Forging Colors

Source: Wikipedia

The practice of using colours to determine the temperature of a piece of (usually) ferrous metal comes from blacksmithing. Long before thermometers were widely available it was necessary to know what state the metal was in for heat treating it and the only way to do this was to heat it up to a colour which was known to be best for the work.

The colours which can be observed in steel

Black heat
(approximately 1200° F)
Not used for working iron. Black heat describes a temperature range from a an almost undetectable red heat (not bright enough to be visible except in a darkened place) to well below red-heat. For safety reasons all iron and tools not glowing red hot, should be treated as black hot until proven otherwise. New smiths should take note that any metal that is still cooling and no longer red hot is still dangerous. And all tools which come into contact with the hot metal are also heated and dangerous.
Dark Red heat
(approximately 1300° F)
For finish forging which means smoothing out dents and sharp corners. Do not forge alloy steels below this temperature. Mild steel and wrought iron may still be worked somewhat.
Red heat
(approximately 1500° F)
For final straightening of wrought iron and mild steel. Put back in fire at this heat.
Orange heat
(approximately 1800° F)
Forging of alloy steels is done at this color. Forging of mild steel and wrought iron at this temperature is beginning to become more difficult than at yellow heat.
Yellow heat
(approximately 1900° F)
For forging wrought iron and mild steel.
Light Yellow heat
(approximately 2000° F)
Best heat for forging wrought iron and mild steel. Try to obtain this heat before removing from the fire for forging. Sometimes this is described as a light welding heat.
White heat or welding heat
(appoximately 2200° F)
For welding mild steel and iron. Remove from fire when sparks begin to fly and stop forge welding when down to light yellow heat.
Never used. Sparkling or sizzling heat should be avoided. The sizzling heat pretty much describes the visual effect. Bright sparks similar to those from grinding steel are flying off the material and the effected area is making a sizzling noise. The sizzling is literally the steel burning. At this point the steel should be considered destroyed, and the effected area is of no further use. Cut off the effected area from the bar and throw it away. To try to save it or use it will end in failure of the piece because the grain structure is destroyed and the metal will crumble during forging and be brittle when cooled. Avoid sizzling heat.

If colors are a problem

It's not always practical to use color to determine temperature. Five to ten percent of the male population are color-blind; further, colors of hot steel are much harder to judge in the sun if you do your heat treating outdoors, which you should unless you have a ventilation hood and chimney in your shop. Tempering can be done in an oven with an accurate thermometer. For hardening temperature, there are several solutions:

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