turtle

good morning!

I am ill, not sparking on all six cylinders. two weeks to firing time and many pots have not been made. but perhaps the kiln will fire more evenly if loosely packed- now is the time to find out! other aspects are coming together nicely, though- we have a new firing partner, Alyssa, who has been giving me some of her time to help rebuild the aweful bourry-boxes that I slapped together last firing to figure out an appropriate scale. basically, a "standard" bourry-box for a wood-firing kiln is a firebox exterior to the chamber which is the primary if not sole engine for the kiln. the basic idea is that the wood is loaded onto supports (hobs) from above, and the primary air is also above, so the air is warmed as it is drawn downward through the stack, on its way past the coals below, and on into the chamber beyond. it is a design that allows the potter to load the firebox pretty full and then rest a while as it burns down, controlling the rate of burn with the volume of air passing through, instead of the more demanding rythmns of other wood kilns. Kilns of this design include the one I've posted above, designed and built by Coll Minogue and Robert Sanderson, as pulled from Jack Troy's book on woodfired stoneware and porcelain. Linda Christianson has a bourry-box style kiln, and Neely's design of the train kiln also makes use of the bourry-box to very different effect.

I am making use of the design by mishap and default: in my very thorough design research and thinking about every little thing, I managed to completely forget about the essential first part of the firing, in which we use wood to bring the kiln up to a temperature at which the oil will ignite and take over. most people said this would occur at 1850 degrees farenheight, (it's much lower in my kiln) which does not occur in a small open tunnel under a large kiln.... for the first firing, I cobbled together a little structure which proved itself too small. for the second, I made something too big, just to find out what the limits were. so hopefully this version will be Just Right.... the deciding factors include primarily that they are not designed to take the kiln to cone ten. if they needed to serve this purpose, they'd probably be much too small, probably four or even five times too small, judging by the advice of Nils Lou. we are aiming for 1000 degrees comfortably. then we switch to a combination of the wood and oil, at which point they are not used anymore. then here's the other factor in their design: pizza oven! slide a clean kiln shelf in that big door, and mmmmmmmmm..


the images above show you what's going on- they show up in code on this editing page, so I'm not going to try to move them down here- sorry. the first one is the brick base, on which is placed the removeable grate bars slid into little four inch sleeves. we anchored the bars in place with bricks inside so that they wouldn't shift during the casting process, and then packed the top part with the clay soil to make a form on which the concrete would rest. put plastic over the form, and then castable over that. it's a hand-pack kind of castable, from the nice guys as harbison walker, and it's a little weird- it sets up cold. as in, it begins to set up pretty fast, but there's no heat generated by the reaction. threw me off, for sure, and I don't think we got a perfect seam in one place because of it. we made the castable three to four inches at the bottom near the grates and lower walls, and then tapered it up to two or a little less at the very top. (HW recommended four inches for best strength, but I'm going to risk it, given it's low demand setting).

another note on structural castable. in conversation with Bruce at HW yesterday as we picked up the goods, I got a better idea of something that had puzzled me. they had told me that it was highly likely that my dome would crack in places- it is a monolithic structure six feet in diameter, four inches thick , more at the edges, and it is subjected to 2300 degrees of heat. that's a lot of stress. the cracks that are there are a quarter inch at the widest, starting at the periphery. there are three of them, dividing the dome loosely into thirds, and they do not extend to the top. so they are built-in expansion joints, and this is the tidbit that I learned yesterday. they have cracked in this configuration probably because of the giant steel band that supports the dome from the outside. as the kiln heats, the steel heats, and radiates it back to the concrete, causing the edge to heat disproportionally fast. so it is expanding and seeks release. I did work a quarter inch expansion joint into the space between the steel and the skew-back that the dome lands on, but either this was not enough or the heating issue is unavoidable. either way, that roof ain't going nowhere, so I'm not worried.

oh, and Bruce said do not fill the crack. not only will it only fall out, but it will only make it worse, because then you have a solid object in an expansion joint, making the situation worse. he said leave it alone, it will not be a problem. i'm the guinea pig- I'll let you know if he's right (he's the man, though, so he's prolly right)