maggie j jones on mon 16 mar 98
What do you do when your supplier changes the formula of a clay that you
have been using for 18 yrs and you begin to get serious breakage due to
dunting? testing is undrway, so far glaze changes in flint specifically
do not effect it. Mags
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the cat lady on tue 17 mar 98
At 11:36 AM 3/16/98 EST, you wrote:
>----------------------------Original message----------------------------
>What do you do when your supplier changes the formula of a clay that you
>have been using for 18 yrs and you begin to get serious breakage due to
>dunting? testing is undrway, so far glaze changes in flint specifically
>do not effect it. Mags
>
>_____________________________________________________________________
For starters, you get the supplier to replace *for free* all the
clay that they sold you before they had the courtesy to inform
you that they changed formula.
As well, if they are responsible, they should provide glaze
adjustments by a trained professional (ie. our own Ron Roy).
Please tell us the supplier that all may avoid this hassle
in the near future.
sam - alias the cat lady
Melbourne, Ontario
SW Ontario CANADA
http://www.geocities.com/paris/3110
maggie j jones on wed 18 mar 98
Thanks for your input...Tom Buck, KLesleur and the cat lady.
I've been using Brownstone at cone 5 from Highwater Clays in Asheville
NC for 18 yrs...It's boring so perhaps this is natures way of shoving me
into change! I am still testing, had to wait while Brian mixed up a
batch of the old formula. It occurred to me I need to test his current
supply too, perhaps i just got a bad batch.
Firing without the glaze is a great idea... and I am waiting on ring
tests for expansion results. Probably will take care of reimursment
after NCECA when he gets back. So far i have not added up my loss but
it's 1 ton of clay in the form of biskware and broken and in boxes! Plus
the fact that production has halted. So far he has mixed 1000 of the
old formula for free. Testing every new Batch????? Guess I'll have to
work that one in!........ It has forced me to play with porcelain and
some wild clays for sculpture work!
Mags, on Stone mtn in NC
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Ron Roy on wed 18 mar 98
Hi Mags,
What clay company? How did they change the body? What did they say they
would do to help you? Why did they change the body? What cone are you
firing at? Are all your glazes cracking pots or just some?
If you want me to assess your glazes from an expansion point of view send
the recipes directly to me - don't worry about me keeping your glazes
confidential - just make sure you send them to my E-mail address.
>----------------------------Original message----------------------------
>What do you do when your supplier changes the formula of a clay that you
>have been using for 18 yrs and you begin to get serious breakage due to
>dunting? testing is undrway, so far glaze changes in flint specifically
>do not effect it. Mags
>
>_____________________________________________________________________
>You don't need to buy Internet access to use free Internet e-mail.
>Get completely free e-mail from Juno at http://www.juno.com
>Or call Juno at (800) 654-JUNO [654-5866]
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
Janet H Walker on wed 18 mar 98
...You find out that your supplier has changed the clay
formula,( his formula), so you discover you've been working with 2
clays and didn't know it.
...Any feedback apreciated!...
Life is short. Change suppliers. Choose one that tests, tests, and
tests again, aims for a consistent product, knows what its
characteristics are, and doesn't pull the rug from under you by
changing formulae or ingredients without notice.
Are there such suppliers? I don't know for sure I would venture to
guess that either Plainsman Clay or Pottery Supply House a.k.a.
Tucker's would be a good place to start asking questions. At least
as long as Tony Hansen and Ron Roy (respectively) work there. Those
two fine Canajun gentlemen have done much to educate me about the
nature of clay bodies and about my expectations/responsibilities as
a buyer.
If there are other such suppliers, I await information about them
with eagerness and I wish their technical people were active on
ClayArt in order to help Tony & Roy in educating us consumers.
I am in the midst of waiting for my sample bags of Tucker's clay
(ordered through Bailey). I have decided that I don't care what it
costs (shipping adds about 50% to the cost) since the clay cost is
really such a tiny tiny piece of the whole cost picture. The time I
have to put into playing catchup/detective with clay changes every
few batches is completely wasted time. And I'm tired of having it
wasted. I want to move on to playing glaze detective...
