Charcoal, crushed to dust, is employed for bluing steel gun-barrels. Iron can be blued as well as steel. The barrels must be very highly polished, and previous to being immersed in the charcoal dust, which is made hot, must be rubbed with whiting to remove all grease; after removal from the charcoal they are dusted with whiting. When being blued, and as soon as the colour is deep enough, allow the barrels to cool, after which oil them thoroughly.