Hendrix, Taylor J. on tue 2 mar 04
Howdy y'all:
I have another question for the class. Am I right=20
to think that an under fired clay will have a larger=20
COE and might craze a glaze that, had the clay been=20
fired to it's correct cone, would not have crazed=20
the same glaze? If so, how much difference in=20
firing are we talking?
I am asking because my first glaze tests are showing=20
some crazing. This body and this glaze have a=20
history of fitting, so I am running down the=20
possibilities. I am leaning toward a weighing=20
variance in glaze materials rather than the firing=20
thing, but I am curious about it. I have put up cute=20
little pictures of my cone packs at=20
http://www3.baylor.edu/~Taylor_Hendrix/catopics.htm=20
so anybody interested take a look and tell me if that=20
is a significant temp difference. I think that is=20
just a few degrees, but I could be wrong. The test=20
tiles were fired on the top shelf.
If the problem persists in my next glaze firing, I=20
will think about the clay/silica or the boron addition=20
to cure the crazing. I am going to mix 1000g of my=20
base glazes this time to minimize any measurement=20
problems I might have had and then take test quantities=20
out of that for my variations. It is quite possible=20
though I'm a pretty old lab rat) that I had fat thumbs=20
when I measured out the individual 100g test batches.
As an aside, I found it quite impossible to see the cones=20
once cone 4 started bending well. I thought I had set=20
the cones with no background pots, but I'll be hanged if=20
I could see anything. I melted a fist full of straws just=20
trying to catch a glimpse. Should I try iron-marked brick for
background?
Taylor, in Waco
Dave Finkelnburg on thu 4 mar 04
Daniel,
If you take a piece out of the kiln, and the glaze on it is crazed, then
obviously delayed crazing is not at work. And no, you won't get moisture in
the ware in the kiln, at least not unless you leave it there after the kiln
has cooled to the air temperature around the kiln.
Glaze fit at low fire is a challenge.
Good potting,
Dave Finkelnburg
----- Original Message -----
From: "daniel"
Sent: Thursday, March 04, 2004 2:36 PM
> I guess I'd be happier with that if it crazed over time with use but if
it
> s crazed directly from the kiln is this so likely the cause ? Is water
> absorption inside the cooling kiln that significant ? That said, in this
> case I do not know whether the pieces were crazed at the kiln opening.
daniel on thu 4 mar 04
Hi Taylor,
I'll take a swipe at this but with the disclaimer that I too am learning.
> I have another question for the class. Am I right
> to think that an under fired clay will have a larger
> COE and might craze a glaze that,
I believe that the COE basically rises with temperature. You can therefore
expect that an under-fired body will have a lower than expected COE. This
will tend to craze an otherwise good-fitting glaze. (Ultimately you want the
glaze in slight compression which implies a slightly lower COE in the glaze
than in the mature body, but not enough to shiver.) Lawrence and West in
Ceramic Science for the Potter show a table of body constituents and all of
the COEs rise with temperature by 1 to 2 orders of magnitude.
had the clay been
> fired to it's correct cone, would not have crazed
> the same glaze?
The reason the glaze crazes is because the COE of the glaze is higher than
that of the body. Ultimately underfiring will lead to a larger discrepancy
in the COEs (as the body COE is lower than expected) tending to crazing.
This led to the interesting thought this morning that the maturation point
of both glaze and body should be the same in the craft low bisque high glost
method. A new thought to me though no doubt commonsense to others.
If this combo had worked before and the firing and the glaze prep are not
that far off I would wonder if the glaze COE is already a might high, a bit
borderline. Do you have any of the previous ok pieces lying around ? It
might be fun to boil/freeze them and see if crazing results. Fun for me of
course, not perhaps so much for you :)
Now bear in mind that I'm still learning this stuff too and lets see how the
ones who really know respond.
Thanx
D
Ron Roy on thu 4 mar 04
Hi Taylor,
Low expansion clays are harder to fit glazes to that won't craze. If the
glaze has a higher COE and the body a lower one you get crazing. What it
means is - the glaze contracts more on cooling then the body.
If the clay has a higher COE than the glaze - the clay contracts more than
the glaze - the glaze is put under compression by the body - no crazing -
but if too low shivering and/or dunting.
It is not possible to predict how under or over firing a body will affect
clay COE - more melt may lower the amount of free quartz and lower the over
all COE - but - with some high fire bodies you may increase the
cristobalite which will raise the COE.
If you want to see what happens with your clay body I will measure the clay
for you - just one stipulation - I have to know the recipe. If it's Tuckers
or Axners clay then I already know.
Try some more substantial "straws" - like from a plumbing shop - maybe copper?
