Laurie Kneppel on mon 29 sep 03
Hi Everyone,
I have asked a lot of different people about a cracking problem I am
having with some 14 inch diameter circular plaques. I never get the
same answer twice, so i am hoping that by posting it to a large group I
may get enough similar answers to form a consensus as to what I need to
do. I'm still sort of new to Clayart, but I have a feeling you all can
help with this problem. I've been doing something with clay since high
school when I was making little Easter Island statues for all my
friends. Been throwing since the mid-70's, took a decade off in the
80's, went back to sculpting in the 90's and now I am throwing again.
Once clay gets under your fingernails you can never quit it!
Anyway, the problem is I am getting cracks in most of these plaques.
They are about 3/8 to 1/2 inch in thickness, about 14" in diameter. I
have a nice Celtic spiral design carved into the clay. It's not very
deep and the cracks don't occur in the same place twice. I used to hand
carve all of the plaques and some of those were quite thin. They didn't
crack as often, but were overall more fragile. Since the handcarving of
Celtic designs is extremely time-consuming I made a mold of one of them
and that is what I have been using. The first three or four cracked
during drying, but I solved that by drying them very slowly between
plaster bats. I usually bisque to cone 04 in an electric kiln. The
plaques appear to bisque okay and I can't find any hairline cracks at
this point. I have fired them to cone 5 and cone 10 and find I am
getting two types of cracks - either a hairline going inwards from the
edge or a crack slightly off the center that may or may not extend to
the edge. Some just crack completely in half.
I like to use red clays for these plaques and inlay a green translucent
glaze into the design. People love them for their patios and gardens.
When they survive the firing, that is. I have tried IMCO's Quarry Tile
(discontinued), Navajo Wheel 412 and 8-11 red. IMCO's Sculpture 412
doesn't seem to crack, but also doesn't give me that terra cotta look
I'm after. Sculpture 412 is also an excellent raku clay. As for forming
methods I was just pounding the clay into the mold with a rubber mallet
and then using a large wooden rib to smooth it. Another potter
suggested rolling out a thick slab, turning it and rolling it in the
other direction to really compress the clay and to switch to the 8-11
Red, since it has a fair amount of grog. I used that method and so far
I have only fired a couple plaques to test and they both cracked in a
cone 10 reduction - one with a hairline through the middle (washed with
iron oxide only) and the other cracked nearly in half (covered with a
tenmoku glaze). I usually lay the plaques flat on a shelf. I asked a
couple other people about putting them up on a plate setter or
something so they are off the shelf, but two out of three said that
probably wouldn't help. I can send photos of my latest "victims"
showing the cracks if that might help. Otherwise my next test will be
the same method and clay but only fired to cone 5 or 6. I will also try
the sculpture clay to cone 10 to see if it survives.
Sorry this is so long, but I wanted to try to cover everything I've
tried
Thanks,
Laurie
Sacramento, CA
http://rockyraku.com
Paul Gerhold on tue 30 sep 03
Laurie,
Try to find a clay body that fits your visual requirements that contains
significant ( 12 or more ) percent kyanite. The Kyanite should preferably be 35
mesh although the 48 mesh might work. Kyanite is infinitely superior to grog in
reducing cracking.
For edge cracking you need to compress the clay inward somewhat at the edge.
For interior cracking you can reduce occurence by compressing into the mold in
both the radially outward and radially inward directions.
Also I suspect you do have bisque cracks they are just very small. Tap to see
if the clay rings or examine wet with a magnifying glass.
Paul
Valice Raffi on tue 30 sep 03
Hi Laurie,
I used Sculpture 412 as my primary clay for 15 years, and loved it's
versatilty. I used it low-fire (I know it's a cone 10 clay), cone 6 work,
raku, pit, wood - you name it. I did very tiny burnished things to large
sculptures and large slab pieces, and it took everything I did to it very
well.
