Annealing. When a slab of glass or metal is allowed to cool down rapidly from its melted state, the constituent particles near the surface become differently arranged from those in the interior. The molecules next the skin are in a different state of tension from those inside. Annealing is a process of slow cooling of a body from a high temperature, by which there is secured a more or less uniform arrangement of the particles or molecules throughout its mass. Glass is in this way made strong and able to resist changes of temperature. The mere dropping of a small angular fragment of some hard substance, such as flint, into a glass vessel before it is annealed, usually makes it fly to pieces. A still more striking example of the unstable nature of unannealed glass is seen in Prince Rupert's drops. These are drops of glass which have fallen in a melted state into cold water, and have assumed a tadpole-like shape. If the point of the tail of one of these be nipped off with the fingers, the whole of it will fall into dust with a loud explosion. This shows that whenever the skin is broken, the particles beneath it are acted on by a repellent force, and fly away from one another.
An annealing kiln or oven is usually of some length, and the glass vessels or sheets placed in it are raised to near their melting-points at its hottest portion, and then moved away at intervals to cooler and cooler parts of the chamber. It takes twelve hours to anneal wine-glasses, but much longer for large objects. Plate-glass requires to be two weeks in the kiln before it is properly annealed. Badly annealed glass shows itself in numerous ways. A basin of thick glass left in an ordinary room will sometimes break spontaneously during a cold night. Plates of glass placed, on account of their apparent strength, in floors to admit light to cellars, have occasionally cracked to pieces during sharp frost. Hot water, as is well known, often breaks tumblers.
Metals under various circumstances require to be annealed. Hollow ware (q.v.) of cast-iron, before it can be turned bright for tinning, must be softened by the annealing process. The old way of doing this was a rough and ready one. Large and strong iron pots which contained the ware were placed on gratings in the open air, and the whole covered over with coke, all interstices both within and without the pots being filled up with coal-dust, to prevent as far as possible the access of air. The coke was then fired and kept at a red heat for about twenty-four hours, after which the pile was allowed to cool. For many years, however, hollow ware has been annealed in an oven not much unlike that used for glass. Large iron-castings are kept covered up in their moulds, to prolong the time of cooling—sometimes with hot cinders—for a month or more. Like thick glass, these occasionally break spontaneously.
What are called malleable-iron castings are articles usually of limited size, made of cast-iron, which are afterwards annealed. They are covered over with powdered hæmatite ore, and subjected to various degrees of heat for about ten days, when they become quite malleable.
Metals, when undergoing the process of rolling, hammering, or stamping, require annealing. In the manufacture of sheet-brass, the rolling, by which it is gradually reduced in thickness, makes it so hard that it has to be annealed several times during the operation. But in this case the annealing is conducted in a reverberatory furnace, and lasts only a few minutes. The sheet-brass is simply raised to a blood-red heat and then withdrawn, this being sufficient to restore the ductility of the metal. Articles made of brass and other metals by stamping, and particularly such articles as require many blows of the stamp to bring them into shape, are repeatedly annealed during the process. In the case of coins, as they receive only one blow of the coining-press, the metal blanks are annealed before they are stamped. A steel matrix, from which die-punches are impressed, being usually a work of much labour, is put through the annealing process after every few blows in the die-press. German silver, which is composed of three kinds of metal, is difficult to anneal from its tendency to crack in the process.
Annealing is also used in gold-beating, in wire-drawing, in nail-making, and many other arts. Tin, lead, and zinc are annealed by the use of boiling water, and steel tools by immersion in hot oil, both liquids being allowed to cool slowly. Many experiments have shown that steel boiler-plates and ship-plates are made stronger by annealing them in oil, or in melted lead, or by simply heating them to redness in a slow furnace, and afterwards covering them up with sand or ashes to prevent them cooling rapidly or unequally. Tempering (q.v.) has been called the inverse process of annealing.