when it is rolled out into thin sheet, the value falls to 7·6; when drawn into thin wire, to 7·75. The following table gives the specific gravities of many metals. Where special statements are not made, the numbers hold for the ordinary temperature (15° to 17° or 20° C), referred to water of the same temperature as a standard, and to hold for the natural frozen metal.
| Name of Metal. | Specific Gravity. |
| Lithium | ·594 |
| Potassium | ·875 |
| Sodium | ·978 |
| Rubidium | 1·32 |
| Calcium | 1·578 |
| Magnesium | 1·743 |
| Caesium | 1·88 |
| Beryllium | 2·1 |
| Strontium | 2·5 |
| Aluminium, pure, ingot | 2·583 at 4° |
| Aluminium,„ ordinary, hammered | 2·67 |
