Annealing, by artificers called Heating, is a part of the process of making, or finishing, glass ; and consists in placing bottles and other vessels, while hot, in a kind of oven or furnace, where they are suffered gradually to cool.
The difference between unan-nealed, and annealed glass, is very remarkable. When a glass vessel that has not undergone this process, 5s broken, it often flies into a small powder, with a violence apparently disproportionate to the stroke which it received. In general, it 5s in greater danger of being broken from a very slight blow, than from a more considerable one. Such vessel 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 break it into small fragments. This sometimes takes place immediately on dropping the sand into it, but the vessel will frequently remain apparently sound, for several minutes after; when, without the least touch, it will suddenly fly to pieces. If the glass be very thin, this effect, does not take place ; and, on the contrary, it seems to possess all the properties of such as are annealed.
Glass is one of those bodies which increase in bulk, on 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; but, when a mass of melted glass is suddenlv exposed to a cold temperature, the surface crystallizes, and forms a firm shell round the interior fluid parts, by which they become solid, and are prevented from expanding.
By the process of annealing, the glass is preserved 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 fibres have an opportunity of expanding and forming a regular crystallization.
A similar process is now used for rendering kettles, and other vessels of cast iron, less brittle; which admits of the same explanation as that above stated. The greater number of metals diminish in bulk when they pass from a fluid to a solid state. Iron, on the contrary, expands.