A couple of times over the years, I would have a problem with it, no
biggie, Eric always made it right for me. But around the beginning of last
year, I started having some serious problems; slab work cracking apart in
the bisque fire, slip flaking off, and underglazes not adhering. There
were spiral and "straight line" cracks in the pugs, and sometimes a batch
of paperclay got mixed in which interfered with my slab work and carving.
Eric was great and said to bring it back, but the new batches were not any
better for me. All this started (I think) when the Ione Kaolin mine
closed. From things I've heard, the recipe for 412 contained very few
ingredients, so when one of those wasn't available any more, the whole
recipe became something else.
Admittedly, I don't wedge, so maybe the pugging problems are a non-issue
for you. But... I've cut 412 off the pug for 15 years. IMCO has always
been a great company to work with, and still is, and how wonderful to live
so close to the maker! My only problem with them has been with the 412.
AND, I always knew that I shouldn't rely on just one clay, gosh - what if I
moved!
I finally decided to try some other clays; Claymaker's Stout (^10) is very
similar to 412, even to the bisqued color, I've been using it (low-fire)
for my children's classes as it's very forgiving. Haven't tried it yet in
raku but it seems to have enough grog that it would do well. I did make
some very large slab pieces (body castings) with it and they came through
just fine. It's supposed to take wood fire nicely, maybe Paul Herman will
let me try a piece in his next fire (hint, hint!). I tried IMCO'S Concow
clay in Paul's last wood fire, it was nice with Gustine Shino on it, but I
didn't care for the large speckles in the unglazed areas (some people would
probably love that tho!).
I didn't really want to have a lot of different temperature clays hanging
around, but as long as I'm diligent about marking them, I should be ok.
For low-fire work, I've tried Claymaker's San Jose White and Laguna's
EM210, I'm just now working my way through a bag of Steve's White (not sure
what maker). I've liked them all. A more versatile (higher cone) white is
Ardvaark's Bee Mix w/sand. Large slab plates and bowls were fairly
vitrified (by the tongue test) at ^5, burnished well enough, didn't crack
and took glaze nicely.
I bought two sample boxes (4+#'s each of 8 clays) from Laguna, low fire and
cone 5, and I've been working my way though those too. 4lbs of each is not
enough to do much with, but I start with a pinch pot to see how I like the
feel and how well the clay responds, then make a stamped slab plate to see
how well it holds up to that. I've been lovin' working with the ^5
"porcelains".
I hope this has been a help, and if you want some 412, I have some to give away!
Valice
in Sacramento
Ron Roy on tue 30 sep 03
Hi Laurie,
I need more informatiom - look at the glaze where is is cracked - are the
edges sharp or not?
If sharp they are cracking on the way down in the glaze firing - if blunt
they have already cracked on the way up in the glaze firing.
Tell me if the partial cracks are open or closed when you unstack.
RR
>Anyway, the problem is I am getting cracks in most of these plaques.
>They are about 3/8 to 1/2 inch in thickness, about 14" in diameter. I
>have a nice Celtic spiral design carved into the clay. It's not very
>deep and the cracks don't occur in the same place twice. I used to hand
>carve all of the plaques and some of those were quite thin. They didn't
>crack as often, but were overall more fragile. Since the handcarving of
>Celtic designs is extremely time-consuming I made a mold of one of them
>and that is what I have been using. The first three or four cracked
>during drying, but I solved that by drying them very slowly between
>plaster bats. I usually bisque to cone 04 in an electric kiln. The
>plaques appear to bisque okay and I can't find any hairline cracks at
>this point. I have fired them to cone 5 and cone 10 and find I am
>getting two types of cracks - either a hairline going inwards from the
>edge or a crack slightly off the center that may or may not extend to
>the edge. Some just crack completely in half.
>
>I like to use red clays for these plaques and inlay a green translucent
>glaze into the design. People love them for their patios and gardens.