Jan Walker
Cambridge MA USA
JLHclay on wed 18 mar 98
What would I do? Scream, swear, moan, curse the supplier, stamp my feet. I
bet you already thought of those. Ask the supplier if they plan to return to
the old mix, request a refund, I know you only get the price of the clay
return to first 5 suggestions, test out new bodies, raise your prices, extend
your order times, take a break, perhaps use 2 or 3 standard clay bodies
instead of depending on one, ask for moral support from friends & family. Joy
in Tucson waiting for my wild poppies to pop forth.
Gavin Stairs on thu 19 mar 98
At 09:00 AM 3/18/98 EST, Janet wrote:
....
>guess that either Plainsman Clay or Pottery Supply House a.k.a.
>Tucker's would be a good place to start asking questions. ...
PSH carries Plainsman clays and bodies, as well
as their own, and a couple others. They are in Oakville and London, Ontario.
Tucker's carries it's own line of bodies. Ron Roy does
their quality control. They are in Markham or Vaughn, Ontario, just above
Toronto off Hwy. 7 and Leslie St.
Plainsman makes bodies and
wholesales its own clays. They deal through Greenbarn and PSH, among
others, and would love to get into the states more, I think. Tony Hansen
does their quality control.
Gavin
Stuart Ridgway on thu 19 mar 98
Mags
What is a ring test for expansion?
Thanks in Advance
Stuart Ridgway
>----------------------------Original message----------------------------
>Thanks for your input...Tom Buck, KLesleur and the cat lady.
> I've been using Brownstone at cone 5 from Highwater Clays in Asheville
>NC for 18 yrs...It's boring so perhaps this is natures way of shoving me
>into change! I am still testing, had to wait while Brian mixed up a
>batch of the old formula. It occurred to me I need to test his current
>supply too, perhaps i just got a bad batch.
> Firing without the glaze is a great idea... and I am waiting on ring
>tests for expansion results. Probably will take care of reimursment
>after NCECA when he gets back. So far i have not added up my loss but
>it's 1 ton of clay in the form of biskware and broken and in boxes! Plus
>the fact that production has halted. So far he has mixed 1000 of the
>old formula for free. Testing every new Batch????? Guess I'll have to
>work that one in!........ It has forced me to play with porcelain and
>some wild clays for sculpture work!
>
>Mags, on Stone mtn in NC
>
>_____________________________________________________________________
>You don't need to buy Internet access to use free Internet e-mail.
>Get completely free e-mail from Juno at http://www.juno.com
>Or call Juno at (800) 654-JUNO [654-5866]
>
maggie j jones on fri 20 mar 98
On Thu, 19 Mar 1998 10:13:42 EST Stuart Ridgway writes:
>----------------------------Original
>message----------------------------
>Mags
>
>What is a ring test for expansion?
>
>Thanks in Advance
>Stuart Ridgway
>
>
The first time i have done it...rings of clay with a slot and glazed on
one side. Measure the space before and after fire to see if glaze is
shrinking more or less than the clay. less will close the gap and more
will pull it open. My tests were so close it was difficult to tell. Now
i'm wondering...can a glaze craze so much on a flat piece ie. pie plate,
that it breaks the clay? clay pulling in opposite direction of glaze?
mags in NC.
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Kristin J Busch on fri 7 aug 98
I was wondering if anyone could give me some advice about a dunting
problem I'm having on plates and wide bowls. I'm using floating Blue as a
base glaze with a store bought glaze from Minnisota Clay Called Nebula, on
the rims and spattered over it. Both glazes are cone 5/6 and I'm firing
them to this temp in an electric Kiln.
the recipe I have is:
gerstly borate 27
Neph. sy. 47.3
epk 5.4
flint 20.4
cobalt carb. 2
iron ox. 2
rutile 4
The clay body is a brown stoneware from AMACO in Indianapolis rated cone
5-7. The info they supply about the clay is 14.2% shrinkage and 3%
absorption at cone 5 and 14.4% shrinkage and .4% absorbtion at cone 7.
