Galen Kirkwood on sun 3 jan 99
------------------
Hi,
Big problems with bisqueing some 12-14=22 plates. Stacked 3 high total of =
6. All
cracked, some straight down the middle,
some in spiral and/or S shape. Also, where the foot of one rested on the top=
of
another one, some discoloration,(lighter
colored) where the foot rested on it. All the large plates were made with
recycled clay. The rest of the load,
bowls(small to med), goblets(stem and cups made of different bodies,same =
cone),
4 8=22 plates stacked 4 high(new clay) all
turned out just fine. The large plates were on the top and bottom shelves of=
the
kiln, so placement in the kiln(Skutt
KM1027 with Envirovent, allways on) does'nt seem to be issue. Fired to =
cone07
with a 10 min soak at 250, 1000, and 20
min at final temp. Total time of 9 hrs., 14 hrs cool down to 150. Since the
smaller plates(new clay) came out fine, I'm
thinking it has something to do with the recycled clay. Also, large plates
stacked 3 high while drying for months. Any
clues/suggestons? Sorry for the long post, just wanted to add all the =
variables.
TIA,
Galen
Mike Obrien on mon 4 jan 99
You didn't note your forming method. I know slip casting, especially cone 6.
A lot of the books recommend no more that 10% recycled clay in a casting slip.
I can tell you that we cast very large pieces at times with 100% recycled
clay, and get almost the same success rate.
Ron Roy on mon 4 jan 99
Hi Galen,
I am assuming the cracks are closed -
The problem is probably what we call bisque dunting - outside of the
platter goes through the silica quartz inversion (gets smaller) at 573C
(during cooling) before the inside of the platter - so the rim tries to get
smaller - the inside clay won't let it cause its hotter and the quartz
inversion is delayed. Has to crack. Platters are some of the worst forms
for this because the rims are up where they can cool faster and the feet
are on the kiln shelf - which is a heat sink (stores heat) anyway.
These kinds of cracks are open when they first happen - when the inside
clay cools enough for the quartz inversion to happen there - the cracks
will close up - of course.
The cure - bisque higher and make the clay stronger - say cone 04. Find a
way of preventing the rims from cooling down faster than the middle clay -
stacking rim to rim will probably work - as Vince will probably say, raise
the platter off the shelf with some shims so the foot will cool faster,
make your rims thicker and the bottom cross section thinner, ramp down from
say 700 to 500 so there is more even cooling.
The Hamer book covers this - 4th edition page 111 under Dunting - biscuit
dunting.
Now that you understand why it's happening you can figurer out you own
solution - which is what potters do most of the time - problem solve. Helps
if you know some of the science of course.
The discolouration is actually reduction - when certain volitiles in clay
ignite - like lignite (coal) the carbon wants some oxygen to become CO2. If
none is around it will take it from any iron in the clay - so in that
enclosed space there is not enough oxygen to prevent the reduction.
>----------------------------Original message----------------------------
>Big problems with bisqueing some 12-14" plates. Stacked 3 high total of 6. All
>cracked, some straight down the middle,
>some in spiral and/or S shape. Also, where the foot of one rested on the top of
>another one, some discoloration,(lighter
>colored) where the foot rested on it. All the large plates were made with
>recycled clay. The rest of the load,
>bowls(small to med), goblets(stem and cups made of different bodies,same cone),
>4 8" plates stacked 4 high(new clay) all
>turned out just fine. The large plates were on the top and bottom shelves
>of the
>kiln, so placement in the kiln(Skutt
>KM1027 with Envirovent, allways on) does'nt seem to be issue. Fired to cone07
>with a 10 min soak at 250, 1000, and 20
>min at final temp. Total time of 9 hrs., 14 hrs cool down to 150. Since the
>smaller plates(new clay) came out fine, I'm
>thinking it has something to do with the recycled clay. Also, large plates
>stacked 3 high while drying for months. Any
>clues/suggestons? Sorry for the long post, just wanted to add all the
>variables.
>
>TIA,
>Galen
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Craig Martell on mon 4 jan 99
Hello Galen:
My advice would be to raise the bisque temp to cone 04, especially if this
is a high fire body.
When you say, recycled clay, does this mean a mix of whatever has been used
here and there, or does it mean that you are using remixed production scrap
of a known, single clay? If you don't know the composition of the clay,
it's difficult to make sense of what's going right or wrong.
You can also box plates, which means setting them lip to lip, instead of
nesting them which gives a hot core and rims that cool faster. The main
thing, I think, is to raise the bisque temp to give the ware enough fired
strength to survive the quartz inversion during cool down.
regards, Craig Martell in Oregon
B. C. Tanzer on wed 6 jan 99
Try bisqueing the larger plates rim to rim and then the third one foot to
foot, if you must do a stack of three. Otherwise do 2 - 2 - 2 , rim to
rim. This has always worked for me.
