Tempering colors

Tempering colors

 

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 causeds colors to appear on the surface of the steel. As temperature is increased, the iron oxide layer grows in thckness, changing the color. These colors, called tempering colors, have been used for centuries to gauge the temperature of the metal. At around 350。F the steel willl start to take on a very light, yellowish hue. At 400。F, the steel will become a noticeable light-straw color, and at 440。F, the color will become dark-straw. At 500。F, steel will turn brown, while at 540。F, it will turn purple. At 590。F the steel turns a very deep blue, but at 640。F it becomes a rather light blue.