This section is from the book "The Tinman's Manual And Builder's And Mechanic's Handbook", by Isaac Ridler Butt. Also available from Amazon: The Tinman's Manual And Builder's And Mechanic's Handbook.
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.
A very pale straw -
Darker straw -
All kinds of wood tools Screw taps.
Hatchets, Chipping Chisels,
Slightly tinged purple
All kinds of percussive tools.
Dark purple -
Soft for saws.
To Temper by the Thermometer.
Put the articles to be tempered into a vessel containing a 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to the hardness required, by means of a suitable thermometer, till heated equally throughout; the articles are then withdrawn and plunged into cold water.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to smoke at 430o or straw color, and that it takes fire on a light being presented, and goes out when the light is withdrawn, at 570o or blue.