>When they survive the firing, that is. I have tried IMCO's Quarry Tile
>(discontinued), Navajo Wheel 412 and 8-11 red. IMCO's Sculpture 412
>doesn't seem to crack, but also doesn't give me that terra cotta look
>I'm after. Sculpture 412 is also an excellent raku clay. As for forming
>methods I was just pounding the clay into the mold with a rubber mallet
>and then using a large wooden rib to smooth it. Another potter
>suggested rolling out a thick slab, turning it and rolling it in the
>other direction to really compress the clay and to switch to the 8-11
>Red, since it has a fair amount of grog. I used that method and so far
>I have only fired a couple plaques to test and they both cracked in a
>cone 10 reduction - one with a hairline through the middle (washed with
>iron oxide only) and the other cracked nearly in half (covered with a
>tenmoku glaze). I usually lay the plaques flat on a shelf. I asked a
>couple other people about putting them up on a plate setter or
>something so they are off the shelf, but two out of three said that
>probably wouldn't help. I can send photos of my latest "victims"
>showing the cracks if that might help. Otherwise my next test will be
>the same method and clay but only fired to cone 5 or 6. I will also try
>the sculpture clay to cone 10 to see if it survives.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Kevin Ritter on tue 30 sep 03
Laurie,
I've had the same problem with my plaques. We've narrowed it down to two
problems. Moisture in the piece when it is fired and thermal shock. We found
that by getting the pieces off the shelf during the glaze firing, we have
virtually eliminated the problem. The shelf holds the heat while the air is much
cooler, the piece is hot on the bottom and cooler on the top, the stress is too
much and they crack. I still loose one once in a while, but not entire kiln
loads like before. Good Luck, Kevin, www.clayspirit.com
Kathie Wheater on tue 30 sep 03
Hi Laurie,
Have you tried drying the slabs between pieces of
drywall (sheetrock, gypsum board)? How about adding
grog to your red clay body you like so much? I noticed
that the clay bodies you've used that didn't crack were
specifically for sculpture. How large are the slabs 14"?
It sounds like the edge cracks maybe from uneven drying.
The edges dry faster. You could try rewetting the edges or
painting the edge with wax resist. It burns out in the bisque.
Cracks can happen towards the center for the same reason
especially if the thickness is greater towards the center. I
have to be very careful when drying double and triple wall
pieces and go as far as turning them over twice a day.
The crucial stage is between the leather hard and bone
dry phase I've noticed. That's when you need to make sure
it's even in color wet to dry. This is when warping and cracking
occurs. When I speed dry pieces that I know will warp a certain
way I slap them to strech the clay the opposite way of the warp
factor and then prop them. I do the same thing with the areas that crack.
At the ' tween phase I'll recompress the piece and flip it or
lay it on dry newspaper or board. I don't know if you can get
this out there but try Tucker's ^6 mid red if you can. It behaves very
well with the abuse I give it and fires to the color of a toasty
yummy terracotta that swoons with a spanish iron & rutile stain
I use. It even gets a slight olive color with that stain. Mossy.
When you fire your pieces make sure the shelf is flat
and try placing the pieces on a bed of grog or silica sand.
i get these cracks in about 10% of my porcelain slab pieces.
Then again, I'm pretty rude to them. They get put together
on the weekend, bisque on monday, glazed and fired wednesday
and at the market friday. Crack in green - into the bucket. Crack
in bisque - test tile. Crack in glaze fire - I get to use them (if not
too severe). I can see it in the headlines:
Potters Kids Eat With Crackpots!
Hope that helps because I just broke my own personal record
for typing (outside of doing medical transcription)!!! Yeeech!
