I learned to truly love the art of wood-fire in an Anagama kiln at school after I had built a very rudimentary wood-fire kiln in my backyard a year before. Working mainly from my own engineering knowledge and a few books it was essentially a ‘mud’-based design built in the Anagama style. In Michigan there is plentiful clay in the ground. Luckily the kiln didn’t achieve true wood-fire temperatures; the local clay starts to melt at about 2100 F and I don’t think the kiln would have been able to take that heat though it did reach lower bisque temperatures.

Shortly after starting as the studio manager at the Ann Arbor Art Center’s ceramic/sculpture studio, I set about on my second attempt at a wood-fire style kiln. With a few old electric kiln-shells sitting around along with broken kiln shelves and the like, the materials were available for a new design. Knowing the time and labor associated with wood kilns, the goal was to search for a way to achieve those results without requiring such a dedication. Also, still with lots to learn, a quicker turn-around than the typical months to a year between firings was sought making lots of glaze tests and delicate adjustments possible.

So I broke down the basics of a wood kiln in my mind; cone 10 temperature capability along with directional ash settling on work inside at red-hot temperatures. The design which surfaced was to fire an electric kiln-shell with propane from below with ports on the side to blow the wood ash directly onto pieces. The first plan was to use sawdust. While it is a readily available material, the total volume needed was so large; the majority of it destined to become nothing more than completely burnable carbon matter with only trace amounts of sodium, potash, iron and all those fluxes and oxides we love so much. So wood ash was the logical alternative; the ‘heart’ of the material needed.

As far as consistency it is well known that there is much variation between woods and likewise wood ash, even from the same wood. The random fireplace cleaning can provide a bit here and there, but the mix will be different every time. Not that this should necessarily be a bad thing; but when dealing with wood kiln design a little consistency can be a nice thing with all the other factors out there. I was lucky to score a sizeable amount of ash from a firewood business; they had two large burn barrels fully loaded with wood ash from the burning of scrap wood. Even more importantly this batch of ash could be mixed together to make a large amount of consistent wood ash to begin my firings with, eliminating one of the many variables in the chaos of the fire.

So, I had my ‘glaze’, I had my fuel (propane), and I had my kiln parts. The design was very simple: a hole was cut in the side of the kiln shell with a 3” hole-saw for the burner inlet; the bottom of the hole right at the floor of the kiln. A few soaps were used for posts, about 6 inches tall, and kiln shelves for the ‘bagwall’/shelf. Gaps were left between the shelves and walls of the kiln; these would be my inlet ‘flues’ from the firebox. To avoid the need for wadding, 30-35 old circular 2” posts were placed on the shelf, the kind with a raised section on top (recessed on bottom for stacking). These would be my pot ‘stilts’. Being a kiln of 24” high, minus 8” for the firebox (shelf and posts), that left room for 16” pots. At the time, I was in the early stages of learning how to throw, having been a hand-builder for years, so the majority of my pieces were bottle and vase forms app. 12-14 inches tall. They made for a good practice pieces, were an appropriate woodfire form, and were fun to throw. One row of these forms would make for a fairly efficient packing. I also have a few extension rings to the kiln for firing larger pieces.

The real advantage to the no wadding approach was the ability to fast-fire. While now I do believe one could make a wadding to withstand the 4-hour fastfire, this system works well for consistent forms. To prevent sticking, the posts are coated with a 100% alumina wash before each firing. To date, the only times I get much fusion is when I throw salt in the kiln… Even then the posts do well, although I’m sure tumble-stacking with wadding would be possible in a fast-fired kiln.

Firing the kiln is easy, for power a 1/50 horsepower squirrel-cage motor is used with propane coming in through a simple pipe-fitting tee to a burner. Air is controlled at the intake by means of a swinging flap and at the kiln exhaust with a damper over a 4” hole in the center of the lid. While I initially start the burner very low, I reach 1800 F in about one hour, 2100 in 2 hours, 2250 in 3 hours when I introduce the wood ash and off in 4 hours. Plenty of variations of this firing cycle are still to be explored.

