Sandra Talarico on sat 18 jan 97
I am a potter at a fairly large studio run by the
local county. Both myself and the instructor of
the class I attend have had teapots crack on us.
All the teapots were made with a stoneware fired
to a reduction cone 10 - glazed on the inside and out.
Admittedly, I am not a "teapot" expert, though I have
been throwing for many years. The instructor has also
been a potter for many years.
My teapot cracked as soon as I poured the boiling water
in it to make tea. Two of my instructor's teapots did
exactly the same thing when boiling water was poured
into them.
I'm having trouble understanding what caused this. I've
poured boiling water into many things that I have made:
cups, colanders, and bowls.
Does anyone have any theories on this?
Thanks,
Sandy Talarico
sandy@quartet.ho.att.com
Sam Cuttell on sat 18 jan 97
>
>My teapot cracked as soon as I poured the boiling water
>in it to make tea. Two of my instructor's teapots did
>exactly the same thing when boiling water was poured
>into them.
>
>I'm having trouble understanding what caused this. I've
>poured boiling water into many things that I have made:
>cups, colanders, and bowls.
>
Hi Sandy:
As anyone of British descent knows, you have to "hot the pot" before
adding the boiling water.
That is....place t-pot under kitchen tap and turn on the hot water
(at this point the water coming out will be cold to room temp).
Let the water run until it is steaming hot. Empty the t-pot and
add your favourite tea and boiling water.
My fave t-pot is used daily and is about 5 years old. My grandma
used her fave porcelin pot for over 50 years.
sam - alias the cat lady
Home of Manx cats, Cavalier King Charles Spaniels and the odd horse
Melbourne, Ontario, CANADA
(SW Ontario)
http://www.geocities.com/paris/3110
"Madness takes its toll. Please have exact change."
Margaret Arial on sat 18 jan 97
Hi, Your problem could have several problems.Send out more technical
information like the clay receipe, the glaze receipe, the bisque temperature,
and the glaze firing cycle and certainly someone will have suggestions. Now
for a simple suggestion it is best when using a teapot to prewarm a pot by
pouring a small amount of hat water into it and swirling it around then
dumping it out to warm the entire inside. however you seem to have such
serious problems that that simple act wouldnot suffice in the marketplace.My
mother in law had a real stubborn streek and insisted on heating the water in
one of my stoneware teapots it never blew up but i certainly would not been
surprised, so the moral is make the product idiot proof because stickers nor
tags last the lifetime of some of our waares. Good luck. Margaret Arial at
MTU19354@aol.com
Bill Amsterlaw on mon 20 jan 97
re: Black coring ?
On 18 Jan 1997, Sandra Talarico wrote:
>>
All the teapots were made with a stoneware fired to a reduction cone 10....
....
My teapot cracked as soon as I poured the boiling water in it to make tea.
Two of my instructor's teapots did exactly the same thing when boiling water
was poured into them.
<<
Hi Sandra:
Take one of the shards and look at the color of the clay. If it is dark gray
or black, you have "black coring" - the only thing I know of which will cause
clay to be so brittle and sensitive to thermal shock.
My understanding of black coring is that in a very low oxygen atmosphere,
between 700-900c, any carbon which is unable to combine with oxygen reacts
with Fe2O3 to produce (black) FeO. The FeO is a potent flux which results in
tiny islands of melted claybody which trap gases being liberated from the
clay.
The following conditions promote black coring:
1. High iron content in the clay (It doesn't happen with porcelain)
2. Large amount of carbonaceous material in the clay.
3. Bisque firing not high enough to burn off all the carbonaceous material in
the clay.
4. Heavy reduction between 700-900c = 1300-1650F = cone 017-010
The most important consideration is the temperature at which you begin to
reduce the kiln. The lowest temperature at which you can start reducing
without risk of black coring is cone 010.
It is possible to get black coring in an electric kiln if it is poorly
ventilated, the clay contains a lot of iron and carbonaceous material, and the
temperature is rising fast.
Black coring is not reversible.
- Bill Amsterlaw (wamster@msn.com)
Plattsburgh, NY
Dan c Tarro on tue 21 jan 97
Sandy,
You probably will get lots of help on this one.
I also have gone through this one with tea pots mugs... . I hate to see a
customer holding a tea pot at my studio door a week after I sold it, it
only means one thing. I have solved my problem with this and maybe you
may wish to consider my findings.
I mix my clays myself so I can keep an eye on the qualities of the clay.
