Up until the mid 1930's the only type of blueing available was either rust blue or heat blue. Springfield Armory rust blued the bolts and barrels of their early 1903 rifles, but the blued small parts were niter blued which was another form of heat blue. Springfield was not set up to do the type of oven blueing done by Colt, but was set up for rust blueing.
Rust blue does not produce a bright blue like the heat blue, as the blueing mixture which contains nitric acid slightly etches the surface of the metal. While rust blueing is more labor intensive than the heat blue, it produces a very durable blue finish.
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