RR
>I have another question for the class. Am I right
>to think that an under fired clay will have a larger
>COE and might craze a glaze that, had the clay been
>fired to it's correct cone, would not have crazed
>the same glaze? If so, how much difference in
>firing are we talking?
>
>I am asking because my first glaze tests are showing
>some crazing. This body and this glaze have a
>history of fitting, so I am running down the
>possibilities. I am leaning toward a weighing
>variance in glaze materials rather than the firing
>thing, but I am curious about it. I have put up cute
>little pictures of my cone packs at
>http://www3.baylor.edu/~Taylor_Hendrix/catopics.htm
>so anybody interested take a look and tell me if that
>is a significant temp difference. I think that is
>just a few degrees, but I could be wrong. The test
>tiles were fired on the top shelf.
>
>If the problem persists in my next glaze firing, I
>will think about the clay/silica or the boron addition
>to cure the crazing. I am going to mix 1000g of my
>base glazes this time to minimize any measurement
>problems I might have had and then take test quantities
>out of that for my variations. It is quite possible
>though I'm a pretty old lab rat) that I had fat thumbs
>when I measured out the individual 100g test batches.
>
>As an aside, I found it quite impossible to see the cones
>once cone 4 started bending well. I thought I had set
>the cones with no background pots, but I'll be hanged if
>I could see anything. I melted a fist full of straws just
>trying to catch a glimpse. Should I try iron-marked brick for
>background?
>Taylor, in Waco
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
daniel on thu 4 mar 04
Hi Ron,
>
> It is not possible to predict how under or over firing a body will affect
> clay COE - more melt may lower the amount of free quartz and lower the over
> all COE - but - with some high fire bodies you may increase the
> cristobalite which will raise the COE.
This begs the question then why a number of books suggest that an underfired
body is often connected with crazing. Some go so far as to say that it is
the number one cause.
In my response to this question I said that there are tables that Lawrence
and West for example show which indicate that there is an increase in COE
for common body components as they rise in temp. Your comments in
conjunction with this lead to the following question. I hope I can state it
clearly. I have wondered whether the COE's of the body as it matures the
first time are really an accurate representation of the COE of the matured
body under subsequent temperature changes. It seems from what you say that
this may not always be true. Dunting and so on also suggest that this is not
true. (This is an analogous question to the eutectic behaviour of a glaze
melting the first time and melting a second time in a refiring I think. )
On the cristabolite question, it would depend on the amount formed as you
mention but if it increased then the COE would increase, tending to make it
fit better, wouldn't it ? This leads me to the further thought that if the
glaze itself has not matured then its COE is probably off the mark too.
Would this be fair to say ?
Apologies if I am merely clogging the thread with ramblings. Anyhow you'll
be rid of me for 11 days tonight as I'm off on vacation. Wa ... hey !!!!
Thanx
D
Dave Finkelnburg on thu 4 mar 04
Daniel,
If I may summarize briefly, Taylor asked if his underfired clay body had
a lower COE and therefore was more likely to have a glaze on it craze. Ron
said "It is not possible to predict...." You ask, "....why a number of
books suggest that an underfired body is often connected with crazing...."
The very short answer is an underfired clay body has the ability to
absorb moisture. IF the body absorbs enough moisture, pressure develops
from inside the body and causes the glaze to craze. This is usually
observed with somewhat porous, low-fire bodies and also explains why on such
bodies crazing is often delayed.
I work at high fire, so I'm inexperience with low fire bodies. Can
someone tell me what is a reasonable absorption target for a cone 04 clay
body? Thanks!
Good potting,
Dave Finkelnburg
daniel on thu 4 mar 04
Hi Dave,
I guess I'd be happier with that if it crazed over time with use but if it
s crazed directly from the kiln is this so likely the cause ? Is water
absorption inside the cooling kiln that significant ? That said, in this
case I do not know whether the pieces were crazed at the kiln opening.
> The very short answer is an underfired clay body has the ability to
> absorb moisture. IF the body absorbs enough moisture, pressure develops
> from inside the body and causes the glaze to craze. This is usually
> observed with somewhat porous, low-fire bodies and also explains why on such
> bodies crazing is often delayed.
Thanx
D
Ron Roy on sat 6 mar 04
Hi Daniel,
I've been thinking about how to answer this in the best way but I'm still
thinking - so ask again if my explanations aren't good enough.
Many glaze books seem to focus on low fire glazes and of course the
generalizations don't apply to high fired vitreous bodies. Because low
fired clays are always under vitrified water gets in the clay during use -
under the glaze - if it can - and the clay will rehydrate - get bigger and
the glaze will then craze. It is a huge problem at lower temps. Industry
solves the problem by using high talc bodies - which raise the expansion
and eliminate the rehydration problem.
Much low fired ware is red earthen ware and I have never heard of the water
absorbency problem being solved in this way - perhaps it would work -
anyone know?