I'm using this glaze combonation on vases ranging in size from small
bottles to 2 foot tall vases, pitchers, mugs, soup size bowls and am
having no problems with these forms. The glaze is crazed on these forms
but no other problems.
When the plates and bowls come out of the kiln the glaze is crazed, but
hours or sometimes days later I will hear a loud crack and the plate or
bowl is history. At first I thought I was opening the kiln to soon, but
then I let this last firing cool until it was room temp and I still had
the dunting. Also I'm using a vent attached to the bottom of the Kiln for
the firing and cooling and was told by the guy I purchased it from that
if plates are on the top shelf they may crack from the air coming in the
vent holes in the lid, so I make sure there is a shelf above the top
plate.
So hear are my qusetions, why is this happening only on the plates
and bowls and not the other pieces? Also it doesn't happen with all the
pieces, so are the uncracked ones bound to crack eventually?
From what I have read in the Hammer book the glaze hasn't shrunk as much
as the clay so it pulls on the clay until it gives, but why doesn't the
crazing relieve this tension?
And finally how should I adjust the glaze to rid myself of this problem?
I've looked through the archives about this glaze and dunting and found
some help, but any other advice would be greatly appreciated.
Kris Busch
bloomington In
Gavin Stairs on sat 8 aug 98
At 01:59 PM 8/7/98 EDT, Kris Busch wrote:
....
>I'm using this glaze combonation on vases ranging in size from small
>bottles to 2 foot tall vases, pitchers, mugs, soup size bowls and am
>having no problems with these forms. The glaze is crazed on these forms
>but no other problems.
>
>When the plates and bowls come out of the kiln the glaze is crazed, but
>hours or sometimes days later I will hear a loud crack and the plate or
>bowl is history. ...
Usually, crazing and dunting are at opposite end of the glaze fault
spectrum. Since you have both problems, I suspect one of two cases:
1. Your crazing glaze is weakening the body, which then gives in to slight
body stresses.
2. It is a body problem unrelated to the glaze.
I note that you see dunting only with the open forms, plates and bowls.
This leads me to suppose that your problem is one of lack of "compression".
That is, your forms have been thrown or formed in such a way as to cause
them to dry with residual tension, probably in the centre of the form. The
solution may be to rib or otherwise compress the clay at the centre while
throwing, rather than to simply open and pull.
The reason this would be more of a problem with the open forms than with
more strongly curved ones is a matter of geometry. With the strong curves,
the tension developed in the centre is less than it is with a flatter
curve, since the line of the force follows the curve, and ends up pulling
up rather than out. So you should do your compression ribbing after you
have made the final form, not before. Just reach in and push down with a
rib, preferably while moving in rather than out.
As to whether the unbroken pieces will eventually succumb, I would guess
that they are probably weak, and susceptible to failure later on, as when
the plate is full of hot food. You should test them by the cold/hot
method, taking care not to scald yourself if a bowl should break while
being filled with boiling water. Put the plate/bowl in the freezer
overnight. Next day, boil enough water to fill the bowl, or some extra to
pour over the plate. Then take the pot out of the freezer, place it in
your kitchen sink or in a large tub, and pour boiling water into it. Don't
let the pot warm up before pouring the boiling water. I would expect that
many of your plates and wide bowls will fail this test, and perhaps a few
of the closed forms, too. Better you should find this out yourself than
after a customer has tried to fill a cold bowl with hot soup.
If you want to see whether the body has the problem independent of the
glaze, fire some pots to maturity without the glaze, and test as above. If
the unglazed pots survive, then you must correct the glaze. Note that a
single pot may not be enough, if you are seeing the problem only some of
the time. First try it on the glazed forms and see what percentage of them
survive. Then you can figure out how many unglazed pots to try: if
80-100% fail, then two or three pots will suffice. If 50-80% fail, then
add a couple more. And so on.
