sam wainford on sun 16 mar 97
Hi again Bill,
Whew! I'm not sure I have answers to all those questions. There are
potters with much more experience than I have on the Clayart list that
could probably give you more informed answers. It would really help if
you could look at some photos of the process. FIND Jack Troy's book
called "Salt-glazed Ceramics".
Here's a start on some answers. If you try any of this stuff, remember
these are just my opinions; travel at your own risk.
Bill Amsterlaw wrote------
1. Are there limits to the size of kiln you can make with this process?
2. Are there guidelines as to how thick the wall needs to be according
to the
size of the kiln - i.e., is a certain thickness required for a certain
span of
arch? Is this the stuff that can be used to make a "flat-top" kiln?
How
thick does a flat top need to be? How wide can it be?
~~~~~~ All materials have limits. I have no idea how large you could go
with that recipe. I'm sure there is a way to test for strength, load,
modulus of rupture etc, but I tend to go more intuitively. My kiln is a
catenary arch 4 ft high by 4 ft wide at base, and 5 inches thick. I'd
be afraid of a flat top made from this homemade mix without the benifit
of engineering. I guess if you made it thick enough, you could do it.
The longer the span, the thicker it would need to be. I used a castable
from A.P.Green (Mizzou, I think)to span about 3 feet over the collection
box of my old salt kiln. It's 5 inches thick and hasn't shown the first
sign of stress of any kind. But there's all kinds of specs available
from the manufacturer on their castables so you can find one for the
span you need for a flattop. Somehow homemade castable lends itself to
a catenary arch. It's such a clean design needing no steel for support.
3. Does the material have a pretty good insulation value?
~~~~~~~ I haven't fired it yet, so don't know.
4. Is the use of a 5-inch wall covered with kaowool less expensive?/more
insulating? than an 8-inch wall without kaowool?
~~~~~~~ One inch of 8# density kaowool is equal to 9 inches of
insulation brick. It would definitely be more insulating to use some
kaowool. Don't know about the cost. Grog is the most expensive
material in the mix. I used about 1200 lbs of grog alone in my kiln
including the floor.
5. How far apart would you recommend putting joints?
~~~~~~~ About every 3 to 4 feet--maybe one on each side for a catenary
arch.
6. Do you use the same material to make a stack/chimney?
You could. I'm using hi-heat brick, then thinking of switching to pipe
after about 6 feet to go up another 10 feet. This castable recipe may
not withstand bad weather if used in the chimney. My kiln is under a
pole building with tin roof. Chimney for a downdraft would extend out
over the roof and be exposed to the elements.
7. If you put stucco on the outside, do you need to build a shed over it
to
protect it from the weather?
~~~~~~~I always use a shed. The storms here are intense.
8. Will a kiln like this hold up well if exposed to harsh winters?
~~~~~~~????????I dunno
9. How do you mix it and wet it? Do you use a cement mixer, clay mixer,
hoe,
pugmill, or what?
~~~~~~~I did the floor with wheelbarrow and hoe. Mix dry, then when
thoroughly mixed, add water. Be careful. A little goes a long way and
it's easy to over wet it. I mixed one batch at a time. Being a woman
quickly approaching the half century mark, that's about all I could
handle by myself with one helper. If you're in good shape you can
prbably mix two. For the kiln, I rented an industrial cement mixer and
mixed two batches at at time. Your mix will be too stiff to pour so
transferring from mixer to form then ramming takes longer than you
think. Best if you have lots of helpers. No pug mill! Don't get cement
in your clay equipment.
10. In your previous post, you gave a recipe and also talked about
Pacocast 30. Which is better and how do they compare in cost?
~~~~~~~Pacocast 30 is a great material. It's more expensive than
homemade. I would have used it, but I already had about a ton of grog
that I got free, so I went with the homemade. I'd love to get my hands
on some PACO.
11. If you were making a castable kiln for salt, would you add alumina
hydrate
to the recipe you posted and if so, how much? Would you add alumina
hydrate
to Pacocast 30?
~~~~~~~I would definitely add alumina hydrate to the homemade probably
about 1/2 a bucket instead of the 1/2 buck misc in my recipe.
I had left over bags of high alumina castables(misc) that were very old
and partially set up. I used them for the alumina content and (after
breaking them back up)as an aggregate. If you use Pacocast30, DON'T ADD
ANYTHING TO IT.
12. Tell me if I have this right (because I am using my imagination to
fill in
some details).... You put up the inner arch covered with tarpaper and
shimmed
up on knockouts. Then you build one level of outer arch form, ramming
in the
castable between the inner and outer form and placing separation
material
where appropriate. When you have one level rammed, you build the outer
form
for the next level and ram that, and proceed level by level until you
get to
the top.
a. Is that right?
~~~~~~Yes, right but cut all your lumber ahead of time and fit it to
your arch. You need a front and back panel to screw it to. See Jack
Troy's book for photos.
b. Do you need to allow each level to harden before you build up the
next
level?
~~~~~~~No--do NOT allow it to harden. it will be much stronger if you
make a continuous pour. If it sets up before the next layer, it will
tend to crack there. It sets up pretty fast, so work fast ramming; have
your lumber cut and ready for your next layer. I put 3" to 4" wide
boards on about one foot at a time. If the kiln is deep, they may need
additional bracing to support the weight of the castable pushing out
against them..
c. How do you maintain the distance between inner and outer arch forms
as you
go up? Is the outer arch form bolted or otherwise attached to the inner
arch
form - or do you just support the outer form at the front and rear of
the
kiln?
~~~~~~~Yes at front and rear with plywood panels cut to shape and size
of outer arch. These panels determine the wall thickness of your kiln.
Screw flat side of panel to end of inner arch at front. You can cast
the back in place by leaving space there and holding the back panel up
with your outside layers of boards--which are screwed to the cut edge of
the plywood panel. This would all be clear if you could see Troy"s book.
d. How high should each level be?
~~~~~~~About a foot, or whatever you can reach down into to ram. If you
cant reach it you may get air gaps.
Bill, it's difficult to describe without benefit of visuals!
Maybe some of the pros out there can offer some additional ideas.
Sam Wainford in Portal Georgia where it just rained cats and dogs.
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