In doing this I find that there is more than just the workability of the
clay when raw that has to be considered. What happens when the clay is
fired and that wonderful thing that goes on inside that makes it almost glass ha
Simply- Clay, like glaze, is made up of three types of material, alumina
(refractory), Silica(glass former), and flux (lowers the melting temp of
the glass former). In glazes you can see what is going on most of the
time just by looking at the surface of the item, with clay it is a little more d
measurement as to how much the of the silica has actually been converted
to glass by the fluxes within the clay body. This can be tested by a
water soaking process which is discussed in any good ceramics book. If
you can't find any info I can help if needed. But the key words here are
silica and flux. Silica(free silica) is an odd item, when it is still in its ori
can be described that way, it jumps.(If there are chemists out there
maybe they can describe what goes on better.) But, when this same silica
combined with a flux and converted to glass(no longer free silica)
becomes more stable and looses it's desire to dimensionally jump. A lot of potte
this and get away from this cracking. I have been adding about 8%, and
am firing ^10-12. I have heard concentrations up to 11% used by others.
Too much may tend to warp your work at higher temps(slump). It will help
some of your drying, reduce your plasticity to some extent, and make your
green ware slightly more brittle. Warpage during drying is lessened and
additions at leather hard is improved. Lots of pros and cons. Best of
all your items become harder, crack less, and last longer. Happier
customers if your producing to sell.
Other things to consider, Iron content of your clay should be kept below
2% if possible, especially if you are reducing. Iron does weird gassy things in
the addition of earthenware clays for darker clay bodies at stoneware
temps. Too much junk in the clay and it also makes for a brittle body.
Watch your cooling at about 1000 deg. and 500, this is where the silica
does its jumping the most, something called quarts inversion, go slowly
past these temps..
I will stand back now after all this and see what other response we can
get.
What do others think? Am I all wet?
Lets help Sandy out.
Best of luck,
Dan Tarro
Oak Tree Stoneware
Ham Lake, Minnesota
--------------------------------------------------------------------------------
Clothes make the man, naked men have little influence on society. Mark
Twain
Cathy Nelson Hartman on wed 22 jan 97
Sandra,
It sounds like over reduction of the clay which can cause the defect
called black coring. The over reduction causes the fired clay to be
susceptible to heat shock due to the fired clay being brittle and less
resilient. Crack open the pot and see if the inner matrix of the clay is
dark gray to black as an indication. If so, try less severe reduction.
Walter
Walter Hartman full time studio potter since 1973
cone 10 porcelain
90% functional work
Hartman Pottery oxidation in gas kiln
1600 Willowwood sell all work at art fairs
Denton, TX 76205 work by myself
(817) 382-1083
e-mail: cnh0002@jove.acs.unt.edu
ROGER KEANE on wed 22 jan 97
----------------------------Original message----------------------------
Sandy,
The lack of thermal shock resistance in the tea pots is due to the body
having too high an expansion from cristobalite silica formation in the
body. Whilst the glaze fits the body it is not possible to visually
determine whether the glaze fit is due to a high quartz silica content
or a high cristobalite silica but I suspect the glaze fit and the poor
thermal shock resistance is due to cristobalite formation.
To increase the thermal shock resistance the body must be altered to
eliminate or reduce cristobalite formation.This is usually done by the
addition of Feldspar to convert the cristobalite to a low expansion
silica glass,the addition of flint to adjust the expansion if required
and a possible clay adjustment to see that the body does not over
vitrify.
If altering the body is too much of a hassle it may be easier to change
to another clay. Select several low iron bodies and do some thermal
shock
tests to determine the most suitable for tea pots.
Changing the firing schedule,the firing temperature,the kiln atmosphere
and the glaze can help to increase the thermal shock resistance.I can
not
give specific advice here with out testing the body because for instance
with some clays firing faster will solve the problem and in others make
it worse,it all depends on the particular body.In any case these can
only partly address the problem,as a body change is required,I
suggest,either adjust the old one if it is worth the time and effort or
change to a body more suitable for tea pots.
Regards
Roger Keane
Keane Ceramics Pty Ltd
Somersby N.S.W.
Australia.
Dan c Tarro on thu 23 jan 97
Thank you Roger,
It seems that Juno or my computer has let me down and trashed my message
by dropping important words in the message I posted. The bit of garbage
that the computer spit out about flux and silica conversion to reduce
the thermal shock, you recovered so well. Free silica is deadly in the
clay body, and sometimes the only solution is the addition of feldspars.
Roger,
thanks
Dan Tarro
Oak Tree Stoneware
Ham Lake, Mn
On Wed, 22 Jan 1997 09:15:44 EST ROGER KEANE writes:
>----------------------------Original
>message----------------------------
>----------------------------Original
>message----------------------------
>Sandy,
>
>The lack of thermal shock resistance in the tea pots is due to the
>body
>having too high an expansion from cristobalite silica formation in the
>body. Whilst the glaze fits the body it is not possible to visually
>determine whether the glaze fit is due to a high quartz silica content
>or a high cristobalite silica but I suspect the glaze fit and the poor
>thermal shock resistance is due to cristobalite formation.
>
>To increase the thermal shock resistance the body must be altered to
>eliminate or reduce cristobalite formation.This is usually done by the
>addition of Feldspar to convert the cristobalite to a low expansion
>silica glass,the addition of flint to adjust the expansion if required
>and a possible clay adjustment to see that the body does not over
>vitrify.
