klay on wed 12 jul 00
* muddy hands up in the air...*
Recently i have been throwing some nice pots but the bottoms are blowing up
in the bisque..
My heart wants to think the bottoms are too thick but they are trimmed so
nicely...and the slab
pieces i am making at the same time are surviving nicely so drying isn't the
problem...
Whatzthisabout?
katie
klay@pcola.gulf.net
Cindy Strnad on wed 12 jul 00
Katie,
It sure sounds like the bottoms are too thick. Have you tried breaking the
blown-out pots to check on the thickness of the bases? Also, you probably
should try firing slower until you reach boiling point. Hold below boiling
for several hours. If you have a small kiln, this may mean firing for a time
with the lid completely open. Use an over thermometer if you need to, but be
sure to get it out of the kiln before you close the lid to do your "real"
firing.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
earthenv@gwtc.net
Bruce Girrell on wed 12 jul 00
Katie, with muddy paws in the air, wrote
>Recently i have been throwing some nice pots but the bottoms are blowing up
>in the bisque..
If the bottoms are indeed blowing up, not just cracking badly, then the only
answer is that there is water remaining in them for whatever reason.
Sometimes I put non-dry, even leather hard pieces in our kiln because we are
short on time. I do an overnight candle (6-8 hours) with a very small
burner, making sure that the temperature gets no higher than 100 oC and I
haven't lost a single one to steam.
Bruce and Lynne Girrell
in Northern Michigan where we will do just exactly that this week as we turn
around from a show last weekend, throw, bisque, and glaze pots this week to
do another show this weekend.
Donald G. Goldsobel on wed 12 jul 00
Katie,
If you let too much water settle in the bottom of the pot while throwing
the moistue differential can cause the bottoms to crack or even separate
near the foot ring. Sponge out the bottom frequently and just before
removal from the wheehead or cutting from your bat.
Donald
At 07:48 AM 07/12/2000 -0500, you wrote:
>* muddy hands up in the air...*
>Recently i have been throwing some nice pots but the bottoms are blowing up
>in the bisque..
>
Pottery by Dai on wed 12 jul 00
Hi, katie - it does sound like a moisture problem. The slab things you make
at the same time will not have nearly the moisture content that a thrown
piece will have --- unless you throw with NO water. Let them dry longer and
see if that solves the problem; maybe turn them over for the last part of
the drying.
Dai in Kelowna---heading for Vancouver Island on Friday, with pots and
teenage granddaughter. I'm learning why women quit bearing children after a
certain age!
potterybydai@home.com
There is an alarming increase in the number of things that I know nothing
about.
-----Original Message-----
From: klay
To: CLAYART@LSV.CERAMICS.ORG
Date: Wednesday, July 12, 2000 6:49 AM
Subject: bombed out pot bottoms..
>* muddy hands up in the air...*
>Recently i have been throwing some nice pots but the bottoms are blowing up
>in the bisque..
>My heart wants to think the bottoms are too thick but they are trimmed so
>nicely...and the slab
>pieces i am making at the same time are surviving nicely so drying isn't
the
>problem...
>Whatzthisabout?
>katie
>klay@pcola.gulf.net
>
>___________________________________________________________________________
___
>Send postings to clayart@lsv.ceramics.org
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melpots@pclink.com.
Ray Aldridge on wed 12 jul 00
At 07:48 AM 7/12/00 -0500, you wrote:
>* muddy hands up in the air...*
>Recently i have been throwing some nice pots but the bottoms are blowing up
>in the bisque..
In my experience, this is almost always due to too-fast firing in the early
water-smoking stage.
Ray
Aldridge Porcelain and Stoneware
http://www.goodpots.com
sandylacross@NETHAWK.COM on wed 12 jul 00
Hello,
I had the same problem with bottoms blowing up a while back and what I
did to solve this problem was to raise the pots from the kiln shelf. I
would make three small piles of grog or silica sand on the shelf and
place the ware on that. longer firing was also helpful. I found that
taking more time to warm the work the fewer the problems.
good luck,
Curt
Pierre Brayford on wed 12 jul 00
If I have any doubts about the dryness of pots or if the pot is in some
other way needing special care I biscuit fire it upside down (if possible).
The base can then lose water(steam) more easily. This in addition to taking
it slowly in the first stages of firing.
An aside, but related!
I have recently been making quite a few "pebbles" (hollow vaguely spherical)
with no holes in. As long as the firing is slow these survive OK, but I have
exploded some of the bigger thicker ones. What has surprised me is the
temperature at which this explosion has happened, much higher than I would
have thought, at about 500 deg C when I had assumed all water would have
departed!
