A/f/TEAL'ING is the process of tempering resorted to in the manufacture of glass and the preparation of several of the metals, whereby these substances acquire a hardness combined with tenacity which renders them much stronger, and consequently more durable. In the making of fliass vessels by the glass-blower, they are of course quickly reduced in temperature whilst the fused glass is being modeled into the desired shape. The atoms of the glass thus rapidly compelled to assume a position, do not seem to be properly and firmly arranged together, and the vessel is very linable to be broken, either by a slight but smart blow, or a sudden increase or decrease in temperature. This brittle ness is very observable in the lacryincs vitrece, or glass tears, known as Prince Rupert's drops, obtained by allowing molten glass to fall into water, when the glass forms pear shaped drops, which arc so brittle that if they be scratched with a file or the end be broken off, the whole bursts asunder and falls down into a fine powder of glass. The same brittleness is exhibited in Bologna jars, or vials, which are small and very thick in the glass; and yet, if a minute angular fragment of any hard substance be dropped into the jar, the latter flies to pieces.
In the A. of glass vessels, they are arranged in iron trays, and placed in a long oven, where they are gradually raised in temperature to near their fusing-point, by the trays being drawn along to the hottest part of the oven; and thereafter, the trays, with their contents, are very slowly drawn into a cooler and cooler part, till they become cold.
The A. operation generally takes 12 hours for small articles, such as wine-glasses; but days, and even a week or two, are required to anneal completely large vessels. Many articles of glass, such as tubes for steam-gauges, lamp-glasses, etc., are annealed by being immersed in cold water, which is very gradually raised to its boiling-point, and thereafter cooled.
The metals are often subjected to the process of A. When medals are repeatedly struck by the die-stamper, the gold or other metal, by the concussion, becomes brittle, and re quires to be now and again heated and annealed. In wire-drawing, also, the metal be comes so hard and brittle that it requires A. to prevent its breaking into short lengths. Boiler-plates, which have been drawn out by rolling, require to be annealed before they are riveted together. The brazier, in hammering out copper and brass vessels, must stop now and again, and anneal the metal. Articles of tin, lead, and zinc can be an nealed in boffin°. water. The tempering of steel is just a process of A. The steel is placed in an oil-bath, or surrounded by a metallic mixture which has a low fusing-point; and according to the temperature to which it is subjected, a steel with various degrees of softness and strength is obtained. Parke's table of metallic mixtures capable of being used in the tempering or A. of instruments made from steel, is as follows: