Smith used the "Carbonia Bluing" process. It used and oil and pine tar-based coating along with a heat treat. Smith produced the deep black blue and a fine gloss. Colt used a similar process on more matte finished guns producing a duller blue.
The Smith blue was very durable, far more so than a charcoal heat blue. My 1917 S&W Still has a great blue although there is holster wear.
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