Editing Heat treating (section)

====Tempering colors====
[[File:Tempering standards used in blacksmithing.JPG|thumb|Tempering colors of steel|300x300px]]
Steel that has been freshly ground or polished will form [[oxide]] layers when heated. At a very specific temperature, the [[iron oxide]] will form a layer with a very specific thickness, causing [[thin-film interference]]. This causes colors to appear on the surface of the steel. As the temperature is increased, the iron oxide layer grows in thickness, changing the color.<ref>''Light, its interaction with art and antiquities'' By Thomas B. Brill - Plenum Publishing 1980 Page 55</ref> These colors, called tempering colors, have been used for centuries to gauge the temperature of the metal.<ref name="New Edge of the Anvil,98-99">{{Cite book
  |title=New Edge of the Anvil: a resource book for the blacksmith
  |first=Jack  |last=Andrews 
  |year=1994 
  |pages=98–99
}}</ref>

* 350˚F (176˚C), light yellowish
* 400˚F (204˚C), light-straw
* 440˚F (226˚C), dark-straw
* 500˚F (260˚C), brown
* 540˚F (282˚C), purple
* 590˚F (310˚C), deep blue
* 640˚F (337˚C), light blue<ref name="New Edge of the Anvil,98-99"/>

The tempering colors can be used to judge the final properties of the tempered steel. Very hard tools are often tempered in the light to the dark straw range, whereas springs are often tempered to the blue. However, the final hardness of the tempered steel will vary, depending on the composition of the steel. Higher-carbon [[tool steel]] will remain much harder after tempering than [[spring steel]] (of slightly less carbon) when tempered at the same temperature. The oxide film will also increase in thickness over time. Therefore, steel that has been held at 400˚F for a very long time may turn brown or purple, even though the temperature never exceeded that needed to produce a light straw color. Other factors affecting the final outcome are oil films on the surface and the type of heat source used.<ref name="New Edge of the Anvil,98-99" />