The so called charcoal bluing on the clones is really Nitre or fire blue that is done with molten potassium nitrate. It is a fragile finish that looks good, but isn't charcoal bluing as Colt did it. Charcoal blue is a process where parts are put in a fire made with high carbon charcoal, periodically remoeved and rubbed with oil/lime on rags and repeated until the right color is reached. Carbonia blue is the same thing except it is done with a gas furnace and Carbonia oil. Hot salt blue is modern blue and looks more black than blue, but seems to be the most durable.