This may be confusion in terminology but I would like some clarification. Most common way I understand is to rust brown a damascus barrel. However some barrels are very dark and appear to be rust blued. Are both methods used on damascus barrels or is the resultant color a product of type of metal in the barrels.
The two are referred to as either brown and white, or black and white finishes. Typically, the brown finish is considered the standard for Brit and Euro guns, the black and white is considered original to American made guns. Technically, both should be referred to as browning, the black color comes from boiling, which turns the brown[red] rust from ferrous oxide to ferro ferro oxide, which is black.
Just a bit confused about the process. Is ferrous oxide boiled to create the fero fero oxide on its own and applied to the barrels afterwards? I assumed that soldered double barrels were not boiled and used a cold rust (blueing/browning) process.
The solution used can be the same for either color, although some solutions are considered best for one or the other methods. In both processes the solution is applied cold & allowed to rust. For the "Black" Browne the part with rust coat is subjected to boiling water or steam to do the conversion prior to carding. This not only converts the color but also makes carding easier as the excess rust is loosened leaving that which is to stay firmly attached. Boiling water does not hurt a solder joint, in fact the temperature of a hot salts bluing tank does not hurt the solder, the chemicals in the hot tank attack the solder, specifically the "Lye". It will also violently attack Aluminum.