So your question about COE does not apply to those statements.
I believe Lawrence and West are saying - as you heat a clay (or a glaze)
the expansion will increase - of course - when we look at a dilatometer
chart we are seeing the expansion in direct relationship to the temperature
and the rate at which it is heated.
No amount of reheating - up to the point when clay or glaze starts to
soften or melt - will change the COE - it is exactly reversible.
Dunting (when hot water is poured into a pot for instance) is simply the
built in strain between clay and glaze - finally breaking the pot. Some
times the strain is so much that it happens in the kiln before unloading.
Sometimes it's just waiting to happen - and the hot water heats the glaze a
moment before the clay causing the glazes to expand (when heated) and the
pot breaks. It can take years sometimes to happen. A knock starts a micro
crack and it gradually works it's way through the whole pot.
This does not happen with crazed pots - even though they are easier to
break than some uncrazed pots - crazing does not push pots apart - but the
crazing does start micro cracks in the clay along the craze lines.
Yes - cristobalite is an anti craze - but there are other problems -
especially if you have a low expansion glaze.
What I was trying to say - if you think that by firing higher or lower -
you can solve fit problems - you had better be aware of all this stuff and
even better - have some dilatometer numbers to help predict what will
happen.
Silica (quartz and cristobalite) in the crystalline forms has a profound
effect on expansion and contraction. Firing higher might generate more
cristobalite and less free quartz for instance - it all depends on the
chemistry of the clay body and the temperature you are firing to. Trying to
say firing higher will cure crazing must always have a qualifier - with
this sort of body at this temperature for instance.
Would I say such a thing - no I cannot predict this yet - which body will
go which way - it's like trying to predict what will happen fit wise with a
matte glaze - no body has this figured out yet.
We do know that adding free silica to a body helps to cure crazing because
the expansion/contraction of the clay is increased - we also know - if that
quartz includes micro fines - we run the risk of increased cristobalite -
if there is not enough KNaO to control it - blah, blah blah - there is so
much to say about this I could go on all night
I have measured over 400 clay, bodies and glazes so far - and I'm still
getting surprises.
Perhaps if we understand how eutectic diagrams are worked out it will help
- they have to do the experiments - still - there is no way to predict.
Anyone who got this far gets a gold star - for what thats worth.
I'm sure this will lead to more question Daniel - that is fine with me. I
often get ideas for experiments from just such a dialogue.
RR
>This begs the question then why a number of books suggest that an underfired
>body is often connected with crazing. Some go so far as to say that it is
>the number one cause.
>
>In my response to this question I said that there are tables that Lawrence
>and West for example show which indicate that there is an increase in COE
>for common body components as they rise in temp. Your comments in
>conjunction with this lead to the following question. I hope I can state it
>clearly. I have wondered whether the COE's of the body as it matures the
>first time are really an accurate representation of the COE of the matured
>body under subsequent temperature changes. It seems from what you say that
>this may not always be true. Dunting and so on also suggest that this is not
>true. (This is an analogous question to the eutectic behaviour of a glaze
>melting the first time and melting a second time in a refiring I think. )
> On the cristabolite question, it would depend on the amount formed as you
>mention but if it increased then the COE would increase, tending to make it
>fit better, wouldn't it ? This leads me to the further thought that if the
>glaze itself has not matured then its COE is probably off the mark too.
>Would this be fair to say ?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sat 6 mar 04
Hi Dave,
I just took a look at my clay body test data - they range from 12% down to
9% at cone 04 - except for one - which has some frit and comes in around
4%.
When paper is added the absorption jumps from an average of 11% up to 18%
using the boil for two hours test on a 1" by 6" by 1/2" bar.
The reason the absorbent clay causes delayed crazing is because the water
causes the underfired clay to rehydrate and swell a bit.
I don't know if this happens with absorbent high fired clay by the way - an
interesting subject for an experiment - or at least some research into
existing data.
Another reason for fireing to enough vitrification to stop leaking?
RR
> If I may summarize briefly, Taylor asked if his underfired clay body had
>a lower COE and therefore was more likely to have a glaze on it craze. Ron
>said "It is not possible to predict...." You ask, "....why a number of
>books suggest that an underfired body is often connected with crazing...."
> The very short answer is an underfired clay body has the ability to
>absorb moisture. IF the body absorbs enough moisture, pressure develops
>from inside the body and causes the glaze to craze. This is usually
>observed with somewhat porous, low-fire bodies and also explains why on such
>bodies crazing is often delayed.
> I work at high fire, so I'm inexperience with low fire bodies. Can
>someone tell me what is a reasonable absorption target for a cone 04 clay
>body? Thanks!
> Good potting,
> Dave Finkelnburg
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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