ANNEALING, by the workmen called nealing, is a process used in glass-making, and in the manufacture of certain metals. In glass-making it consists in placing the bottles, &c. whilst hot, in a kind of oven or furnace, where they are suffered to cool gradually. They would otherwise be too brittle for use. The difference between unannealed and annealed glass, with respect to brittleness, is very remarkable. When an unannealed glass vessel is broken, it often flies into a small powder, with a violence seemingly very unproportioned to the stroke it has received. In general it is in greater danger of breaking from a very slight stroke than from one of some considerable force. One of these vessels will often resist the effects of a pistol bullet dropt into it from the height of two or three feet; yet a grain of sand falling into it will make it burst into small fragments. This takes place sometimes immediately on dropping the sand into it; but often the vessel will stand for several minutes after, seemingly secure; and then, without any new injury, it will fly to pieces. If the vessel be very thin, it does not break in this manner, but seems to possess all the properties of annealed glass.

The same phenomena are still more strikingly seen in glass drops or tears. They are globular at one end, and taper to a small tail at the other. They are the drops which fall from the melted mass of glass on the rods on which the bottles are made. They drop into the tubs of water which are used in the work; the greater part of them burst immediately in the water. When those that remain entire are examined, they discover all the properties of unannealed glass in the highest degree. They will bear a smart stroke on the thick end without breaking; but if the small tail be broken, they burst into small powder with a loud explosion. They appear to burst with more violence, and the powder is smaller, in an exhausted receiver, than in the open air. When they are annealed they lose these properties.

Glass is one of those bodies which increase in bulk when passing from a fluid to a solid state. When it is allowed to crystallize regularly, the particles are so arranged that it has a fibrous texture. It is elastic, and susceptible of long-continued vibrations; but when a mass of melted glass is suddenly exposed to the cold, the surface crystallizes, and forms a solid shell round the interior fluid parts. This prevents them from expanding when they become solid. They therefore have not the opportunity of a regular crystallization, but are compressed together with little mutual cohesion. On the contrary, they press outward to occupy more space, but are prevented by the external crust. In consequence of the effort of expansion in the internal parts, the greater number of glass drops burst in cooling; and those which remain entire are not regularly crystallized. A smart stroke upon them communicates a vibration to the whole mass, which is nearly synchronous in every part; and therefore the effort of expansion has little more effect than if the body were at rest; but the small tail and the surface only are regularly crystallized. If the tail be broken, this communicates a vibration along the crystallized surface, without reaching the internal parts. By this they are allowed some expansion; and overcoming the cohesion of the thin outer shell, they burst it, and are dispersed in powder.

In an unannealed glass vessel the same thing takes place. Sometimes the vibration may continue for a considerable time before the internal parts overcome the resistance. If the vessel be very thin, the regular crystallization extends through the whole thickness; or at least the quantity of compressed matter in the middle is so inconsiderable as to be incapable of bursting the external plate.

By the process of annealing the glass is kept for some time in a state approaching to fluidity; the heat increases the bulk of the crystallized part, and renders it so soft,, that the internal parts have the opportunity of expanding! and forming a regular crystallization.

In the manufactures in which the malleable metals are employed, annealing is used to soften a metal after it has been rendered hard by the hammer; and also to soften cast-iron, which is rendered very hard and brittle by r a . pid cooling.

In the manufacture of steel goods, which are firstform. ed by the hammer, and require to be filed or otherwise treated, and in which softness and flexibility are essential to the change, annealing is absolutely necessary. This is particularly the case in making files and scissors, that the metal may be left sufficiently soft for cutting’the teeth, and for filing off those parts which cannot be ground. Annealing is not less necessary in the drawing of wire, whether iron, copper, brass, silver, or gold. The operation of drawing soon gives the wire a degree of hardness and elasticity which, if not removed from time to time by annealing, would prevent the extension of the wire, and render it extremely brittle. The same operation is also necessary in rolling or flatting those metals which are in a cold state, such as brass, silver, gold, &c. The brazier who forms vessels of copper and brass by the hammer, can work upon it only for a little time before he is obliged to anneal it. ∙

The common methods employed for annealing iron and steel are very injudicious, and materially injure the latter when it is used for making cutting instruments. After they have been formed by the hammer, they are generally piled up in an open fire, slowly raised to red heat, and then allowed as gradually to cool. By this method the surface of the steel will be found considerably scaled, from the action of the oxygen of the atmosphere. When it is remembered that steel consists of iron joined to carbon, it will be evident that the steel immediately under the sealyoxide will be deprived of its carbon, which has been carried off by the attraction of the oxygen; and, in consequence, will lose the property of acquiring that degree of hardness necessary to a cutting instrument.