>
>If altering the body is too much of a hassle it may be easier to
>change
>to another clay. Select several low iron bodies and do some thermal
>shock
>tests to determine the most suitable for tea pots.
>
>Changing the firing schedule,the firing temperature,the kiln
>atmosphere
>and the glaze can help to increase the thermal shock resistance.I can
>not
>give specific advice here with out testing the body because for
>instance
>with some clays firing faster will solve the problem and in others
>make
>it worse,it all depends on the particular body.In any case these can
>only partly address the problem,as a body change is required,I
>suggest,either adjust the old one if it is worth the time and effort
>or
>change to a body more suitable for tea pots.
> Regards
>Roger Keane
>Keane Ceramics Pty Ltd
>Somersby N.S.W.
>Australia.
>
Craig Martell on fri 24 jan 97
Hi Sandy, Roger, Dan and other interested parties:
I'm in agreement with Roger and Dan in regard to the cracking teapot post. I
had a similar problem some years back when I was the Resident Potter at an
Art Center in Oregon. When I first assumed the job, I tested the claybodies
that were being used by the students at the pot shop and found that one
particular clay cracked very consistently when exposed to thermal shock from
boiling water. I used Jim Robinson's glaze fit series (see "Fear of Silica",
Studio Potter, June 1981 and "Body Building for Potters", Studio Potter, June
1988) to determine if the clay was developing cristobalite. Jim developed a
series of 10 glazes from high to low expansion that will give information
about any clays behavior due to silica inversions. Glaze #1 is high
expansion and they progress through the numbers to the lowest expansion at
#10. If a body is developing cristobalite, you will see some shivering in
the low expansion glazes. The offending clay that I tested shivered tiles
8,9, &10, indicating that there was a lot of cristobalite forming, and the
body was deficient in feldspar. I did absorbtion tests on the clay as well,
and these tests indicated that the body was too porous for stoneware and was
indeed underfluxed. I obtained random samples from the supplier, and ran
further tests which corroborated the initial findings. The thing that really
bent me outta shape was that the company that made the clay wasn't real
receptive to the testing even though I wasn't the only one complaining, I
didn't expect them to take the clay off the market on my say so, but I did
feel they should have been interested in pursuing the problem. Jim's glaze
series can be used on any commercial clay to determine its suitability for
teapots and ovenware, mugs etc.
I think that the cracking of teapots and mugs is due to the fact that
crystalline (free) silica has a higher coefficient of expansion than fused
silica that has been involved in a melt with feldspar. However, the cracking
is not due to a cristobalite inversion which takes place somewhere between
438 and 540 degrees F. Cristobalite is nasty stuff! It will invert
somewhere between these two temperatures, at a point randomly determined by
itself, which is usually never the same, and it will change volume by about
4.5% which is enough to demolish an ovenware piece. And unlike Quartz,
cristobalite goes through its expansion instantaneously. Boiling water is
only a bit above 200 degrees F. so teapots and mugs are not at risk from a
cristobalite inversion. When you pour boiling water into a pot, there is
uneven heating for a short time and if the piece is replete with high
expansion, crystalline silica, it will probably crack. When I was testing
the offending clay, I destroyed two teapots even though I preheated them with
very hot tap water. When I poured the boiling water in the pots, they were
quickly and badly cracked.
I hope I haven't put you all to sleep. This was longer than I thought.
Craig Martell-Oregon
John Tilton on sat 25 jan 97
I suspect that the problem you are experiencing with the teapots is due
to cristobalite inversion.
Silica exists in three forms: Quartz, Cristobalite and Tridymite.
Tridymite is very rare and never happens to potters. Cristobalite forms
at 1300C when there is free silica which is not combined into the glass
matrix of the clay. By free silica, we mean the microscopic stuff which
is combined with ball clays and such, not any 200 mesh silica you might
have added to the body. Just as quartz has an alpha and beta phase,
changing around 570C, cristobalite has these phases also and they are in
the range of oven use and boiling water,and the percentage change is
much greater with cristobalite inversion than quartz inversion.
I once visited a friend and presented her with a new teapot which
promptly and theatrically cracked the first time we poured boiling water
into it.
What to do?
In Studio Potter 16-2 there is an article by Jim Robinson which
addresses this issue, "Body Blending for Potters" I think is the name. I
went through all the steps of the article and now have a clay body which
has been not only cristobalite free, but it takes into account the
glazes I use, so that materials can change a bit without affecting the
final result. ( knock on wood ). It has worked for many years.
HARD DISK CRASH-------------------------------------
Yep, I've had another one and have been offline for about a month. My
Compuserve address is now invalid- Compuserve had just become so
maddening to use. Please change my address to
tilton@atlantic.net
Thanks.
John Tilton
Alachua, Fl
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