Construction Tips - High-Temp Salt Pots

The design criteria for a high temp pot are:

(1) something that can take temperatures of 1600 -1800 F without problems;
(2) something that can generate the necessary heat but in a very controlled fashion; and
(3) something with a decent volume and a long enough length to handle your longest blades.

Because of the danger involved, safety must be the first consideration. During the design phase for my pot, I tried to anticipate everything that could go wrong and build in counters to those problems. I'm not claiming that I was totally successful, but I'm still around.

The pot is really a union of a dedicated gas forge and a stainless steel pipe. The forge is made of a 24" tall x 12" diameter well pipe, welded to a 1/4" plate base on casters. At the top of the pipe is a side vent and a clamshell closure - more on that later. The refractory is 3000 F type K bricks - cut to allow flumes to exit out the side vent. The bottom is lined with castable refractory. The burner is a standard venturi / atmospheric burner using a 0.030" orifice (for 1500 F) running at 2..3 psi of LP. I found that a 0.040" (#60) orifice ran a bit hot at 1600F.
The tube is a 4" diameter x 24" long piece of stainless pipe with 1/4" wall thickness. The bottom is a 1/2" SS plate welded (with SS rods) to the base. Several sections of pipe were cut, split and welded to the top to form a 1/2" wide 'shelf' at the top and a set of SS rings (more pipe sections) were welded to the thickened opening. While I did the best welding I could, I would recommend getting a TIG or MIG weld for the end cap - mine leaks just a little bit. Other folks use black pipe and electric heating elements - but the combination of leaks, corrosion, and blown elements persuaded me to go the SS-gas route.

The reason for the rings and thickened opening was to provide someway to lift the tube and someway to suspend the tube in the forge without actually sitting it on the bottom. The thought of the tube spilling was not acceptable.

 

At the top of the forge is a set of hinged clamshell closures. The bricks lining the forge stop 1" shy of the top and the gap is filled with sections of bricks. In the pictures to the right, you can see the system in place and also see the component pieces. Below you can see the tube in place. The tube hangs on the clamshells and is suspended in the body of the forge.

 

The forge is lit (drop a hunk of burning newspaper in and open the gas valve), the tube is slipped in, the bricks are placed around the tube, the clamshells are swung shut, and the tube is lowered the last inch to rest on the clamshells. Gas pressure is set to 8 psi and in about an hour, the tube is filled with molten salt. The pressure is dropped to 5..6 psi and the meter (from Omega with a type K probe) is used to monitor the temperature. When it reaches 1525 F, it's time to get to work. When the last blade is done, the gas is shut off and after a little time (dull red salt, still fluid), a tapered steel rod is inserted into the salt to create (when the taper is removed) a hole for expansion when the tube is next heated.

In the picture to the right, you can see the cavity after the taper was removed. Removing the taper may require putting a stout bar through the lifting rings, suspending the tube from some convenient bench, and bashing the rim of the tube with a sledge hammer. If it's inserted when the salt is just hardening, it may just slide out. Some folks don't use the taper - call me paranoid but if there is a volume change on melting, I would prefer not being near a molten salt bomb.

While corrosion is not as severe as with the low-temp salts, it it still a problem. The taper MUST not -- repeat MUST not -- be rusty. Water expands ~1500 times its volume when it goes from liquid to gas. That can throw a lot of molten salt and rust contains water. After I get the taper out, it is wire brushed, waxed and stored in an oil-soaked pants leg (those old jeans are good for something). If there is a hint of rust, it gets removed before using.  The tube is also stored in an oiled pants legs (right or left at your option). All of the pieces are stored in a large ammo container (Surplus Center - Rocket box). 

The salt is basically NaCl - available from a number of sources but usually in drum amounts. I got mine from another smith (Kevin Cashen). A charge last a reasonable amount of time - especially if you stir it with a carbon rod (from the local welding company - just remove the copper jacket from the electric-arc rod). Now -- ask yourself -- How do I get the salt out of the tube? When you have a good answer, let me know.

Final words - salt pots are very nice but very dangerous. Water is NOT your friend. Due to the hassle and time involved, I tend to run batches of blades when I use mine. I do wish I knew about them before I bought my Paragon.