Encyclopædia Britannica, Ninth Edition/Annealing
ANNEALING is the process of removing the brittleness peculiar to glass vessels and metal castings immediately after manufacture. Newly-blown glass is so fragile as to be altogether unfit for use; and the common occurrence of the breaking of a lamp-glass on lighting the lamp, or of a tumbler on pouring in hot water, shows the want of proper annealing.
The process consists generally in heating the vessels up to a red or white heat in a close oven, and allowing them to cool gradually over a space of several hours or even days. The brittleness of rapidly-cooled glass is exhi bited by the philosophical toy known as Rupert s drops or glass tears, said to have been first brought over to England by Prince Rupert and exhibited at the court of Charles I. They are small tadpole-shaped pieces of glass, which have been formed by allowing fused glass to drop into water. A pretty hard blow may be given with impunity to the thick part or head of the glass tadpole; but the mere breaking off of the tail causes the whole to fly to pieces, or rather to dust, with a sharp explosion. An illustration of the same kind is given by what are known as Bologna phials. These are vessels of thickish glass about half the size of a common Florence oil-flask, with no apparent flaw or cause of weakness. Though they will stand the shaking of a small bullet within them, yet the shaking of a few particles of sharp sand hardly visible will cause the bottom to fly to pieces. It is the mere scratch ing of the skin that is fatal; and the explanation is easily found. The particles of the glass have a cohesive polarity which dictates a certain regularity in their arrangement, but which requires some time for its development. When the vessels are suddenly cooled, the surface molecules only can have had time to dispose themselves duly, while those within are kept by this properly formed skin in a highly constrained situation; and it is only so long as the surface film keeps sound that this constraint can be resisted. In the Rupert s drops it is plainly visible that the interior substance is cracked in every direction, and ready to fly to pieces.
time in a state approaching fluidity, and admitting of a uniform molecular arrangement throughout. Of course the thicker the glass the more careful must be the anneal ing and the longer the process; very thin glass requires little or no annealing, because it is, so to speak, all skin ; and glass thread requires no such treatment to make it
possess the exquisite flexibility of silk.The large sheets of plate glass, now used for shop windows require to be very carefully annealed before being ground and polished, otherwise they would be un- uble to stand the scratching of the surface. Imperfection in this respect is often revealed by the spreading in every direction of a crack once begun. These plates require as long as a fortnight to complete the annealing; and smaller articles from six to sixty hours, according to their thickness and weight.
In the manufacture and working of the various metals, annealing is frequently employed to reduce the brittle- ness usually resulting from a rapid or important change of molecular structure, such as is produced by hammering, long-continued vibration, rolling, and sudden cooling. The coppersmith who hammers a flat sheet of copper into a vessel of any description he pleases, must leave off ham mering at intervals, and anneal the piece of metal to pre vent it going to pieces under his blows. The extreme degree of tenuity to which gold is reduced by heating, is attainable only by very frequent and careful heating and slow cooling at each stage of the processes of forging, rolling, and beating.
Perhaps no better example can be given of the importance of annealing in metals that have to be subjected to frequent concussion than the art of die-forging. The matrix, from which the die-punches are impressed, is the product of many months labour, and the subject of tender care in its manufacture. Being a mass of steel of con siderable dimensions, it is extremely liable to fracture after the operation of hardening, so much so that it will not bear even sudden changes of atmospheric temperature. This brittleness is removed by putting it in water, gradu ally raised to the boiling-point, and then allowed slowly to cool. The matrix is used, not so often for the stamping of medals and coins directly, as for the manufacture of die-punches, which shall be used to produce any required number of dies exactly like the original matrix. For this purpose a conical plug of fine soft steel is compressed by powerful machinery against the matrix, but in receiving the impression its texture is condensed and hardened, so . that the annealing process has to be repeated after every few blows in the die-press, to prevent cracking of the sur face Coins, whether gold, silver, or copper, are usually impressed by a single blow, and in their case the blank metal is annealed before coining But in medals, where the design is in bolder relief, a number of blows, some times as many as thirty, is required; and in this case it is necessary to anneal after every third blow.
Wheels and axles of railway carriages, from the constant vibration to which they are subjected, become, in course of time, dangerously brittle; and they require to be re-worked and annealed anew to restore the requisite toughness to the material. So also in the processes of metal rolling and wire drawing, frequent annealing is necessary to allow the cohesive force to produce the most stable arrangement of the particles. The soft metals, such as lead, tin, and zinc, are annealed by immersion in water, which is made to boil and then to cool slowly. Sudden changes of temperature have the effect, almost invariably, of rendering metals brittle. And, as sudden cooling is required to give to steel its peculiar and invaluable hardness, so unfortunately it renders it at the same time brittle as unannealed glass, from the unnatural strain which the inner substance of the metal possesses. Annealing must, therefore, be employed to reduce this molecular strain, and to give the steel a workable flexibility. This is done by raising it to a red heat within a close vessel, and then allowing it gradually to cool. Heating in an open fire is very injudicious, because the carbon which enters the steel as an element combines with the oxygen of the air to the detriment of the quality of the steel. Usually steel goods are heated in an iron oven with charcoal powder, or placed in the heart of hot cinders, so as to exclude all air and oxy gen. Large castings, such as are used in the manufac ture of cannon, are kept covered with hot cinders and allowed to cool down sometimes over a space of about three months.
In proportion as this brittleness and hardness are reduced by annealing, the flexibility is increased, which allows of the employment of steel for an infinite variety of purposes. A bath of oil, which boils at about 600 Fahr., is often used for the annealing of steel tools, when a tem perature between the boiling-point of water and 500 or 600 Fahr. is required. Annealing is also employed to render cast-iron malleable or less brittle than it is naturally, so that a dozen of small articles such as buckles and knife- blades can be manufactiircd at less cost than a single article can be otherwise.
Annealing may be said to be the inverse process of tempering, which is the fixing of the molecular condition of steel by more or less sudden cooling from a particular temperature.
The real nature of the change which metals undergo by annealing is not thoroughly understood. Most of the malleable metals are susceptible of two distinct forms , one, called the crystalline form, which they assume by slow cooling; and the other, the fibrous, which is acquired by hammering or rolling. When this, however, is carried beyond a certain point, the metal becomes so hard that it is not capable of being bent far without breaking, and recourse must be had to annealing or slow cooling.
On the other hand, if the annealing be long continued the malleability diminishes, and themetal again has a crystal line fracture. Zinc by wire-drawing becomes very flexible, and possesses a degree of tenacity not inferior to that of copper; but, if it be kept in boiling water for a length of time, it will resume its original brittleness, and show a crystalline appearance when broken. Thus it appears that little can be said of the theory of the subject; little more-, indeed, can be attempted than the enumeration and classification of the facts.