Galen Kirkwood on wed 6 jan 99
------------------
Thanks for all the advice/pointers, folks=21 Sounds like several things may =
be the
culprit, but I think a lot of it may
have been rough handling before bisque. 14=22 plates stacked 4 high may =
have
been to much stress. Ron, you mentioned the
cracks being closed. Actually, six of the eight plates cracked down the =
middle
into two pieces. The other two were of
the spiral sort. Going over my records it looks like I fired at cone 04, not=
07.
I had heard that cone 04 can make some
glaze application diffucult. Been thinking about bisqueing to 06 cuz of =
that.
Should I stick with 04? Anyways, thanks to
all. I'll start by not stacking so high when drying and bisqueing, loading
plates on the middle shelves only, up on
stilts,rim to rim, and turn off my Envirovent to slow down the cooling.
Galen
Dale A. Neese on thu 7 jan 99
In as much as I love to glaze large platters and plates like a painting,
they are sure difficult to get them through the bisque sometimes. I have
attempted to cut my losses by taking more care in the handling, drying and
firing. When I bisque fired in the electric kilns, I always lost some. I
think the 1/2 shelves, three posted, flexed causing the cracking. Now I only
bisque in the gas with thicker, larger, some 24"X24" monster shelves. When
throwing, I really compress the bottoms, but leaving them thick enough to
trim a smooth foot ring on the bottom. Then I cover and dry slowly until
time to trim. I always place another bat on the rim to turn them over to
trim the bottoms. Next I use several layers of newspaper under the plate on
another bat while drying under plastic. I change the newspaper under the
foot when it absorbs moisture from the platter. Always using another bat to
turn, trying not to handle them. Slow, even drying keeps the platters from
warping. When completely dry I load the platters onto shelves sifted with
fine grog or silica sand. Just one on a shelf with platters over 20 inches.
When I stack plates, I use small squares of Kaowool spaced evenly around
the plate above the foot ring and place another plate on top. The next
squares of Kaowool go directly above the ones underneath. Three plates is
all I chance. I bisque fire to cone 08 and cool until I can unload with my
hands. Plates and other pots are just still warm enough for me to brush
plain wax from an electric skillet on the bottom as I unload. I don't have
to turn the skillet up where there is a lot of fumes. But I wear a dust
mask and ventilate the room avoiding waxy stiff nose hairs. Now applying the
glaze to the round canvas is really enjoyable. Sometime dipping the plate in
a large bottom of a cut off 55 gallon plastic barrel. Pouring glaze from a
pitcher or ladle. Or giving a splash with a turkey baster, kissing it with a
fling of glaze from a large bamboo brush. Then I reduction fire to flat cone
10. Three days before I even pull out the peeps. Nice!
Dale Tex
Predicting 70 degrees tomorrow in south Texas
-----Original Message-----
From: Galen Kirkwood
To: CLAYART@LSV.UKY.EDU
Date: Wednesday, January 06, 1999 2:32 PM
Subject: Re: Problem bisqueing plates
----------------------------Original message----------------------------
------------------
Thanks for all the advice/pointers, folks! Sounds like several things may be
the
culprit, but I think a lot of it may
have been rough handling before bisque. 14" plates stacked 4 high may have
been to much stress. Ron, you mentioned the
cracks being closed. Actually, six of the eight plates cracked down the
middle
into two pieces. The other two were of
the spiral sort. Going over my records it looks like I fired at cone 04, not
07.
I had heard that cone 04 can make some
glaze application diffucult. Been thinking about bisqueing to 06 cuz of
that.
Should I stick with 04? Anyways, thanks to
all. I'll start by not stacking so high when drying and bisqueing, loading
plates on the middle shelves only, up on
stilts,rim to rim, and turn off my Envirovent to slow down the cooling.
Galen
Ron Roy on fri 8 jan 99
Hi Galen,
I don't think it's rough handling - this sounds like a typical bisque
dunting problem - I should have included in my prevention list - shut off
the vent - and I would stay at cone 04 - I always have and it is most
important if it's porcelain or any other high silica body.
Cone 04 does not make glaze application difficult - as I said - I have
allways done it that way for cone 04, 6, 8 and 10 but you do have to make
your glazes a bit thicker. If you were only making small pots then it
probably does not matter what you bisque to - but the bigger the pots the
more likely you will have dunting simply because of the size and
differences in temperatures as the ware goes through the quartz inversion
at 573C.
RR
>----------------------------Original message----------------------------
>------------------
>Thanks for all the advice/pointers, folks! Sounds like several things may
>be the
>culprit, but I think a lot of it may
>have been rough handling before bisque. 14" plates stacked 4 high may have
>been to much stress. Ron, you mentioned the
>cracks being closed. Actually, six of the eight plates cracked down the middle
>into two pieces. The other two were of
>the spiral sort. Going over my records it looks like I fired at cone 04,
>not 07.
>I had heard that cone 04 can make some
>glaze application diffucult. Been thinking about bisqueing to 06 cuz of that.
>Should I stick with 04? Anyways, thanks to
>all. I'll start by not stacking so high when drying and bisqueing, loading
>plates on the middle shelves only, up on
>stilts,rim to rim, and turn off my Envirovent to slow down the cooling.
>
>Galen
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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