Burnished steel components, screws particularly, are used in watchmaking industry for their higer resi stance to corrosion, beyond being far more elegant in their periwinkle blue.
How is this colour obtained? Material is heated, generating a thin oxide layer. The highest is the temperature, the thicker is the oxide layer. The thickness of the layer produces the colour of the steel surface.
How is it possible to obtain the periwinkle blue, distinguish sign of fine watchmaking? It is necessary to heat the metal at a temperature of 300 °C (572°F). Maximum precision is required, since at 290 °C a dark blue is obtained, while at 320 °C a pale blue. And for lower temperatures, you get to violet, purple, until light yellow; for higher temperatures, to grey, brown and red.