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kiln floor for wood/salt bourry box kiln

updated sun 22 aug 04


Geralyn W. Barry on thu 19 aug 04

Hello all,

I am working through the final design details before starting to build a
wood/salt kiln. This is a group project (10 people). Most of us have helped
build a few kilns but none of us has ever been responsible for the entire
process til now. I am the main person "in charge" of the design and
building, and I hope someone can answer a few questions I have...

It seems the only books I read these days are about kiln building. I've
searched through the Clayart archives repeatedly and think I have probably
have read every post there about wood kilns, kiln construction, propane,
etc., though maybe I've missed a few! I have tried to sort through the
various ideas I've read about to come up with a plan that makes sense to
me, and would appreciate any comments from more experienced folks. Most of
what I've read pertains to straight wood kilns or gas-fired salt kilns -
ours is a combination.

We will be building an external Bourry box kiln very much like Robert
Sanderson's kiln as depicted in Ian Gregory's book "Kiln Building" (p.
70-75 and cover photo). That kiln is in several other woodfire books,
including those by Jack Troy (p. 62-65) and by Coll Minogue and Robert
Sanderson (p.28-31, 162). The main difference is that ours will include
burner ports in the ware chamber for the option of doing a straight salt
firing with propane only, with the Bourry box blocked off - no wood. Our
walls will be 9-inch thick hard brick with 4.5-inch soft brick exterior on
the ware chamber (we like the effects slow cooling gives us).

We have a concrete pad and an open-sided shed built over it. I have
recently been researching what to use for the layers under the various
parts of the kiln. We already have some large hard firebrick (approximately
12.5 in X 8.5 in X 4.5 in) to use as a foundation under the entire kiln,
where concrete block is often used. The firebrick has 6 long parallell
slits or "holes" that run through the entire 4.5-inch thickness; we plan to
lay the brick on the concrete slab with these holes facing vertically.

The interior kiln floor is low from the Bourry box into beginning of the
ware chamber as far as the bag wall, then it steps up 15 inches. The
stacking area is on this higher level. This higher level continues into the
flue and chimney. In the Bourry box area, I plan to lay a 2.5 inch layer of
high duty hardbrick (old Columbia-X) on top of the "holey" brick (which is
4.5 inches high), with a 4.5 inch layer of super duty hard firebrick as the
top layer (hot face). The bricks in this top layer are 9 X 4.5 X 3 super
duty Clipper bricks - "tongue and groove" bricks, with ridges and
indentations along the 9 inch length so that the bricks lock together when
set on top of each other.

So, the hot face floor of the entire kiln (both levels) will be these
Clipper bricks laid on their sides, making a layer 4.5 inches thick, backed
by the 2.5 inch Columbia bricks. In the ware chamber only, there will be a
layer of 2.5-inch thick soft brick behind that (Babcock and Wilcox K-23 -
old but still in the box).

These are the questions I have:

1) Should there also be a layer of soft brick under the Bourry box? Or
should just the chamber be insulated? I read about insulating the floor of
the kiln, but was not sure if that was just the ware chamber in this case,
or the Bourry box, flue and chimney also. Right now, the plan is to onll
insulate the ware chamber.
2) Might the salt penetrate down to the soft brick layer in the ware
chamber (2 layers of brick, 4.5 inches plus 2.5 inches)?
3) There is no vapor barrier under the concrete slab. I have read
suggestions that one should be installed between the concrete and the kiln
base, but I have never seen a kiln where this was done. Is this something
you would recommend for an outdoor kiln (shed over it, but not much side
protection) in rainy (and last year, snowy) western Oregon? Which layers
should I put it between? Right on the slab or on top of the "holey"
bricks? Someone has recommended a couple of layers of heavy duty aluminum
foil one on top of the holey bricks as a vapor barrier. Does this make
sense? Will it be too hot for the aluminum (melting point 1220 F)? Or
should I use heavier sheet metal (how heavy?), or should I just forget it?
4) I think I should fill the holes in the big "holey" bricks from the
Bourry box to the bag wall to protect the concrete slab from heat. Does
that make sense, or not? If I fill them, what should I use? Silica sand?
Grog? Broken soft brick (we have some bits, but not enough for the entire
area)? This is probably the hottest part of the kiln, but I am not sure
what kind of temperatures to expect in the different layers beneath the
kiln. Any ideas? We will also be introducing salt into this area.

Any comments appreciated!

Geralyn Wood Barry, usually a lurker, in the Coast Range of western Oregon

Michael Wendt on fri 20 aug 04

Industry always insulates wherever possible on the heat input side. Once the
heat has done its work, the chimney is less critical to insulate. If you
don't insulate the floor and walls of the bourry box, you have to heat up
all that mass before you can expect much heat gain in the ware chamber. My
recommendation is to veneer the bourry box with high heat refractory and
insulate it and the ware chamber as much as you can afford to do. As you
decrease the dead mass, the fuel consumption goes down and the rate of
firing proceeds faster. Moreover, if you can make the kiln very air tight
when done, you can get super slow cooling rates without all the wasted fuel.
Tech talk: each gram of hard brick requires about 0.35 calories to raise its
temperature 1 degree Celsius. A soft brick also requires 1 calorie per gram
of temperature rise but it weighs 1/3 or less than a hard brick.
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501

Larry Davidson on sat 21 aug 04

Geralyn, one thing you didn't mention that you need to consider is the
tightness on the kiln. with a bourry box kiln the draft is drawn through
the kiln where as a forced draft kiln the draft is blown through the kiln.
What this means is that you probably need to consider mortaring the bricks
together so you don't get any air sucked into the kiln. I have found a mix
of fire clay and sand works well, make it a little on the sandy side and
wet, so a hard brick will sink if set in it.
If you haven't gotten Layed back wood Firing by J. King and S. Harrison
you probably should get it.