With years of experience firing atmospheric kilns full of pottery, Justin Rothshank shares tips on key steps in preparing for soda firing.
I’ve been using CoreLite kiln shelves exclusively for the past ten years, and sporadically for even longer. These shelves are extruded with a highly thermal-shock-resistant refractory material composed of rich mullite and cordierite. They are suitable up to cone-10 firing temperatures, and I use them in all my kilns—electric, soda, and wood—regardless of the temperature I’m firing to. They are lightweight and come in a variety of sizes and shapes.
CoreLite shelves can be more fragile than traditional mullite/cordierite shelves, and thus may not work well in an environment with a high volume of student users. Atmospheric firing can be hard on shelves, and cracks can develop through normal wear and tear. Because of their extruded quality, it can be difficult to see these tiny cracks when they run the length of the shelves along the extruded lines. In recent years, I’ve experienced minimal shelf collapse during long firings because of cracked kiln shelves failing. In 15+ years of use, I’ve lost approximately six kiln shelves, and completed a minimum of 60 atmospheric firings, and hundreds of electric kiln firings (1).
The fragility of the shelves is the biggest downside I have found to these CoreLite shelves. I’ve rectified the situation by carefully stacking the kiln and posting the shelves to minimize collapse impacts. This means adding additional posts when a shelf collapse seems possible. I have stopped cleaning kiln shelves with a hammer and chisel, and instead use an angle grinder and diamond grinding pad. Lastly, I carefully inspect the shelves both during the cleaning of them, and before loading them into the kiln. Older shelves are stacked higher in the kiln so that they are not responsible for carrying as much weight.
If CoreLite shelves aren’t for you, then any shelf of mullite/cordierite composition will do nicely in your soda kiln. I have avoided silicon carbide shelves, because they are expensive, and tend to corrode, bubble, and drip onto pots when used in an atmospheric environment. However, many people happily fire with silicon carbide shelves and find these shelves can stand up to the heat and atmosphere. If you choose to use silicon carbide shelves, using a kiln wash on all the shelf surfaces will prevent silicon carbide drips onto your pots.
Wadding is a refractory material that keeps the pieces from sticking to each other, to kiln shelves, or the kiln post. I work with 2 types of wadding and mix both recipes roughly by volume using the unsophisticated “handful measuring” technique. This wadding is also used to make cone packs, and any excess is kept in a sealed container for use in future firings.
The Alumina Wadding/Kiln Wash, while more costly, provides a great barrier between the artwork and the kiln shelf. This wadding is highly effective, but its biggest drawback is that it can leave a distinct white mark on dark clay bodies. This mark can be quite distracting against the chocolate brown colors of atmospheric earthenware.
The Fireclay Wadding is less costly and can be a less effective barrier, especially in areas with high concentrations of soda spray. This can result in the need to spend more time cleaning and grinding fired wares. However, it leaves a darker mark and can be less distracting on darker clay bodies.
Once pots are ready for the soda kiln, there are several more steps to prepare for the firing. Some artists apply kiln wash to the kiln posts or even the kiln shelves. I avoid kiln wash on all my shelves, as this prevents the option of flipping the shelves between firings to minimize warping. When a glaze runs, I use an angle grinder to clean it. This is a quick and easy process, and my glazes are formulated so glaze runs are minimal. I do kiln wash the tops and bottoms of each kiln post. This aids in the separation of wads from posts after each firing and keeps the posts level and free from wadding debris.
Each piece that will be fired needs to be prepared with wadding. When wadding pots, I prepare a bucket full of pre-rolled wads in different shapes and sizes. Sausage logs, jellybeans, marbles, and walnuts are my default sizes and shapes. I also make randomly shaped shields for deflecting ash and vapor in intentional patterns. These are thin slabs of wadding shaped in different ways to create interesting flashing patterns.
I prefer to glue the wads onto the pot using white Elmer’s glue or some similar cheap white craft glue (2, 3). The glue allows potters to pre-wad their pieces before loading the kiln without fear of the wads falling off during the loading process. Glue will keep wads in place for several hours, days, or even weeks until the pots are ready for loading. The glue burns off in the kiln.
Moistening the wads with only water will also work but the glue is more durable than water. Wads are placed intentionally, considering the pattern they will leave on the clay surface after the firing. During the firing, the glue burns away, and the wad is fired while it lifts the piece off the kiln shelf. Upon the completion of the firing, the wads are easily removed and discarded, or saved for possible use in future firings.
I have found that wads leave varying marks, depending, of course, on shape and size, but also on fired state. I keep some clean, fired wads for use in future firings. A wad applied as wet clay will form a tighter seal against the vessel’s surface, thus leaving a more defined mark. Applying a wad that has been previously fired will leave a more organic mark, as it does not conform exactly to the vessel surface.
Part of the glazing process of atmospheric firing, wadding pots and loading the kiln have an enormous impact on results. These are important decisions with lasting visual impact. Wherever you place a wad, you’re placing a mark on your work. Stacking pots, using wads or bricks as shields, and deciding the placement on the kiln shelf are all opportunities to consider when planning your firing.
Noting where pots are placed within the kiln is important, too. Pots closer to the soda ports will get blasted with more soda. Pots closer to the burners may get hotter or display more reduction effects. Pots nearer to the center of the kiln may get less soda or have more oxidized surface effects.
Excerpted from Low-Fire Soda by Justin Rothshank, published by The American Ceramic Society and available in the Ceramic Arts Network Shop: mycan.ceramicartsnetwork.org/s/shop.