The
work should be held where it is reasonably free from cold air currents. If,
upon touching a pine stick to the piece being annealed, the wood does not
smoke, the work may then be cooled in water.
Better annealing is secured and harder metal may be annealed if the cooling
is extended over a number of hours by placing the work in a bed of
non-heat-conducting material, such as ashes, charred bone, asbestos fibre,
lime, sand or fire clay. It should be well covered with the heat retaining
material and allowed to remain until cool. Cooling may be accomplished by
allowing the fire in an oven or furnace to die down and go out, leaving the
work inside the oven with all openings closed. The greater the time taken
for gradual cooling from the red heat, the more perfect will be the results
of the annealing.
While steel is annealed by slow cooling, copper or brass is annealed by
bringing to a low red heat and quickly plunging into cold water.
_Hardening._--Steel is hardened by bringing to a proper temperature,
slowly and evenly as for annealing, and then cooling more or less quickly,
according to the grade of steel being handled. The degree of hardening is
determined by the kind of steel, the temperature from which the metal is
cooled and the temperature and nature of the bath into which it is plunged
for cooling.
Steel to be hardened is often heated in the fire until at some heat around
600 to 700 degrees is reached, then placed in a heating bath of molten
lead, heated mercury, fused cyanate of potassium, etc.
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