The tempering colors are used for judgment regarding tempered steels` final properties. Steel that is very hard is normally tempered within the light-to-dark straw range. Spring steels are often tempered towards blue color. There is variance regarding the final-hardness of tempered steel, which depends upon composition of steel. Thickness of oxide film also increases over time. Due to this fact, steel held longer at 400F turns purple or brown. This happens even when the temperature remains lower than that required for producing light straw color. Final outcome is affected by other factors are heat source type and oil films used on surface.