Tarnish

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Locally heated steel belt
Annealing colors for
unalloyed tool steel
colour temperature
White yellow 200 ° C
Straw yellow 220 ° C
Golden yellow 230 ° C
Yellow brown 240 ° C
Brownish red 250 ° C
red 260 ° C
Purple 270 ° C
violet 280 ° C
Dark blue 290 ° C
Cornflower blue 300 ° C
Light Blue 320 ° C
Blue-gray 340 ° C
Gray 360 ° C
Bismuth crystals grown from the melt at approx. 300 ° C. Only then do the typical tarnish colors arise from oxidation

Tarnish colors , also temper colors , are superficial, iridescent bright colors of a fabric that are caused by interference on thin layers. They are mainly found on metals, but also on minerals. This interference is very similar to that found in oil stains on puddles or in the lamellae of soap bubbles .

Origin and examples

On metals, the annealing colors usually arise from an oxidation of the surface. The thickness of the oxide layer is determined by the depth to which the oxygen atoms can diffuse . This depth is strongly dependent on the temperature. This makes it possible to determine the temperature to which a metal was exposed, for example during welding or tempering . This is important because important material properties such as hardness and toughness also depend on the temperature.

In the case of steel , for example, when heated to 200 ° C, pale yellow tempering colors are found, at 300 ° C cornflower blue and at 500 ° C gray (too thick for a layer). This can be used to color stainless steels . A black coating of silver sulfide, which often shimmers in bright colors, also forms on silver in air (due to hydrogen sulfide ). In the case of minerals, tarnishing occurs due to weathering. An example of this is the colored copper gravel, which is colored by tarnishing .

In the case of titanium , a targeted coloring is achieved in a similar way, for example for jewelry, by specifically creating an oxide layer by means of anodizing . At 10-25 nm the result is a gold color, at 25-40 nm purple, at 40-50 nm dark blue, at 50-80 nm light blue, at 80-120 nm yellow, at 120-150 nm orange, at 150-180 nm purple and at 180-210 nm green.

See also

Web links

Individual evidence

  1. ^ Ulrich Fischer: Metal table book . 41st edition. Verlag Europa-Lehrmittel Nourney, Vollmer, 2001, ISBN 3-8085-1721-2 , p. 128B.
  2. ↑ euro-inox.org ( Memento from February 23, 2014 in the Internet Archive ) (PDF)
  3. ↑ Information on the coloring of titanium on tiananodisiert.eu