Simple kilns.............Q&A

Questions
Having looked through a lot of references on the web regarding kilns and building them, I don't see mentioned much the type of brick folks use in building their kilns, no matter what kind (anagama, saggar, electric, gas). Are the bricks you used in your early kilns (the quick-n-dirty designs) regular building/construction brick, or is it specialty refractory brick, or is it just a grade up from construction brick, i.e., fireplace brick? I'm guessing that for your anagama kiln, you probably invested in specialized refractory brick that might be very similar to the refractory material found in electric kilns? Can one find all of these brick materials at a construction yards, or for the more sophisticated kiln you probably need to order from a specialty house? Correct?

I'm entertaining the thought of the simple small scale one just to learn and play. I have some spare brick and pavers from a remodel that was done to my house and I was thinking of using these, but I'm wondering if they can't take the 1000+ C temperature that I would like to shoot for. Perhaps they can't hold in enough heat, or the size design won't allow that high a temperature, but half the fun of doing this is trying and failing...........................BA, California

Answers
The simple, low temperature kilns were built from common house bricks. Any clay bricks would do the job so go ahead and give it a go with the ones left over from your house. At the very least it will be an interesting experience, even if you attract scorn and derision from your nearest and dearest. In Australia most brickworks produce house bricks but not necessarily anything more refractory. It depends on the clay source for the brickworks. Some lucky people, living near Gulgong in NSW or Cooroy in QLD, have local production of house bricks which are refractory enough to use in high temperature kilns. Have a talk to someone in the production side of the brick business and you will probably find out a lot of interesting stuff.

You can always buy good high temperature bricks from specialist suppliers, but these are not necessary for the simple kilns. Our woodfire kilns are made from used dense (that is, standard firebricks, not insulating) refractory bricks, mostly purchased at the sites where sugar mill boilers are demolished. The local agriculture is mainly concerned with sugar cane. We have usually had to extract them from the old boilers ourselves. The main consideration with used bricks is the amount of mortar stuck to them and the difficulty of removing it. There is another problem. Once you start hoarding used refractories you start to get a whole lot of different brands, all at slightly different sizes. This makes construction tedious as you frequently have to compensate for different levels.

Dense bricks do transmit and radiate heat readily, so you should not try to stretch the natural upper temperature limits for the simple kilns. Remember, the walls of these kilns are only 4.5 inches thick. The production of durable table ware is probably out of reach, but quite beautiful things can be made using low temperature raku type glazes. Plenty of recipes are available on the Web. A possible source of better bricks, even insulating refractories as used in electric kilns, is from the newsletters of potters' groups. People go into and out of pottery all the time so bargain priced materials are frequently available.

Question
Embarrassment is the path to knowledge..
I'm really more curious about what happens to kiln walls and bricks when you actually are able to fire it higher than the rating of the brick. Do the bricks melt or just lose some of their structural integrity and begin a process of crumbling? Those of you who have built multiple kilns must surely have had the experience of 'overdoing' it, or 'using the wrong material'. These experiences and failures are surely useful knowledge.. Can't really learn anything unless you wander into unchartered territory and push a material to it's limits.
Thanks for the reply and answer. I really found your pictures of that early kiln and design enlightening and most instructive as to what can be done. Although I think the construction of the larger, and more involved anagamas are things of beauty and I appreciate the art of construction that goes into these, they seem somewhat too large and complicated and out of reach of the average person to get access to and learn things about. I would be very interested in whatever studies and demonstrations that have been accomplished on the 'small scale' side of the kiln designs. For example, how small of a kiln can you get away with and still accomplish reasonably high temperatures (> 1000 deg C)?........BA

Answer
If you heat any brick to a high enough temperature it will indeed melt, but at temperatures well below the actual melting point of the brick it will become overfired and brittle when cold. This will show up as breaks in the brick at points of stress, as when it is supporting the structure above it. The next stage of deterioration is pyroplastic deformation so that arches will tend to collapse, and walls can bulge sideways. The only kilns I have fired to destruction have been salt kilns, and the presence of salt fumes complicates the issue, as they will start to attack and glaze the outer surfaces of the bricks.

The photo at right was taken after the fortieth and last firing of our second salt kiln. The wood came at the expense of a wrecked Land Rover gearbox, destroyed while charging up the hill to the kiln with a full load of wood on a 2 ton trailer. On the 24^th April, 1986, we watched an eclipse of the moon while stoking. The latest apparition of Halley's comet was fading from view. The firing was slow and difficult with prolonged stalls in the temperature rise. After unpacking it became clear that the exit flue had collapsed.
I am not sure where all those bricks came from. Some have fallen from above and some may have come from a structure in front of the exit flue. All I am prepared to say about that structure is that it seemed like a good idea at the time. The lintel brick above the hole was a large slab and which looked underfired when put in place. It has partially melted, and chunks of it have fallen down. Looking on the bright side, it was not too bad for 39 firings with very heavy salting.

As for small kiln size and high temperatures it must be said that in this context 1,000 C is not very high. With insulating refractory bricks in layers 7 to 12 of the second of the simple kilns mentioned should do well over that, but simplicity will be diminished, expenses will have been incurred, the temperature will tend to be uneven, and the payload will be very small. The learning experience will remain, but the main objective should be to produce finished work. Better to move on to a design with more potential than to keep flogging a reluctant horse.

Question
Should I use mortar and cement the brick as I build the kiln? NH, California

Answer
A mortar made of clay and sand would be desirable to bed the bricks down. Do not use ordinary cement, and do not use mortar on layer 12 as bits of it will probably drop onto the pots.

Question
Do I need a piece of iron for the top of the fire box and if so, how thick? NH

Answer
Iron above the firebox is not a good idea as it will quickly corrode if it does not actually melt. The bricks above the firebox are held in place by the weight of the bricks on higher layers.

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