Case-hardening
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Case-hardening or carburization is the process of introducing carbon to the surface of a low-carbon iron, or more commonly a low-carbon steel object, to harden the surface.
Colt Peacemaker, showing discoloration from case-hardening
A flame-hardened sprocket. The discoloration around the teeth delineates the area that was rapidly heated and then quenched.
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Bluing (steel)
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Bluing, sometimes spelled as blueing, is a passivation process in which steel is partially protected against rust using a black oxide coating. It is named after the blue-black appearance of the result
Case-hardening
…pattern of black, blue, and purple caused by the various compounds formed from impurities in the bone and charcoal. This oxide surface works similarly to bluing, providing a degree of corrosion resistance, as well as an attractive finish. Case colouring refers to this pattern and is commonly encountered as a…
A 19th century blued steel button
Acid solution applied to bare metal
After boiling rusted parts
After eight rust, carding and oiling sessions