This is apart from the glaze crazing, which you should attempt to correct.
I will leave that part of the problem to the glaze gurus.
Gavin
Ron Roy on sat 8 aug 98
Hi Kris,
This is curious because crazed glazes don't crack pots (you are right - the
crazing should relieve any stress.) The calculated expansion of the glaze
you listed is not low enough to cause dunting unless the clay you are using
has an unusually high contraction on cooling.
How long is your glaze firing and cooling?
I also wonder if the crazing you see is the opposite - It is possible to
have cracks in a glaze that can be mistaken for crazing - any spiral or
vertical cracks? Have you tried glazing without the commercial glaze on
top?
Could it be you are leaving your vent system on while cooling - if so you
should shut it off and see if that makes a difference.
>----------------------------Original message----------------------------
>I'm using this glaze combination on vases ranging in size from small
>bottles to 2 foot tall vases, pitchers, mugs, soup size bowls and am
>having no problems with these forms. The glaze is crazed on these forms
>but no other problems.
>
>When the plates and bowls come out of the kiln the glaze is crazed, but
>hours or sometimes days later I will hear a loud crack and the plate or
>bowl is history. At first I thought I was opening the kiln to soon, but
>then I let this last firing cool until it was room temp and I still had
>the dunting. Also I'm using a vent attached to the bottom of the Kiln for
>the firing and cooling and was told by the guy I purchased it from that
>if plates are on the top shelf they may crack from the air coming in the
>vent holes in the lid, so I make sure there is a shelf above the top
>plate.
> So hear are my qusetions, why is this happening only on the plates
>and bowls and not the other pieces? Also it doesn't happen with all the
>pieces, so are the uncracked ones bound to crack eventually?
>
>From what I have read in the Hammer book the glaze hasn't shrunk as much
>as the clay so it pulls on the clay until it gives, but why doesn't the
>crazing relieve this tension?
>Kris Busch
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
(Kathe Umemoto) on sun 9 aug 98
Kris,
I'm wondering - on these plates and bowls...I assume there is a foot ring; if
so, are you glazing inside the foot ring? If not, perhaps this is a factor...
Kathe
David Hewitt on mon 10 aug 98
In message , Gavin Stairs writes
>----------------------------Original message----------------------------
>At 01:59 PM 8/7/98 EDT, Kris Busch wrote:
>...
>>I'm using this glaze combonation on vases ranging in size from small
>>bottles to 2 foot tall vases, pitchers, mugs, soup size bowls and am
>>having no problems with these forms. The glaze is crazed on these forms
>>but no other problems.
>>
>>When the plates and bowls come out of the kiln the glaze is crazed, but
>>hours or sometimes days later I will hear a loud crack and the plate or
>>bowl is history. ...
>
>Usually, crazing and dunting are at opposite end of the glaze fault
>spectrum. Since you have both problems, I suspect one of two cases:
>
>1. Your crazing glaze is weakening the body, which then gives in to slight
>body stresses.
>
>2. It is a body problem unrelated to the glaze.
>
>I note that you see dunting only with the open forms, plates and bowls.
I may be completely of track with this problem, but a not too distant
experience of mine comes to mind. I was making some vases and only
wished to glaze the inside. I expereienced far to many of them cracking
when removed from the glaze firing. The glaze was crazed. I now have a
glaze that fits the body well and I do not get any of the cracking.
Is there a difference between the glazing on the top of your open forms
and the underside? Is the underside largely unglazed?
Also, do you want the crazed finish? If not, I am sure that to change
the glaze to avoid this would be a step in the right direction.
>This leads me to suppose that your problem is one of lack of "compression".
> That is, your forms have been thrown or formed in such a way as to cause
>them to dry with residual tension, probably in the centre of the form. The
>solution may be to rib or otherwise compress the clay at the centre while
>throwing, rather than to simply open and pull.
