|How to Build a Catenary Arch Soda Kiln
(or more to the point, How I Built Mine.)
Step one : come up with the bricks. There are books and plans
and on line info telling you how many of which kind of brick you need to
purchase for various sizes of kiln, but if you're like many people, you
will scrounge or luck into some used odds and ends and make do with
what you have on hand. I got my mix of new and used bricks on ebay,
when a potter was moving and needed an unfinished, never-fired kiln
removed from her back yard within days.
I bought it for a hundred bucks. But buying a kiln is kind of like buying
an in-ground swimming pool. First you dig the new hole, then you start
carrying buckets of water...
We would need a support form to take the kiln apart. I went to see it,
and took a piece of plywood. I had my kids hold the board over the
open end of the kiln while I squatted inside and traced the arch with a
I went home and cut out two arches in the shape of my marker line,
and built a kiln form by attaching the two end boards with 1X2s (you
can see it in the picture here, sticking out of the kiln we bought as it is
disassembled.) Usually you make the arch by hanging a chain from
two nails on a piece of plywood and tracing its curve -- that's the
magical catenary arch, the shape that allows stone to stand without
mortar (in, for instance, the Roman aqueducts) so that the thrust of the
weight goes into the earth. You can read more about catenaries here.
My husband, my kids, my mom and I made a couple of trips in a
borrowed truck, boxed up bricks in whatever we could find, and hauled
them to the family's cottage at Wolf Lake (Irish Hills, Michigan.) And
there they sat, all winter, under a tarp.
Disassembling the unfinished kiln I bought on ebay.
Taking the tarp off the bricks in spring, I evicted some frogs.
Step two: Thinking about the size. In my case, the kiln bricks I bought didn't include a front wall or door, so I
knew I had to build a smaller arch. That suited me fine, since I have no guild, class or co-op firing with me. I will be
filling this kiln by myself, and given the learning curve with any new firing method, I expect to generate a lot of
So I borrowed a small kiln form from Diana Pancioli at EMU, a yard wide, a yard deep and a yard high. That would
leave me enough extra bricks to build a front and a door.
Step Three: Where to put the kiln. I had dumped the
bricks at the cottage just because it was close to the place
where I bought the bricks, but my parents suggested that I go
ahead and build it there. They pointed to a size yard (left) at
the bottom of a hill, facing the lake. It was near enough to
keep an eye on from indoors, shaded by big old oaks, and
the view of the lake (below) was lovely.
Step four: Digging the hole. I had 36 cinderblocks. I drew out the foundation on graph paper. I used twine and
long nails to draw the square on the grass, and just started shoveling. Family pitched in when I wore out -- hubby,
the kids, even my mom at 70 took a few scoops with the turf shovel. "Just like cutting out brownies." I made the hole
deepest on the two sides where the weight of the arch would rest.
Step Five: Pour the slab. I figured I'd need 32 sixty-pound bags of cement to make the slab. Quickcrete was
about $130 total, and it would cost $35 to rent a cement mixer for the day. Since grandma was the only one with a
Michigan drivers license (she got pulled over for speeding in her 90s) she rented it for us, so we called it
"grandma's mixer" and made her the foreman. My cousin Dan helped heft the bags. I put rebar in the slab as we
poured, and my dad got into the spirit and tossed in any old metal posts, tools and junk he could find.
Part Six: stacking blocks. Once the slab was done, I covered it with heavy duty aluminum foil -- in theory, to retain
heat and resist moisture. I started with the cinder block layer. There is some argument on clay discussion lists about
whether blocks should go holes-up (which is the strongest weight support) or holes-sideways, to keep the concrete
slab from heating up and exploding. I chose holes-sideways, for the following highly scientific reason: Chipmunks
lived under the one at school, and I would be happy if chipmunks lived under this kiln also. I like chipmunks. You
can't argue with that kind of logic.
Disclaimer: I have no idea what I am doing. I am
making this up as I go. I haven't even fired this
kiln yet. I present only a possible trail of bread
crumbs for others traversing the dark forest. No
guarantee that you won't be eaten by a bear.
No chipmunks were harmed in the making of this kiln.