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
A 19th century blued steel button
Acid solution applied to bare metal
After boiling rusted parts
After eight rust, carding and oiling sessions
Heat coloring of metals
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Heat coloring of metals is a relatively simple process that, in principle, produces interference colors on the surface of the metal. The colors depend on the temperature to which the metal is heated.
Bluing (steel)
…Electrochemical coloring of metals Heat coloring of metals Patina…
Scale of colors obtained by heat coloring of steel.This process is known as tempering when it is applied to the hardening of tools made of steel.
The various colors produced indicate the temperature the steel was heated to. Light straw indicates 204 °C (399 °F) and light blue indicates 337 °C (639 °F)
Blackened steel test plate, oiled with linseed oil and then heated to 300 - 400 C
Titanium test plate, colored by heat