Pierre
http://www.ukpotters.co.uk
>My heart wants to think the bottoms are too thick but they are trimmed so
>nicely...and the slab
>pieces i am making at the same time are surviving nicely so drying isn't
the
>problem...
>Whatzthisabout?
>katie
>klay@pcola.gulf.net
>
>___________________________________________________________________________
>Recently i have been throwing some nice pots but the bottoms are blowing up
>in the bisque..
>My heart wants to think the bottoms are too thick but they are trimmed so
>nicely...and the slab
>pieces i am making at the same time are surviving nicely so drying isn't
the
>problem...
>Whatzthisabout?
>katie
>klay@pcola.gulf.net
>
>___________________________________________________________________________
___
Andrew Buck on wed 12 jul 00
Katie
It has been my experience that along with going slow through the 212 degree
F. boiling point of water, you should avoid sudden increases in temperature
at 500 deg. also. I believe these are the temperatures at which most of the
actual blow-ups occur. Major cracking will happen if you increase temperature
unevenly through quartz inversion temperatures between 900 and 1200 deg., but
most of the blow-ups happen at lower temperature. If I am wrong about this,
and I well might be, I am sure we will hear about it, however it has been the
rule that I fire by and has worked well for me. Best luck in future firings.
Andy Buck
Raincreek Pottery
Port Orchard, Washington
Bruce Girrell on thu 13 jul 00
Pierre Brayford wrote:
>What has surprised me is the
>temperature at which this explosion has happened, much higher than I would
>have thought, at about 500 deg C when I had assumed all water would have
>departed!
Water exists in two states in clay. Some of the water is held in the pore
space of the clay. That is the water that will form steam at 100 oC. If you
were to drive that water out of the clay at or below 100 oC, then let the
material cool, grind it up and add water, you would end up with clay again.
Even when a piece has been dried to a temperature above 100 oC, there is
still water remaining in the clay. That is the water that is electrically
bound to the surface of the clay platelets. It is necessary to heat the clay
to a temperature of about 500 oC (someone have a more accurate number? Hamer
and Hamer do, but I don't have the book here) to drive off this bound water.
It is this water that caused the explosions that you refer to. If you were
to heat the clay sufficiently to drive off the bound water, then let the
material cool, grind it up and add water, you would _not_ end up with clay
again. You now have grog.
Bruce Girrell
in northern Michigan with the temperature climbing on a set of less-than-dry
pots right now. Bisque this evening, cool tonight, glaze and fire again
tomorrow. Show Saturday - 7:30 AM setup time.
Michael Banks on fri 14 jul 00
Bruce,
Kaolinite -the most common clay mineral in clays that potters use, loses
it's "chemical water" between about 500 to 620oC, peaking usually at about
570oC. Actually this "water" is hydroxyl ions (OH-) which are covalently
bonded to three out of the six apices on the alumina octahedrons in the
kaolinite structure. The temperature range of this loss of hydroxyl can be
measured accurately for a particular clay by differential thermal analysis
(DTA). DTA consists of progressively heating of a sample whilst measuring
the amount of heat absorbed or emitted by the sample. Loss of "water of
plasticity" at ~100 degrees and hydroxyl loss at ~570, is marked by
endothermic peaks.
The hydroxyl groups are there to maintain electrical neutrality in the
octahedral layer and once they boil-off at ~570oC, the clay becomes a new
compound called metakaolinite. This is an unstable mixture of alumina &
silica which is incapable of reabsorbing water (when cooled) to recombine as
clay and recrystallizes into mullite and cristobalite on further heating.
Clay can usually be reconstituted (by adding water) if fired below 450
degrees, but progressively undergoes the irreversible reaction to metakaolin
above this. As 450 degrees is beyond most domestic ovens, they are not
effective at carrying out biscuit firings :)
Michael Banks,
Nelson,
NZ
----- Original Message -----
From: Bruce Girrell wrote:
> Water exists in two states in clay. Some of the water is held in the pore
> space of the clay. That is the water that will form steam at 100 oC. If
you
> were to drive that water out of the clay at or below 100 oC, then let the
> material cool, grind it up and add water, you would end up with clay
again.
>
> Even when a piece has been dried to a temperature above 100 oC, there is
> still water remaining in the clay. That is the water that is electrically
> bound to the surface of the clay platelets. It is necessary to heat the
clay
> to a temperature of about 500 oC (someone have a more accurate number? (Sn
ip)
Daisypet@AOL.COM on fri 14 jul 00
Katie: To help remove or equalize any residual moisture, can you turn the pot
upside down or stand it on a plaster bat? Has anyone mentioned extra
compression of the bottom during the throwing process? What a disappointment
to have the bottoms blow. With all the ideas from this wonderful group, your
problems should be solved.
Phyllis Tilton
Daisypet@aol.com
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