When steel is heated in an oxygen rich environment, an oxidized film is formed on the surface. The color of this oxidized film will change color depending on the temperature of the steel and this can be used to determine the approximate temperature. This is used to determine the correct temper of the steel, but isn't always accurate as the composition of the steel can affect the oxidized film color.
| Degrees | Color of Steel | |
|---|---|---|
| Fahrenheit | Celsius | |
| 430 | 221.1 | Very pale yellow |
| 440 | 226.7 | Light yellow |
| 450 | 232.2 | Pale straw-yellow |
| 460 | 237.8 | Straw-yellow |
| 470 | 243.3 | Deep straw-yellow |
| 480 | 248.9 | Dark yellow |
| 490 | 254.4 | Yellow-brown |
| 500 | 260 | Brown-yellow |
| 510 | 265.6 | Spotted red-brown |
| 520 | 271.1 | Brown-purple |
| 530 | 276.7 | Light purple |
| 540 | 282.2 | Full purple |
| 550 | 287.8 | Dark purple |
| 560 | 293.3 | Full blue |
| 570 | 298.9 | Dark blue |
| 640 | 337.8 | Light blue |