The tempering colors can be used to judge the final properties of the tempered steel. Very hard tool steel is often tempered in the light to dark straw range, whereas spring steel is ofter tempered t the blue. However, the final hardness of the tempered steel will vary, depending on the composition of the steel. The oxide film will also increase in the 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 film on the surface and the type of heat source used.