Salt Firing

The purpose of this series of postings will be to provide guidance and a bit of commentary on fossil fuel kiln firing, starting with salt (or soda) firings. Later posts in the series will be on wood and reduction firings, kiln design ideas, and experimentation. Design and aesthetic considerations of the work being fired in such "atmospheric" kilns will be discussed. Feel fee to add or remark on the ideas presented.

A general overview or outline for firing the prattmwp salt kiln can be found at my blog, bryanmcgrath.blogspot.com. But the rest will be found on this site. Please note that we are discussing the firing of our particular kiln. It is a 40 cu. ft. (stacking space) sprung arch downdraft made of insulating fire brick (IFB). We do not use soda as a rule. Straight salt is less polluting (more on that in a later post) and easier to use. We will organize the firing as follows-

-preparation and starting

-firing (oxidation/reduction)

-salting/soaking/cool down

Preparing the kiln-

Before loading the salt kiln a few important tasks need to be done:

- Make sure the kiln is "clean." Clear debris from the fire box, especially in front of the burners. Also, make sure any large drips of glaze, or in our case brick "peelings," are removed from the arch (more on this later). It is impossible to keep pots completely safe from shelf drippings or other flying debris. Sometimes these accidents add something pleasant to the work, sometimes not.

- Check the exit flue for obstructions and deterioration of flue supports. Be sure the floor and bottom shelf posts are in good shape. It can be hard to predict the failure of kiln furniture in a salt firing. A visual check is often all you have to rely on. You will get better with experience. The risk/reward (or failure!) equation is always a factor.

Most salt kilns require repairs more often. It is best to do the inspection and any repairs well before your firing date. Many repairs will need time to cure. Castable refractory is the best material for filling in holes, cracks and creating areas needing support like portals. We use a commercial mix. You can make a castable from scratch as well. Many recipes are out there. Repairs can sometimes wait a firing or two, but the sodium vapor corrosion is cumulative, so routine inspection after every firing is best. It should be noted that a kiln made entirely from castable may require the least repair work during its life. Factors in deciding on what type of kiln to build will have to wait for another time.

We wash the kiln walls with a commercial mix called "Plistix 300." It is a refractory wash (high in zirconium and alumina) that protects the insulating bricks. We add water and a small amount of kaolin (EPK) to the mix as a binder. Then we brush this wash on the walls (and the arch with crossed fingers) every few firings.

A note is needed here-

As mentioned above, we use insulating fire brick for most of the kiln wall and arch (hot face and outside). This is usually advised against in the books. Being a school, we (read "I") decided to experiment using IFB for the hot face or interior walls. We did encounter a few somewhat expected glitches. After seeing what actually happened to the bricks- they soaked up salt readily and began to glaze over (with the pots left very "dry"). With hard brick this glazing over is normal and part of the kiln aging. These, however, were being eaten away quickly (not good for the high "R" value of the walls- the reason for using IFB in the first place). Our solution was to wash the walls with the Plistix mix and watch the results. In fact, firing a few times before applying the wash was a benefit. The wash fused enough with the slightly glazed insulating brick to eliminate the concern about flaking. The arch is an issue as we get some flaking or peeling. But a hard brick arch would have some dripping. The flaking is definitely (always) worse on the pots than dripping.

We also lined the firebox, ports and the strike plate at the rear of the fire box with the castable. So far, so good! The time and effort to reach temperature has been fast and easy (we have over fired it a few times!) and the results are great. With some trial and error, the result is an efficient kiln, great results, albeit a bit more maintenance (mostly from rebuilding the parts that take the most heat and corrosion), but worth it. I believe this to be a good solution for schools (or anyone) especially since the firing schedule can be compressed to accommodate busy student (con't. ed.) schedules.

- Finally, prep the shelves. We use a combination of silicon carbide shelves (heavy) and the newer nitride bonded shelves. I have done any number of things down through the years to protect shelves and pots. Dusting with silica; washing with a silica/kaolin/alumina slip thickly applied and then grooved with a notched spatula; dusting alumina (oxide or hydrate) over that (on older type shelves). Now we simply dust all shelves with alumina (a bit expensive, true) and stick a marble size ball of high alumina wadding (using liquid wax resist) to the bottoms or feet of the work. When needed, the shelves are scraped down, then a fresh wash and/or dusting is applied. We are currently experimenting with the nitride bonded shelves. We will report when we have some results.

With attention to the kiln condition and castable in the right places, our kiln should last as long as a hard brick kiln, but probably not as long as one with wash on the walls. But I think the benefits outweigh a longer kiln life (if true). It is an experiment- so we will see. Anyone with experience, or ideas pro or con, please add to the discussion!

Our students will be firing the salt kiln for the first time this semester on Oct. 3. We will give an update on the results.

Next- the pots...