When one starts thinking of building a salt kiln and wondering about refractories, three things will determine the outcome. Refractories already available, finances available for purchasing new refractories and the intended lifespan of the salt kiln. At first I used whatever hard brick I could get my hands on and I coated the interior with alumina hydrate. This retarded the reaction between the bricks and the salt vapors, but eventually the salt won. Then I tried coating the interior with a high alumina cement that I trowelled on the surface, but again the salt won. More recently I've been involved with high alumina refractories that would hopefully resist sodium destruction. I much prefer making pots to building and repairing kilns. The chief disadvantage of high alumina refractories is their cost. A. P. Green Mizzou bricks are selling between $1.25 and $1.50 and the Mizzou castable is a little over $20.00 per 100 pound bag.

Several years ago I decided to build a high alumina kiln because all indications were that high alumina was the answer for long lasting salt kilns. The kiln had a complete interior of A. P. Green Mizzou. The floor and walls were Mizzou straights cemented together with A. P. Green #36 High Alumina Cement. The fireboxes and arch were cast with Mizzou Castable and the entire kiln was G-23 Insulating brick on the outside. The Mizzou material ranges from 60-63% alumina and in brick form is very dense, strong, resistant to chemical attack and very refractory, PCE 36-37. In the castable form, it allows you to mix and use it like concrete to cast monolithic units.

The reasoning behind the castable was to cast monolithic units that would be crack free, therefore eliminating sodium and chlorine seepage into my studio. The same reasoning applied to the fireboxes in preventing liquid sodium chlorine seepage through the floor structure deteriorating the refractories. This happened in the past because prior to vaporization, there is always a period of liquefaction even with super hot burners. The liquid can cause more trouble than the sodium vapors. The reasons for using the castable proved to be as weak as the castable.

First, the arch suffered a construction crack that opened in the firings and sifted aggregate onto the ware. This crack resulted from casting from one side of the arch to the center and then from the opposite side to the center. There was enough time lapse to prevent the material from fusing or sealing together and it acted like a scratch on a piece of glass, a natural breaking line. Aside from the construction crack there developed a network of cracks which I've learned are quite natural with castables. They will form their own stress cracks if not cast in sections. My concern at that time was twofold. I couldn't allow the large crack to dribble debris onto my pots. I was also concerned that the arch might collapse. As for the fireboxes, they became very soft and crumbly although they were containing the liquid sodium chloride. So I removed the arch with a sledge hammer to learn several things.

The arch would have never fallen on its own accord. During the removal I could also see that there was a good deal of vapor penetration, even though the arch interior surface revealed no reaction. About an inch inside the arch there was a zone of crumbly Mizzou where the vapors were actually deteriorating the castable chemically. The A. P. Green representative explained that almost all castables are bonded by cement which is fluxed by calcium. The calcium reacts strongly with any alkali, especially sodium vapor. This brings me to the reason for this article. Several disadvantages of castables are: (1) they will crack in use which permits vapor penetration, (2) they are very porous because they are seldom fired to maturity in a cone 10 kiln, and (3) they are fluxed by calcium which reacts strongly with any alkali present. This applies directly to Mizzou castable but relates to probably most others. If you choose a lower temperature castable that will become denser, it will overreact with the salt vapors. If you choose the high alumina castables, they will be extremely porous. Therefore, maybe our concern should be with a refractory's density more than its alumina content. High alumina with high porosity is bad news. High alumina and high density is good. Alumina is a very porous material by itself and firing a high alumina refractory to cone 10 is like bisque firing a stoneware body.

At the recommendation of the A. P. representative I replaced the Mizzou arch with a refractory plastic. A plastic is like an unfired brick with a moisture content of about 10%. Industry uses pneumatic hammers but you can get by with a three pound sledge hammer if you don't mind beating your brains out. We used the Super Hybond Plastic because it was less refractory and we thought it might become denser and seal its surface from the sodium vapors. A real bad mistake! I spent about six hours beating the plastic into place, removed the arch form as directed and fired the arch in place with a 32 hour curing cycle plotted by the engineer. First problem was the arch sagging and the second problem was the sulphur given off by the plastic. But the biggest problem was the many cracks that resulted from insufficient beating or ramming of the plastic into place.

The sulphur chemically bonds the plastic refractory and the A. P. Green representative thought it would burn out in the curing cycle. Unfortunately for me it did not. Instead, it ruined a load of stoneware. The sulphur chemically reacted with the salt vapors and the iron in the body and reduced the pots so much that two subsequent oxidation firings could not lighten the color. The pots were resalted in another firing with little change and finally put through a cone 9 electric kiln cycle which finally made them look nice. The sulphur has been reacting each time I fired so I'm planning on replacing the arch in the near future. I may try a couple vaporized copper red salt firings to see if the sulphur might be of some help instead of just a hindrance.

Disregarding the bad experience with the sulphur, the trial with the plastic refractory substantiated my feelings that density as well as alumina content is necessary for the resistance of sodium vapors in salt kilns.

My next arch will be 4-1/2" of dense Mizzou hardbrick cemented together with #36 high alumina cement and backed up with block insulation, equals 14 inches of hard brick relative to insulating ability. An arch like this would be high in alumina and density, light weight, and I think as permanent as can be at this point in time.

There are other brands of refractories available as well as other types of refractories. This hasn't been an ad for A. P. Green, these are simply the materials I've been using and am familiar with. If you go below the 60-65% alumina range, the refractory will react with the sodium to form glaze on the interior and eventually deteriorate. If you go above 60-65% range you will have problems with porosity as well as increased cost. There is a KX-99 brick that A. P. Green makes that is about their densest high temperature brick but it's been cracking because it can't take quick firings and coolings that studio potters practice.

These are my ideas and experiences with but a few materials and I'm sure many people are experimenting with other refractories and maybe having good luck. My concern is that people starting out might avoid the problems that some of us have experienced. Special refractories are expensive, as is down-time for repairing kilns and sore backs.

Tom Turner teaches at Clemson University and operates a pottery in Liberty, North Carolina