Nothing, therefore, can be more obvious, than that steel particularly should be annealed in close vessels, to prevent that effect. For this purpose the goods should be placed in a trough or recess made of fire-stone or fire-brick, and stratified with ashes or clean sand, and finally covered with a thick stratum of the same; but if the size of the vessel be small, it may have a cover of its own materials. This oven or trough must now be heated by the flame of a furnace passing under and round it, till the whole is of a red heat. It must then be suffered to cool, without letting in the air. The goods so treated will be much softer than by the common method. The surface, instead of becoming scaled, will have acquired a metallic whiteness, from the presence of a small quantity of carbonaceous matter contained in the ashes in which they were imbedded. They will become so flexible also, as to allow them to bend considerably without breaking, which is very far from being the case before the operation. The fracture, before annealing, will be smooth and short; but afterwards it will be rough, exhibiting bright parts, of a crystalline appearance. Wire, especially that of iron and steel, should be treated in a similar way when it is annealed. Th e wire used for some purposes requires to be soft, and is sold in that state. If the wire, after finishing, when it 18 bright and clean, were to be annealed in contact with oxygen, it would not only lose all its lustre and smoothness, but much of its tenacity. The process above mentioned will therefore be particularly necessary in annealing fin 18 * 1 ' ed wire, as well as in softening it from time to time during the drawing.
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Copper and brass suffer much less than iron and steel from annealing in the open air, and do not require to be heated above a low red heat. If, however, the lustre is to be preserved, a close vessel would be desirable. The latter metals, after annealing, although much discoloured by the oxygen of the atmosphere, may be cleansed by immersion in a hot liquor composed of water and a small quantity of sulphuric or nitric acid. Very small brass or copper wire is frequently annealed by exposing it to the flame of hay or straw. In casting minute pieces of pig-iron, which is generally done in wet sand, the metal possesses the property of steel to such a degree as to assume, by the rapid cooling, a degree of hardness equal to hardened steel; at the same time that the articles are so brittle as to break by falling on the ground. When, however, these goods are treated in the way above directed, they acquire a degree of softness which renders them penetrable by the file, and at the same time capable of bending. In this state they are much less tenacious than steel, but still so much so as to have been sold in the form of cutlery for steel.

The change which metals undergo by annealing is not yet 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. All the malleable metals in the ingot or in their cast state are brittle, and exhibit a crystalline fracture. By hammering or rolling they become more tenacious, and break with difficulty, exhibiting what is called a fibrous fracture. At the same time they become stiffer and more elastic. They lose the latter properties by annealing, but become more malleable. If the annealing, however, be long continued, the malleability diminishes, and they again have a crystalline 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. This proves that the particles of metals can change their arrangement without losing their solid form; which is still more strongly confirmed by the fact, that brass wire loses its tenacity by exposure to the fumes of acids, and even by the presence of a damp atmosphere. This is not caused by the moisturc, but by the action of air upon the moistened surface. The manufacturers of common pins are obliged to keep their wire in a dry atmosphere, or immersed in water. If the wire be first moistened, and then exposed to the air, it will assume the brittle state much sooner. In this condition it breaks with a crystalline fracture, similar to that exhibited by an ingot. When a steel plate, such as a watch-spring, has been once tempered, the operation of simply rubbing it bright will render it soft and elastic. The same change is brought about by slightly hammering it. It, however, resumes its elastic state by being carefully heated till it becomes of a blue colour. If the heat be continued to redness, particularly in a close vessel, it becomes perfectly annealed.