The article after being completed, is hardened by being heated gradually to a bright red, and then plunged into cold water; it is then tempered by being warmed gradually and equably, either over a fire, or on a piece of heated metal till of the color corresponding to the purpose for which it is required, as per table below, when it is again plunged into water.

Corresponding Temperature

A very pale straw -

430°

Lancets

Straw - - -

450°

Razors

Darker straw - -

470°

Penknives

All kinds of wood tools Screw taps.

Yellow - -

490°

Scissors

Brown yellow - -

500°

Hatchets, Chipping Chisels,

Saws.

Slightly tinged purple

520°

Purple - -

530°

All kinds of percussive tools.

Dark purple -

550°

Springs.

Blue - -

670°

Dark blue - -

600°

Soft for saws.

To Temper By The Thermometer

Put the articles to be tempered into a vessel cantaining sufficient quantity to cover them, of Oil or Tallow; Sand; or a mixture of 8 parts bismuth, 5 of lead, and 3 of tin, the whole to be brought up to, and kept up at the heat corresponding to the hardness required, by means of a suitable thermometer, till heated equally throughout; the articles are then withdrawn and plunged into cold water.

If no thermometer is available, it may be observed that oil or tallow begins to smoke at 430° or straw color, and that it takes fire on a light being presented, and goes out when the light is withdrawn, at 570° or blue.