The ways of many lathe beds were listed as being Flame Hardened. These were normally made of cast iron & used the carbon in the cast iron for hardening. This was often done by a flame traveling down the ways, followed by a stream of quenching water.
I do not recall having ever seen a reference to a gun frame being so hardened. What is "Frowned" upon here is the process of using the tip of an acetylene torch to Spot Temper Color a frame. Instead of imparting hardness this actually draws the existing hardness & in an irregular fashion to boot & is very rightly frowned upon.
Cyanide hardening is done by submerging the part in molten Cyanide which is above the critical temperature of the steel being treated. Carbon is absorbed into the surface of the steel just as in pack hardening, though generally not as deep. The part is then quenched which imparts the hard case. Cyanide hardening generally has more of a "Striped" appearance than "Color Case Hardening" done by the pack method & "In My Opinion" is not near as attractive.
Cyanide hardening is quicker & less expensive than pack hardening & was used on a lot of lower priced guns, such as single barrels & hardware store doubles etc. It was not used by most of the makers of higher quality guns until cost cutting methods came into effect in their later years when they were for the most part just trying to stay in business.
There is a method of coloring gun parts, including frames, which for want of a better term I will call "Faux Case Coloring". This process was described by Ithaca back when the were importing the Perazzi. The Perazzi was made of a heat treated alloy steel & not suitable for Case Hardening. As described by Ithaca the part was heated to around 200°F or just a bit below the boiling point of water. A Cold Blue solution was then squiggled on with a Q-tip to the desired pattern. With a bit of practise one can do a rather good looking job similar to Cyanide colors. I would not of course recommend it on anything other than an inexpensive gun. I once did a Stevens 94 for a friend this way which had belonged to his father but had been neglected prior to his obtaining it. He just wanted to pretty it back up & hang it on the wall. He was extremely pleased with the way it turned out.
Due to the low temp this method does nothing to detract from the original hardness of the part. Hot bluing is done at near 300°F & even in rust bluing, either slow or express, the part is subjected to water at a full boil.
