Bluing (steel)
Bluing, sometimes spelled as blueing, is a passivation process in which steel is partially protected against rust using a black oxide coating.
Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action.
"Cold" bluing is generally a selenium dioxide-based compound that colours steel black, or more often a very dark grey.
It is a difficult product to apply evenly, offers minimal protection and is generally best used for small fast repair jobs and touch-ups.
Many double-barreled shotguns are soft soldered (lead) or silver brazed together and many of the parts are attached by that method also.
The higher temperatures of the other processes as well as their caustic nature could weaken the soldered joints and make the gun hazardous to use.
[4] Bluing can also be done in a furnace, for example for a sword or other item traditionally made by a blacksmith or specialist such as a weapon-smith.
Blacksmith products to this day may occasionally be found made from blued steel by traditional craftsmen in cultures and segments of society who use that technology either by necessity or choice.
[5] Bluing may be applied by immersing steel parts in a solution of potassium nitrate, sodium hydroxide, and water heated to the boiling point, 275–310 °F (135–154 °C) depending on the recipe.
[8] Modern home hobbyist versions of this process typically use a hydrogen peroxide and salt solution, sometimes preceded with a vinegar soak, for the rusting step to avoid the need for more dangerous acids.
The mixed fumes of the acids produce a uniform rust on the surface of the parts (inside and out) in about 12 hours.
These processes were later abandoned by major firearm manufacturers as it often took parts days to finish completely, and was very labor-intensive.
They are still sometimes used by gunsmiths to obtain an authentic finish for a period gun of the time that rust bluing was in vogue, analogous to the use of browning on earlier representative firearm replicas.
Often it does provide an adequate cosmetic touch-up of a gun's finish when applied and additionally oiled on a regular basis.
The cross section and size of parts affect the outcome of the finish and time it takes to achieve.
If cold bluing is the only practical option, the area should be kept oiled to extend the life of the coating as much as possible.
This is often achieved without chemicals by simply heating the steel until a blue oxide film appears.
Bluing is also used in seasoning carbon steel cookware, to render it relatively rust-proof and non-stick.
A non-linear resistance property of the blued steel of razor blades, foreshadowing the same property later discovered in semiconductor diode junctions, along with the ready availability of blued steel razor blades, led to the use of razor blades as a detector in crystal set AM radios that were built by servicemen (as foxhole radios) or by prisoners of war during World War II.
