What is Annealing

 Annealing  (From the Sax. analan, to  kindle, to heat)

The name applied to the process by which glass or metal-castings are deprived of the brittleness which they exhibit when allowed to cool rapidly by exposure to the air from a state of red heat.

Its consists in placing the materials in a chamber, heated nearly to redness, where the are allowed to cool very slowly. 

All the particles of the glass or metal thus cool and contract to the same rate; on the other hand, by sudden exposure of the air the outer surface cools and contracts, whilst the inner portion remains dilated by the heat and the particles of the latter, when cooled in their turn, are unable to assume their natural positions.

Metals  used for many purposes, such as the wheels and axles of railway carriages, require to be very carefully annealed; as do also large sheets of plate glass.

These plates require as long as a fortnight to complete the annealing; and smaller articles require from six to sixty hours, according to their thickness & weight.