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Glazing Firing Tools + Equipment
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Process: Firing All that being said, it is also exciting, unique and rewarding. Given some experience with a particular kiln, many of the variables can be controlled to a reasonable point, which then opens to opportunity to introduce new ones, in search of something not seen before. Clay bodies and glazes are very dynamic, highly responsive to slight changes in process - that is a large part of their appeal as a medium. Playing with these qualities is the best part of firing. I always glaze fire in a fuel burning kiln, usually natural gas or wood. Unlike electric kiln firing, fuel gives you real flame, atmospheric currents and draft, the ability to fire in a reduction atmosphere, and a nice component of drama. The typical glaze firing is to pyrometric cone 9-10 (about 2350° F), with clays and glazes formulated to mature at this temperature. Examples of pots fired in each of these types of kilns can be found in the Archive Gallery. I currently fire a medium sized gas reduction kiln 4-6 times a year (70-80 pots per firing) and a small soda kiln 3-4 times a year (30-40 pots per firing). Both kilns are pictured below.
Below are two photos of the gas fired fiber kiln at DePauw University - before and after a glaze firing. The kiln is a prefab model by Geil, with a computer controller and pyrometer that can be set to maintain a specified rate of climb through the early stages of the firing - a very nice feature! In the photo at the left, the unfired glazes can be seen in their raw state, pale pinks and reds and whites, with the resulting finished glazes on the right.
The firing cycle lasts 7-8 hours, after an overnight warmup, and takes about 18 hours to cool. The photos below are from another firing in the same kiln, which holds 70-80 pots on average. You can see that I generally use 5 or 6 glazes at a time, and spread them out fairly evenly in any given firing. This is partially because I enjoy the variety of the results, partly due to different pots calling for a particular glaze, and partly to minimize the results of an improperly mixed glaze batch ruining a group of pots - a rarity, but always possible.
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 This is Clary Illian's soda kiln, which I got to fire a couple times in '94-'95, bricked up and ready to fire. It runs on two natural gas burners, and has several ports in each wall to facilitate easy spraying of the soda. This kiln is smaller in cubic footage than the average gas reduction kiln, which allows for smaller loads, faster turn-around time, and more experimentation in firing approaches. An interesting feature of this kiln is that it is built back-to-back with a large gas car kiln, enabling them to share the same stack.
More photos of this kiln and a firing.
To date, I've done 10 firings with a variety of results, but generally good and improving. I'm currently using a combination of salt and soda in about a 50/50 mix; the salt is thrown into the fireboxes in a "salt burrito": mixed with sawdust, wrapped in newspaper and doused in water. The soda is sprayed in with a pressurized garden sprayer. The salt disperses through the kiln, getting some amount of glaze in all parts of the stack, while the soda is more directional, often creating two distinct sides to each pot. |
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St. Earth Pottery | Handmade
functional pottery by Scott Cooper | Indiana, USA | www.stearthpottery.com | © 2000-08 |
What follows are some examples of the the kilns I've fired over the past eight years. The first is a Minnesota Flat Top car kiln at Arizona State University, designed by Nils Lou and built by Kurt Weiser. The car kiln is gas fired, very easy to load, and holds a lot of work - a really nice design. (The small kilns in front are for raku, and an old dough mixer is visible in the background at the right).
Also on the kiln patio at ASU (pictured at left), are four large gas reduction kilns. These are pre-fab hard brick kilns, with burners that sit below the metal frame and a heavy hinged door. I'm not sure what company manufactured them, but they were workhorses - reliable, tough and consistent. These were the kilns that I first learned to fire on my own. 
While at the University of Iowa, I participated in group firings of an anagama kiln, the large wood fired monster illustrated here. This type of kiln is based on the traditional asian model, holds hundreds of pots, and is often fired for up to a week at a time, to encourage ash and color accumulation on the work. Parts of the firebox can get to cone 14, a temperature in excess of 2400° F. While in grad school at Southern Illinois - Edwardsville, I fired both a train kiln, based on John Neely's design, and Dan Anderson's anagama - the nicest wood kiln I've ever seen. The train kiln is sort of a modern-day redesign of a small anagama, very efficient, and capable of amazing results in a (relatively) short 24 hour firing cycle. Some examples of pots fired in these kilns can be seen in my 
A close up of the kiln above, this photo is looking through a spyhole in the door at about cone 9 (2300° F). Just visible are the neck of a vase and a teapot spout. The numbered bricks that make up the door make it easy to unstack and restack them in the right order each time. The kiln has just been charged with soda, which disperses around the chamber and coats the ware in a vapor glaze. 
This is a small catenary arch soda kiln at DePauw University, circa 2003. It uses 2 forced air, natural gas burners, has a castable door, and a removable metal stovepipe for a stack. It fires in 8-10 hours. I use about 4# of soda mixture (50% soda ash/50% baking soda), sprayed in with a pressurized garden sprayer at about cone 9. This gives a light soda glaze to pots on the interior of the stack, with some heavy soda build-up to pots near the spraying ports - all creating a good variety of effects within one firing. 
Here's the kiln being unloaded, with some stoneware pots in the foreground and mostly porcelain pots still stacked inside. The iron in the stoneware darkens the finish, while the porcelain is very receptive to changes in atmosphere and markings on flashing slips. 
This is a small experimental soda kiln I built at my studioin 2005. It is an updraft design, fires with propane gas on one Venturi burner, and has about 7 cubic feet of stacking space. It is a hybrid design of various other kilns, but based primarily on kilns I fired at the U. of Southern Illinois @ Edwardsville. 
Here's the kiln being unloaded, with the door bricks removed. This load was all porcelaneous stoneware, with a variety of flashing slips, underglaze and glazes over the raw clay body.