Some DIY chemistry talk

http://www.blackworksusa.com/thedetails.html
First, black oxide and bluing is the same thing, they are used interchangeably depending upon who is saying it. People in the firearms industry referrer to it as Bluing, mainly because most salts are chemically mixed to provide a slight “Blue” color to the steel rather than a blacker looking color. When it comes to industrial applications, it is usually referred to as “Black Oxide” or blackening. Same thing, same process, just called different names.

… The black oxide solution, regardless of powder or liquid form contains three main ingredients, Sodium Hydroxide (NaOH) (Lye, basically your household drain cleaner as in Drano®), Sodium Nitrate, (NaNO3) and Sodium Nitrite (NaNO2). Along with this chemical mixture, there is also trace amounts of proprietary additives which give you the different colors ranging from light blue to super black.

The combination of chemicals mentioned above are then mixed with water (H2O), which is what is then referred to as a “Salt Bath”. The salt bath is typically heated between 285-310 degrees Fahrenheit. At this temperature and chemical mixture, the surface of the iron metal submerged in the bath is converted into magnetite (Fe3O4). It leaves dark or black like cosmetic appearance and a corrosion resistance finish.

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