MALLEAI3ILITY, a property by virtue of which certain bodies, especially some of the metals, are capable, although in very different degrees, of being beaten out under the hammer (n&leas, Lat.), or laminated between rollers. The following table represents the order of malleability Gold is usually cited as an extreme case of malleability. It is. beaten to the 290,000th part of an inch in thickness, and often much thinner than this, even to the 367,650th of an inch, or about 1200 times thinner than ordinary printing paper, and even this is not the limit of malleability, although there are practical difficulties in the way of further reduction. Comparing equal weights of gold and silver, it will be found that silver can be made to cover a larger surface than gold, so that silver is really more malleable than gold, although the leaf cannot be beaten out so thin. The metals lithium, potassium, and sodium, as well as frozen mercury, admit of being extended under the hammer.
In the production of sheet-metal, the hammer has been to a great extent superseded by rollers. Iron, steel, and copper, are best rolled red-hot ; zinc is most malleable between 300° and 400° ; most of the other metals and alloys are rolled in the cold state. The rolling-mill performs its work in a more uniform and gradual manner than the hammer, and at the same time developer to the utmost the hardness, tenacity, elasticity, and ductility of such metals and alloys as admit of being rolled. During the process, the condensation is such that brittle
ness would be produced unless the process of annealing were resorted to at intervals. In rolling the thinnest sheets of metal, a number of them are often sent through the rollers at the same time. In some cases, gold and silver leaf for example, the metals are rolled as thin as they will economically admit of, after which the hammer is resorted to. Leaf iron has been rolled into sheets, the 2500th of an inch in thickness, and a square inch of the leaf weighed only three-quarters of a grain.
The difference between malleability and ductility is noticed under Duaritrrr. In both processes the particles seem to glide along each other's surface, and " the external layers of the metals are retarded or kept back, as it were, in a wave, whilst the central stream or substance continues its course at a somewhat quicker rate. The necessity for annealing occurs, when the compression and sliding have arrived at the limit of cohesion ; beyond this the parts would tear asunder, and produce such of the internal cracks and seams met with iu sheet metal and wire, as are not due to original flaws and air-bubbles which have become proportionably elongated in the course of the manufacture of these materials." Holtzapfeers Mechanical Manipulation.'