This photo shows a simple forge used for heating high temperature salt for heat treating. The salts I use are suitable for carbon steels and Damascus and have an effective range from 1300F to 1900F. In the photo you can see a small stainless pipe protruding from the center of the lid. This pipe is 18" long and was filled with salt. The probe coming from the pipe is a K type thermocouple that is connected to the digital controller mention in the Sword Class notes. The controller opens and closes a solenoid valve connected to the gas line. I have a switch that allows me to change from the long sword heat treating forge to the salt forge using the same controller and solenoid for both appliances.

The forge itself is made from a 14" diameter pipe and constructed exactly like the blade forging forge mentioned in forge contruction page. Materials and parts list is detailed on that page. You will notice a stack made from channel iron on top of the forge. This covers a vent that has been cut in the cover and takes the heat and flue gases away from your face.

In operation, the set point is programmed into the digital controller and the forge is lit. The burner will fire until it approaches the set point, then the controller will begin to open and close the solenoid valve turning the burner on and off. The burner stays lit because the interior of the forge is above 1500F. On the long forge I have installed a sparkplug ignition and could use the same arrangement here, but it isn't necessary. The controller will ramp up to the set point and then hover at temp. In the salt it holds within +/- 5 degrees.

There are many advantages to using salt for hardening. First, it is a liquid and doesn't the hot spots you get from air furnaces, the blades come up to an even heat and quickly. Also it is a non oxidizing environment and the blades come out scale free. I first heard about salt for heat treating from Henry Frank. Henry makes wonderfully engraved and inlaid folders. All the work is done on the blade prior to heat treating, they come out virtually the way they went it.

There are some precautions you need to be aware of with salt. First, it is a liquid that is hot and any water will immediately boil and can splatter the salt. I wear a full face shield, gauntleted gloves and body apron as a precaution. You should inspect the pipe containing the salt prior to firing to make sure there are no apparent leaks or bulges in the pipe. My pipe is welded stainless steel with a wall thickness of 3/8ths. While it is firing I keep a cover on top of the pipe in case it decides to spit. Other than that it is a joy to work with and it gives me the control required to get the most from the steels.

Tip from Vance Burns:

Salt Rectification

Neutral salts used for austenitizing steel become contaminated with soluble
oxides and dissolved metals during use. As the buildup of these oxides and
dissolved metals renders the bath oxidizing and decarburizing toward steel,
it is necessary to periodically rectify the bath. In the case of salt bath
furnaces with immersed electrodes, daily rectification of the bath is
required. For the recommended barium chloride-based salts (Salt# 1,2,3 in
Table II) rectification should be done by adding 125 gm of boric acid (for
each 100 kg of salt) and inserting a 3-inch graphite rod for one hour for
every 4 hours of operation.

Ref article at Industrial Heating