Your Name on wed 15 jan 97
I'm building another kiln but this time it's on a wooden floor. The
obvious thing to avoid is transmitting the kiln heat to the floor. I
plan to use 8"x 16" concrete blocks with their flat surface on the floor
with a 1/4" ceramic paper layer on top of them. The blocks will be held
tight by a steel strap wrapped completely around them. The first layer
of 2000 degree bricks will be laid on the paper insulation with the next
layer being 2300 degree bricks and the final interior bricks being
2600's. The kiln is to be 30 cubic ft gas fired down-draught. If anyone
knows a better way to protect the floor please pass it on to me.as I
want to do this one right the first time. Thanks for any advise.
Gavin Stairs on fri 17 jan 97
At 09:39 AM 15/01/97 EST, you wrote:
>----------------------------Original message----------------------------
>I'm building another kiln but this time it's on a wooden floor. The
>obvious thing to avoid is transmitting the kiln heat to the floor. I
>plan to use 8"x 16" concrete blocks with their flat surface on the floor
>with a 1/4" ceramic paper layer on top of them. The blocks will be held
>tight by a steel strap wrapped completely around them. The first layer
>of 2000 degree bricks will be laid on the paper insulation with the next
>layer being 2300 degree bricks and the final interior bricks being
>2600's. The kiln is to be 30 cubic ft gas fired down-draught. If anyone
>knows a better way to protect the floor please pass it on to me.as I
>want to do this one right the first time. Thanks for any advise.
Some R values (approximate):
8" concrete block (hollow) 1.1
8-1/2" cinder block (hollow) 2.8
8" clay tile, two compartments 1.8
1" common brick (hard) 0.2
1" insulating brick (soft) ~1
1" ceramic felt (board or paper) ~3 - 4
You don't say how your brick courses will be laid, and whether any will be
soft (insulating) bricks. Suppose you use only hard bricks, and lay them
flat, for 3-1/2in thickness each. That gives a total R to the concrete
block top of
3.5*3*0.2 + 0.25*3 = R22 at cone 10 (say 2500oF), the heat escaping will
be not more than 2500/22 = 114 BTU/ft^2/hr. That gives about 1140BTU/hr
through your floor (10ft^2). That's about 333Watts, which is a few light
bulbs. This might seem insignificant, but if you were to put 5 60W light
bulbs into an unventilated box a yard square for a while (lit, of course)
the box would get quite toasty. In the construction you have described, the
real function of the concrete blocks is to ventilate the box, so to speak.
What you need to do is to line up the holes in the blocks so that air can
circulate freely under the kiln. Also, make sure that you don't block these
holes with assorted junk.
Of course, if you lay the bricks thicker, or use insulating brick, the heat
escaping will be less, and the ventilation need less as well.
Let's look at the temperature profile in the floor. The inside is 2500oF.
The outside is a lot less, but we don't know it exactly. We certainly need
to have it below 200oF or so, so let's choose that as the outside temp.
Since the three courses of bricks all have the same insulating value, and
the ceramic paper is about the same, we know that the temperature drop
across each one is just 1/4 of the total, which is 2500 - 200 = 2200oF. So
the drop across eack layer is ~550oF. So, the profile looks like this:
inner wall 2500oF
first joint 1950oF
second joint 1400oF
inside of ceramic paper 750oF
outside of ceramic paper 200oF
This means that you can save a bit on the bricks. You need the 2600's on
the inside, but the next layer never gets hotter than 2000oF, so you can
drop right to 2000's and the final layer might even be common bricks. To be
sure, you could put the ceramic paper layer between the common bricks and
the first layer of fire bricks, which would keep the common bricks below
800oF or so. Aside from insulation, the role of the ceramic paper is to
keep the refractory bricks from feeling the joints in the concrete blocks
too much. A sort of soft pad. The common bricks probably won't mind that
too much, but you should try to keep the whole foundation layer flat. You
can set it in sand or grout to help if the floor is uneven. The other thing
the ceramic paper does is to catch drips. It helps to arrange the bricks so
the cracks don't line up. Also, you might want to use higher refractory
bricks near the burners, depending on how they are arranged.
Perhaps someone else on the list can check my numbers and comment?
Bye, Gavin
LOWELL BAKER on sat 18 jan 97
I would be as much concerned about rips in the paper as you are about
insulation. Anytime you get a tear or a rip you will have no
insulation value. The bricks and blocks will tend to move as the
kiln heats and cools. I would think building the kiln on a frame,
which would allow you to monitor the effects of the heat on the wood
floor, would be the best bet.
If you do not want to do that or cannot then I would suggest at least
two layers of insulation Paper where you indicate one now. You might
consider aluminum foil between the layers of paper or one layer of
foil and one of paper. Foil works well because if tends to reflect
some of the heat.
I would not sleep too well at night knowing I had a brick kiln on a
wooden floor.
Two years ago I was teaching a little workshop at a local high school
.. I had a small raku kiln (commercially built) on the back of my
flat bed trailer( wooden floor). The kiln had three layers of brick
for the floor and a layer of foil. after about an hour of firing I
started to smell burning wood. I washed the area under the kiln down
with water while the kiln was hot. after the workshop was over I
removed the kiln to find burning wood under the kiln floor. The
point on elevating the kiln is that if you can see what is going on
under the kiln you are safer. You can extinguish the fire or add
reflective material. If it is on the floor you might not notice the
fire until it is too late.
Build it up on a frame like an Alpine kiln and don't worry.
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