Annealing.— To avoid sudden fracture in glass, most careful attention is necessary in the annealing, or tempering. The operation must be commenced as soon as the fashioning of the form is completed. Glass expands with heat and contracts again upon cooling. Thick glass is more liable to sudden fracture than if blown thin and an uneven thickness is still more sus ceptible. Unequal contraction, due to the thicker parts retaining the heat longest, is the cause of fracture, and slow cooling to allow of gradual contraction the only safe remedy. Heated air is the most convenient and most generally used annealing medium, but hot water and heated sand have been successfully em ployed for the purpose. Occasionally all these mediums have been requisitioned in the perfect annealing of one article. 'A flint glass form judged to have been insufficiently annealed by the heated air means has been immersed in a bath of sand and water —of the same temper ature as the glass — raised to the boiling point and kept there for a few hours, then very grad ually cooled off. Glass shrinks very slightly upon cooling and the shrinkage is more per ceptible after slow than quick cooling. Glass of uneven thickness, either blown thick and thin, or made thick in places by parts applied to the surface, will not contract uniformly and requires a slow annealing. Annealing in oil increases toughness in glass. There are dif ferent forms of annealing ovens to meet the requirements of quick or slow cooling periods. Heavily-made glass— especially that intended for cutting—needs very careful and long an nealing and for this a kiln is generally used and is considered the safest. The whole lining of the kiln is of fire-brick. Fire boxes are built on either side of the one opening through which the glass is passed in and taken out. Several hours' firing is necessary to raise the tempera ture of the kiln to that of a little below the melting heat of the glass to be annealed. The glass should be of about the same temperature when it is first placed in the forepart of the kiln, between the two fire-boxes. As the floor space in the front becomes covered with the glass articles the individual pieces are taken up on the prongs of an iron fork, kept hot for the purpose and removed further back in the kiln and arranged so as to cover the whole floor space without the articles touching each other. When the kiln contains all it will safely hold — that is, without any of the arti cles being too near the fire-boxes—preparation is made to close up the kiln by means of iron doors which have inner linings of fireclay. The closing is gradual and depends upon the fire still in the boxes and the kind of articles in the forepart of the kiln. The interstices in the door are sealed with clay, so that no cold air can get into the kiln. When finally closed the doors are padlocked for safety. The closed period depends upon the class of goods within the kiln, but rarely less than four or more than six days is allowed.
The opening of the doors is as important as their closing, and, like the annealing, must be gradual. On removal the glass should be of the same temperature as the outside air to be quite safe.
Should any of the glass be insufficiently annealed— the sign of which is given by the heavier pieces cracking after being taken from the kiln—it may be replaced in the kiln be fore the next firing, with protecting screens of iron around it, and thus gradually heated and cooled again. The lear (or lehr) form
of heated air annealing is for the lighter kinds of glass and such of the heavier forms which do not require so long a cooling period. Cool ing in a lear may be effected in from six hours on, the period being determined and the glass placed in position, according to requirements, giving the heaviest pieces the hottest fire and the longest time.
The lear is in the form of a tunnel, with fire-boxes at the receiving end and a smoke chimney near the discharging end. - It may be straight or circular and usually has a °quick° and a °slow° side, or both sides may be oper ated quick or slow at will. In a straight lear the glass articles are placed upon iron pans— square or oblong—and as each pan— which has a sprinkling of sand upon it — is filled to capacity with glass it has another one linked to it and by means of a windlass and chain is drawn away from its place contiguous to the fire-box and makes room for the next pan and so on continuously to the end of the week's work. After emptying at the cool end, the pans are returned to the fire end again for refilling.
As with the kiln, the lear temperature is first raised to about that of the glass it is to receive. At the end of the making operations for the week the iron doors of the lear are closed tightly and padlocked to prevent any casual opening likely to admit cold air before the cooling is completed.
A lear is arched over for about 15 to 20 feet but the bed is extended well beyond the arch and into the discharging room. A screen of cloth at the discharging end of the arch pro tects the lear from currents of cold air. The discharging (or sorting) room is constructed as nearly airtight as possible, the entrance to it having two pairs of doors, one pair of which must be securely closed before the other is opened. Cold air currents prevent perfect annealing and the glass upon the pans is liable to crack. An insufficient heat does not prop erly anneal and an excess of heat is liable to melt the forms out of shape so that much skill and care is required in regulating the temperature. The circular lear is considered an improvement upon the straight one but it takes up considerably more space. The anneal ing principles are the same but instead of a series of pans linked up one at a time and drawn away from the fire in a straight line, the circular plan provides two continuous tables — no intersections—flanged at the sides and moving free from each other, for slow and quick annealing, upon a tramway principle. Cogwheels underneath turned from a windlass rotate the separate tables past the fire-boxes as required.
The mouth of the lear and the kiln should be near to the furnace where the glass is made and the glass should be as hot as possible— so that it is below melting heat—when placed inside.
A precautionary measure is sometimes taken when the place in the furnace at which the glass is made is some distance from the annoialing oven of keeping up the heat in transit by hold ing hot metal near the part of the glass most likely to be affected; usually where the iron pontil has been attached.