KathieW
Laurie Kneppel on wed 1 oct 03
Hi Ron,
The edges are razor sharp and the cracks are open. This might be a good
clue - one of the recent cracked plaques is cracked nearly in half
(glazed with a tenmoku) and the center of the crack is open wider than
the rim areas. The other one that was in that same firing (cone 10
reduction. but with iron oxide applied, no glaze) appears to be cracked
in the interior only (as far as i can tell by looking without a
magnifying glass) and the crack is barely visible on the top, but is
quite large on the underside. The clay used for these two was IMCO 8-11
red 412 which has a 35 mesh grog and I have never had it crack with any
other shapes except these.
Thank you for your thoughts on this!
Laurie
Sacramento, CA
http://rockyraku.com
On Tuesday, September 30, 2003, at 11:50 AM, Ron Roy wrote:
> Hi Laurie,
>
> I need more informatiom - look at the glaze where is is cracked - are
> the
> edges sharp or not?
>
> If sharp they are cracking on the way down in the glaze firing - if
> blunt
> they have already cracked on the way up in the glaze firing.
>
> Tell me if the partial cracks are open or closed when you unstack.
>
> RR
>
>
>> Anyway, the problem is I am getting cracks in most of these plaques.
>> They are about 3/8 to 1/2 inch in thickness, about 14" in diameter. I
>> have a nice Celtic spiral design carved into the clay. It's not very
>> deep and the cracks don't occur in the same place twice. I used to
>> hand
>> carve all of the plaques and some of those were quite thin. They
>> didn't
>> crack as often, but were overall more fragile. Since the handcarving
>> of
>> Celtic designs is extremely time-consuming I made a mold of one of
>> them
>> and that is what I have been using. The first three or four cracked
>> during drying, but I solved that by drying them very slowly between
>> plaster bats. I usually bisque to cone 04 in an electric kiln. The
>> plaques appear to bisque okay and I can't find any hairline cracks at
>> this point. I have fired them to cone 5 and cone 10 and find I am
>> getting two types of cracks - either a hairline going inwards from the
>> edge or a crack slightly off the center that may or may not extend to
>> the edge. Some just crack completely in half.
>>
>> I like to use red clays for these plaques and inlay a green
>> translucent
>> glaze into the design. People love them for their patios and gardens.
>> When they survive the firing, that is. I have tried IMCO's Quarry Tile
>> (discontinued), Navajo Wheel 412 and 8-11 red. IMCO's Sculpture 412
>> doesn't seem to crack, but also doesn't give me that terra cotta look
>> I'm after. Sculpture 412 is also an excellent raku clay. As for
>> forming
>> methods I was just pounding the clay into the mold with a rubber
>> mallet
>> and then using a large wooden rib to smooth it. Another potter
>> suggested rolling out a thick slab, turning it and rolling it in the
>> other direction to really compress the clay and to switch to the 8-11
>> Red, since it has a fair amount of grog. I used that method and so far
>> I have only fired a couple plaques to test and they both cracked in a
>> cone 10 reduction - one with a hairline through the middle (washed
>> with
>> iron oxide only) and the other cracked nearly in half (covered with a
>> tenmoku glaze). I usually lay the plaques flat on a shelf. I asked a
>> couple other people about putting them up on a plate setter or
>> something so they are off the shelf, but two out of three said that
>> probably wouldn't help. I can send photos of my latest "victims"
>> showing the cracks if that might help. Otherwise my next test will be
>> the same method and clay but only fired to cone 5 or 6. I will also
>> try
>> the sculpture clay to cone 10 to see if it survives.
>
> Ron Roy
> RR#4
> 15084 Little Lake Road
> Brighton, Ontario
> Canada
> K0K 1H0
> Phone: 613-475-9544
> Fax: 613-475-3513
>
> _______________________________________________________________________
> _______
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
Laurie Kneppel on wed 1 oct 03
Thank you Kevin and Kathie and everyone else who has responded!
Based on what you all have said, I am more sure than ever that the main
problem I'm having is with thermal shock and that it is happening
during the cooling down phase.
