Plum coloring of metal will occur in two different scenarios during salt blueing process.
1. Salt blueing heat hardened steel.
2. Salt blueing to hot. Water content% has to be monitored all of the time during process. Temperature is directly dependent on water%, color directly depends on temperature.
There is a third condition as well but very rarely occurs and it will only happen if during process manufacturer run out of appropriate metal pickling solution and substitute must be used. Color outcome in that case is unpredictable since salt bath and neutralising bath is too late to adjust or too expensive or process disruptive.
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