Oil is used to temper by burning it off because it burns at very close to 600 degrees, the spring temper temperature. This is an archaic method of drawing the temper. the resulting blue is also quite beautiful and very close to the temper, nitre or heat blue shown in the above photos.

This temper bluing was Not a method used by Smith & Wesson. It is a method I use on a regular basic for screws and other small parts.