Craig Fent on fri 24 oct 97
Here's one for the kiln gurus. I have a raku kiln (about 6 cf) made of hard
brick. The bricks seem to be moving apart, the corners are not held together
by a frame. An old teacher of mine says that they are moving because of the
way I am firing the kiln. She says that it is building up pressure inside
and spreading the bricks. I say it's just that the expansion and contracting
of the bricks due to the heating and cooling over time has caused them to
crawl apart. BTW it's a flat top with a fiber door, 8"x8" burner opening and
flue. I don't see how any pressure could build in this kiln, not less
enough to push the bricks apart.
TIA
Craig Fent in Norman, OK where it is supposed to rain for days.
Craig Martell on sat 25 oct 97
At 08:00 AM 10/24/97 EDT, Craig Fent wrote:
>----------------------------Original message----------------------------
>I say it's just that the expansion and contracting
>of the bricks due to the heating and cooling over time has caused them to
>crawl apart.
Hi:
You're right. Hard bricks expand a lot. They expand more that soft brick
because of their greater density and they expand mostly along the length.
You might want to make an angle iron frame to hold the kiln together if you
get tired of pounding the bricks back into place.
regards, Craig Martell-Oregon
Cameron Harman on sun 26 oct 97
Further to my earlier comment:
all brick, hard and soft expand the same amount about 6.3 x
10 -6 inches per inch of length per degree F and they expand
the same amount in all directions. Don't forget that or you
can raise the roof when you don't expect it.
It is further complicated by the fact that the temperature
varies from the hot face back, so the average movement of a
soft brick may be less since it has a larger temperature
gradient from hot face to cold face, but the hot face will
move the full amount none-the-less. If the hot face is too
confined it will lead to "spalling" in which the edges or
even the whole face of the brick chips off. This happens
because the face moves and is constrained by other bricks
(no expansion joint) and the force is grater than the brick
can take, so it breaks, or "chips". This effect is sometimes
called "pinch spalling"
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
see our web site at http://www.kilnman.com
THE place for solutions to ALL your kiln and drier problems
**********************************************************
Ron Roy on tue 28 oct 97
I think this would be very difficult to prove. Hard brick and soft brick
come in many variations which means the materials in them varies. We all
know each oxide has a different coefficient of expansion so how could they
all expand and contract at the same rate?
This does not take into account what happens to the crystalline silica in
bricks after repeated firings. If you are over 1100C you are building up
cristobalite and therefore the movement at the beginning and the end of the
firing. The quartz in the brick is also being melted gradually so the
inversion at 573C is being lessened.
>----------------------------Original message----------------------------
>Further to my earlier comment:
>all brick, hard and soft expand the same amount about 6.3 x
>10 -6 inches per inch of length per degree F and they expand
>the same amount in all directions.
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
Cameron Harman on wed 29 oct 97
Ron Roy's reply indicated doubt about the average thermal
expansion of the bricks used in kiln building. Whereas you
are technically correct, Ron, the figures that I used have
been worked out in actual practice by kiln and furnace
builders many years ago. On the average the number is
correct, although it is not exact. It is useful because it
is on average close enough for expansion calculations used
in all types of kiln and furnace applications.
Part of the reason is that the "soft" bricks (IFB) are
nearly all made from a relatively pure fire clay with almost
no free silica. There is the presence of alumina in some of
the higher temperature grades.
The "hard" bricks are also nearly 100% fire clay with
various additions of alumina. So, while the expansion is not
exactly the same it is close and remains close enough over
the life of the brick.
Interestingly enough, when a kiln designer forgets to check
he can get into trouble. In one case a German company
designed a tunnel kiln for a location in Texas and did not
recalculate his expansion for the different brick being
used. That kiln used Chrome-Magnesite hot face lining for
3200 degree F use. The average expansion was so much greater
(than his usual brick) that when the kiln was first heated
to temperature it pushed the front roof section 8" into the
room and bent an 8" "H" beam in the process !!
So Roy, you are correct , its just in practice with the
bricks used for most ceramics the average about the same.
Over the useful life of the brick there is not enough
Cristobalite formed to affect the outcome.
I read your Dilatometery paper and thought it was quite
good. During my early years my father and I ran a Ceramics
laboratory. We developed a dilatometer to used at very high
temperatures (over 2500 F). I still have the 2" long single
crystal ("C" axis) Al2O3 bar we used to calibrate the
furnace with. There is no question that the Dilatometer can
be of great service to the Ceramist. Thanks, again, for your
paper.
regards,
Cameron
--
**********************************************************
Cameron G. Harman, Jr. 215-245-4040 fax 215-638-1812
e-mail kilns@kilnman.com
Ceramic Services, Inc 1060 Park Ave. Bensalem, PA 19020
see our web site at
http://www.kilnman.com/potters.html
THE place for solutions to ALL your kiln and drier problems
**********************************************************
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