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.
Dugo prije nego što su bili dostupni termometri, bilo je neophodno znati koja je temperatura metala zbog potrebne toplinske obrade i jedini način da se to učini bilo je da se zagrije na boju za koju je poznato da je najbolja za taj posao.