>
>The reason this would be more of a problem with the open forms than with
>more strongly curved ones is a matter of geometry. With the strong curves,
>the tension developed in the centre is less than it is with a flatter
>curve, since the line of the force follows the curve, and ends up pulling
>up rather than out. So you should do your compression ribbing after you
>have made the final form, not before. Just reach in and push down with a
>rib, preferably while moving in rather than out.
[snip]
>Gavin
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Kristin J Busch on tue 11 aug 98
Thanks to all who responded to my questions. To answer some of the
questions that arose,
the firing is taking around 11 hours. I'm using 2 different Kilns, an
econo with manual controls and a cress with auto turn ups and it
happens in both kilns. Because of this I don't think it is the speed of
the temp increase. ( in the econo I usually start at a low setting the
crank it after several hours. in the cress it gradually increases in temp
from start to finish) I will try a more gradual increase in temp in the
econo just to make sure though. As for the cooling, it takes about 15
hours. But with these latest plate firings I've been waiting until the
kiln is room temp. before I open the kiln.
I am using the vent during the cooling but according to the person selling
them this should not hurt anything.
As for the crazing, it does look like crazing (almost like a spider web)
not virtical or spiral.
I have not tested the floating blue with out the commerical glaze on
plates yet, but I have tried to alter the glaze to get rid of the crazing
with out much luck so far.
The plates and bowls do have foot rings and they are glazed on the entire
bottoms, but some times the glaze is thicker on the top then the bottom
because the thicker this glaze it the more interesting it looks. But this
is also the case with the more virtical forms, the inside and the lower
half have less glaze with no problems.
As for the clay body, I do make plates and large bowls out of 2 other clay
bodies using different glazes and have no problems with dunting. I will
try these with the floating blue and commerical glaze combonation and see
what happens. I do know other people using the warm brown stoneware to
make plates in this temp range and they seem to not have any trouble.
Any further comments on this problem would be greatly appreciated.
Thanks again,
Kris Busch
Bloomington In
mel jacobson on thu 29 jun 06
i think dunting is the worst enemy of a working potter.
i would do anything to prevent it from happening.
just imagine a customer serving a salad from one of your
most beautiful bowls, and her guest finds a big shard of
broken, sharp glaze in her mouth, and blood running down her chin.
we do not like dunting.
mel
listen carefully to ron roy about dunting.
do what he says. some glazes just don't like
certain clay bodies.
(that is how my love affair with rhodes 32/mel's variety
came into being....it does not dunt on my clay/ever.)
from: mel/minnetonka.mn.usa
website: http://www.visi.com/~melpots/ (new website)
http://my.pclink.com/~melpots3 old
Clayart page link: http://www.visi.com/~melpots/clayart.html
Nils Lou on sun 29 nov 09
Dear Ivor and Neon-Cat,
Ivor's description of iron furnaces certainly aligns with=3D20
my own pottery experiences--especially with hard brick.
One obvious thing to consider is the differential in heat
from the hot face to the cold face in a thick refractory
like hard brick. IFB is a slightly different story as the fabric is
so open, yet it too will dunt from too rapid cooling.
In my suggestion relating to symmetrical cycles of heating=3D20
and cooling I was referring to the product itself. Much depends=3D20
on a controlled environment like commercial tunnel kilns where
the ware is fabricated to exacting formulae, and can be exposed=3D20
to precise heating/cooling conditions.
Of course, as Neon-Cat eloquently explains, so much depends on
clay chemistry. A dense porcelainous body may phase quartz to
cristobolite more than a stoneware body. I would guess that
acid/alkaline ratios will affect ion play on whether or
when in the heat cycle cristobolite is developed.=3D20
I appreciate the contributions you both generously provide. The
knowledge and information we can share via this IT network is
remarkable. I love being informed...nils
nils lou, professor of art
http://nilslou.blogspot.com
www.tinyurl.com/bpc5nm
503.883.2274
"Play is the essence of creativity", and
"What is not brought forward into consciousness....
we later call Fate", Carl Jung
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