Laurie
Sacramento, CA
http://rockyraku.com
On Tuesday, September 30, 2003, at 10:14 AM, Kevin Ritter wrote:
> Laurie,
>
> I've had the same problem with my plaques. We've narrowed it down to
> two
> problems. Moisture in the piece when it is fired and thermal shock.
> We found
> that by getting the pieces off the shelf during the glaze firing, we
> have
> virtually eliminated the problem. The shelf holds the heat while the
> air is much
> cooler, the piece is hot on the bottom and cooler on the top, the
> stress is too
> much and they crack. I still loose one once in a while, but not
> entire kiln
> loads like before. Good Luck, Kevin, www.clayspirit.com
>
> _______________________________________________________________________
> _______
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
Laura Kneppel on wed 1 oct 03
Hi Valice,
Yes, it is nice to live near the manufacturer of your clay - saves a
lot on transportation and shipping, plus you get to talk to the people
who work there and Eric is a very valuable resource. Other potters i
know also had problems with the first batches of the "new" Sculpture
412 after the Ione kaolin mine closed. To be honest i have such a large
"stash" of the older stuff that i haven't used the new version yet. I
have a box of it, though! Like you, I have used the old 412 for just
about everything you can do with clay - certainly an extremely
versatile clay!
I always wedge clay before throwing it, but for sculpting very rarely
unless it was recycled wet scraps (not ribbons, more like chunk-sized).
412 has always been THE raku clay for me. I have tried the Concow and
it goes through the process okay, but for me it doesn't seem to turn
very black and I don't get the great crackle - maybe I need to try
another white crackle recipe that works better on Concow. In spite of
the barium I have always loved Harriet Brisson's Crackle White.
I know several people who like the Stout Stoneware for raku and they
have had great results with it. Since I've been throwing again I've
been trying out a lot of different cone 10 stoneware and porcelain. Got
some nice porcelain from East Bay Clay, don't know what it's called - I
traded some boxes of junior cones for it - but it throws real nice.
Is your give away 412 "old" or "new"?
Laurie
Sacramento, CA
http://rockyraku.com
On Tuesday, September 30, 2003, at 07:44 AM, Valice Raffi wrote:
> Hi Laurie,
>
> I used Sculpture 412 as my primary clay for 15 years, and loved it's
> versatilty.....
>
> I finally decided to try some other clays; Claymaker's Stout (^10) is
> very
> similar to 412, even to the bisqued color,....
> I hope this has been a help, and if you want some 412, I have some to
> give away!
>
> Valice
> in Sacramento
Ron Roy on thu 2 oct 03
Hi Laurie,
It certainly is a good clue - it means the cracking occured after the glaze
had frozen - this can mean only two things - quartz inversion at 573C or
cristobalite around 200C.
High iron bodies are prown to developing cristobalite if they are not
formulated properly - you might ask you clay supplier what % of fine silica
is in that body and what % of spar - that will tell the tale.
Firing lower might help - fire as fast as you can from 1100C - crash
cooling down to 1100C will cut down on the amount of cristobalite formed -
soaking is just giving the cristobalite more time to form.
If you will send me a bar to test in my dilatometer I will do it - if you
are willing let me know and I will tell you how to do it. It's easy - just
a 3" bar but it needs to be made the way I need it.
RR
>The edges are razor sharp and the cracks are open. This might be a good
>clue - one of the recent cracked plaques is cracked nearly in half
>(glazed with a tenmoku) and the center of the crack is open wider than
>the rim areas. The other one that was in that same firing (cone 10
>reduction. but with iron oxide applied, no glaze) appears to be cracked
>in the interior only (as far as i can tell by looking without a
>magnifying glass) and the crack is barely visible on the top, but is
>quite large on the underside. The clay used for these two was IMCO 8-11
>red 412 which has a 35 mesh grog and I have never had it crack with any
>other shapes except these.
>
>Thank you for your thoughts on this!
>
>Laurie
>Sacramento, CA
>http://rockyraku.com
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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