Before the use of instruments such as
pyrometers, colors were used to judge
temperatures when hardening and tempering.
For example, on carbon tool steel where the
tempering range may typically be from 200 deg C
to 350 deg C, the colors change with the rise in
temperature giving Light Straw at around
210 deg C, Purple at 275 deg C, and Grey at 330 deg C.
The practice still continues in workshops
where controlled heat treatment facilities are