The wood ash in introduced by means of a second squirrel cage blower motor. The ash is sieved with an old flour sifter; it sifts quickly and can take the abuse of un-burnt coals and other foreign matter. A stainless steel pipe of about 12 inches comes off the outlet of the blower. More 3” holes are cut in the kiln shell every other facet; 5-6 per kiln, about in the middle of the firing chamber. When the kiln reaches at least red heat, the outlet pipe is put into one of the holes and a cup of the ash is fed directly into the inlet of the blower motor. The correct angle and a slow feed keep the ash from jamming up in the housing of the blower. I go around, hit all the holes with a cup, then repeat this a few times. Usually 2-3 cups per hole is enough for an average build-up of ash. More will give heavy drips and firebox-like results, while less will give the right claybody or glaze a very nice richness.

The temperature will fall a bit during ash loads, but with the right timing you can keep the kiln hot enough during this process. I used to do two cups at a time, one seems to help but quickness is the key. Have everything ready-at-hand when you open the port; motor on, ash in a cup, gloves on, place to set the hot ‘spyplug’. Inject the ash, close the port, then clean up a bit and prepare for the next port; by then the kiln should have recovered temperature and is ready for another round. Of course, ashing can be done at various times, but the important part is that the pieces are red-hot so that the ash sticks to them and doesn’t just blow out the exit. Be sure the kiln has recovered to at least good glowing temperatures before ashing more. The great thing about this design is that you still get the directional aspects of true wood kilns; it can even be harnessed (a bit) with thoughtful placement of the pieces in the kiln.

You might be wondering about the effect of the ash on the softbrick walls. My first enactment of this design was used for many soda, salt and wood ash firings, and the soft brick did start to melt but it withstood a fair amount of abuse. For the really old kiln-shell, I’d bet a good 20-30 firings could be made without any coating. Likewise, a simple box made of soft bricks would suffice. Due to the internal dynamics of this design, I do not believe a round shape is integral. In this case, kiln shells were the material-at-hand. My second kiln was a big larger and was a nicer shell, so I wanted to protect it. I was given the secret to a great coating from J. T. Abernathy in Ann Arbor. It was a mixture of colloidal silica (with a gel-like consistency) and alumina. I sprayed this onto the entire interior of the kiln-shell, inside the grooves and all. After a number of wood ash firings and even a few salt firings, the walls are still chalk-white showing no signs of fluxing, flaking or cracking. I’m sure the coatings available in catalogs would do sufficiently as well.

I speak of doing salt and soda firings – these have gone very well also. To introduce the soda, I use the same blower motor set-up. For salt, a few times I simply dropped a quantity of table salt in through the exit port but a better system has been to set a bowl in the middle of the shelf. The first few times, I dropped the salt into this bowl at cone 6 temps. It vaporized quickly and circulated all around the pieces well. After that there was enough salt left in the bowl to vaporize itself during subsequent firings. While this also worked well, I think the results were more dramatic when the salt was introduced at temperature. (the bowl will only withstand a few firings unless a hearty clay-body is used I would assume)

I have had some amazing results with terra-cotta in soda firings. The salt firings produce classic orange-peel textures along with great results with Albany slips and a locally dug slip I use obtaining excellent results with salt, ash, or just electricity. Any potter should be exploring their local clays if present; mine are very close to Albany with a slightly lower melting point.

As I mentioned earlier, the one great advantage to this kiln (along with the price) is that it can be fired in such a short amount of time. This allows for quick turnaround speeding glaze test turnover and allowing you to keep everything fresh in your head. With some fancy cooling, the kiln can be opened in another 4 hours. A fully cycled woodfire in one workday!

Some might question the thermodynamics and how the kiln circulates. Truth is, the flow of the kiln would make one assume it would have cool spots and no ash/vapor circulation. But the fact that it is such a small kiln makes it work. The heat is distributed through radiation and conduction as much as by convection, which draft kilns rely on. The fact that it is driven by a forced air burner along with a small exit flue makes for a slightly pressurized design, increasing the internal circulation. An efficient draft is not part of the needs of this kiln, so instead the air is forced everywhere. This idea could be applied to larger kilns employing the same blown-ash system. These concepts are partially based on J. T. Abernathy’s kiln designs; one of these is at the Art Factory site, displaying excellent firing characteristics using a more radiation based design. This potential for kiln designs utilizing alternate means of heat and ash/vapor distribution lends much excitement to the future of our field.

You can see some basic photos at http://ccpalmer.com/ashkiln.htm

      Christopher Palmer