The process of changing the colour of polished steel by heating to give a protective film of oxidation which also has a pleasing color, brown, purple and blue. The bluing of clock components is normally carried out in a flat thick-bottomed copper bluing pan, the parts to be blued being first polished and cleaned. Care must be taken to heat the parts evenly, keeping them free from cold draughts. Heating is stopped immediately the desired colour is obtained. Parts which will not lie flat on the bottom of the pan are blued on a deep bed of brass filings placed in the pan.