Blackened gauntlet from Bargello
Blackening steel can be achieved a number of ways. Blacking or bluing is done primarily for two reasons. The first is the appearance, blackening produces a unique appearance that is not producible in any other method. Second is blackening inhibits rust. The color is caused by an oxide layer which acts as a barrier protecting the underlying steel.
Cold Blueing[]
Achieved using chemicals - often selenic acid based- cold bluing and russeting can be achieved without the application of heat and are generally inexpensive. Cold blues generally create a very thin layer that easily scratches off. If the acid or other catalysing compound is not completely removed the armour is likely to quickly rust.
Hot Blacking[]
This is generally achieved with oil and a fire or torch. Many people have reported good results using linseed oil. Oil blackening generally produces a thicker more resistant layer to the piece than cold blues and will usually work as a rust inhibiter.