Craig Martell on sun 21 sep 03
At 01:48 PM 9/21/2003 -0400, you wrote:
>Does anyone know why a porecelain body can be formulated to have close to
>zero absorption rate when fired, but a stoneware should have about 2%?
Hi:
Porcelain can have zero aborbtion because of the low iron content. When
stonewares are fired to the point of zero absorbtion, or close to it, they
become very brittle due to a higher iron content. This is especially true
with pieces that are fired in reduction. Oxidized bodies usually don't
develop the brittleness when fired to total vitrification.
that's the short answer, Craig Martell Hopewell, Oregon
Paul on sun 21 sep 03
Does anyone know why a porecelain body can be formulated to have close to
zero absorption rate when fired, but a stoneware should have about 2%? I
haven't found the answer to this in books yet. What would happen if a
stoneware had enough feldspar so that it had closer to 0% absorption and was
used for functional ware, would it break?
Paul B
Falmouth, KY
Ababi on sun 21 sep 03
Out of my experience, it would bloat and probably will be too fragile.
The why in not clear to me-too
Ababi
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Paul
Sent: Sunday, September 21, 2003 7:48 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: ABSORPTION OF PORCELAIN AND STONEWARE
Does anyone know why a porecelain body can be formulated to have close
to
zero absorption rate when fired, but a stoneware should have about 2%? I
haven't found the answer to this in books yet. What would happen if a
stoneware had enough feldspar so that it had closer to 0% absorption and
was
used for functional ware, would it break?
Paul B
Falmouth, KY
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Malcolm Schosha on sun 21 sep 03
Paul,
By increasing the vitrification, you would be also increasing the
shrinkage, which you might not want. Also there is less margin of
firing error allowable because porecelain is usually very close to
the point where it might slump. However, I worked for a factory in
Israel for three years that made fully vitrified stoneware. It was a
very hard product.
Malcolm
.................................................
--- In clayart@yahoogroups.com, Paul wrote:
> Does anyone know why a porecelain body can be formulated to have
close to
> zero absorption rate when fired, but a stoneware should have about
2%? I
> haven't found the answer to this in books yet. What would happen if
a
> stoneware had enough feldspar so that it had closer to 0%
absorption and was
> used for functional ware, would it break?
> Paul B
> Falmouth, KY
>
>
______________________________________________________________________
________
> Send postings to clayart@l...
>
> You may look at the archives for the list or change your
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> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@p...
Paul Herman on sun 21 sep 03
Hello Paul,
I use a claybody that fits your description below. It's a white
stoneware, very porcelain-like, and subject to warping and plucking
(sticky foot which chips in cooling). It fits the glazes well, and
handles the oven, but it's not the easiest clay to work with. Strong and
servicable though. I use it because I think the fired result is
'pretty', and the small amount of fireclay in the recipe improves the
plasticity a lot.
I don't think there's any "have to" about the 2% absorbtion.
Good mixing,
Paul Herman
Great Basin Pottery
423-725 Scott Road
Doyle, California 96109 US
potter@psln.com
----------
>From: Paul
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: ABSORPTION OF PORCELAIN AND STONEWARE
>Date: Sun, Sep 21, 2003, 10:48 AM
>
> Does anyone know why a porecelain body can be formulated to have close to
> zero absorption rate when fired, but a stoneware should have about 2%? I
> haven't found the answer to this in books yet. What would happen if a
> stoneware had enough feldspar so that it had closer to 0% absorption and was
> used for functional ware, would it break?
> Paul B
> Falmouth, KY
Cindi Anderson on sun 21 sep 03
I have seen stonewares that were 1%, but usually you are right, they say
2-4% and most people think that is close enough.
According to the texts, there is a point as you approach zero absorption
that the piece gets brittle. That might be one reason why you see
stonewares typically higher. Also perhaps manufacturers want a little
margin so if they overfire slightly the clay doesn't melt. I could imagine
that grog might limit the absorption you could reach in stoneware but I am
not sure about that.
Cindi
Fremont, CA
----- Original Message -----
> Does anyone know why a porecelain body can be formulated to have close to
> zero absorption rate when fired, but a stoneware should have about 2%? I
> haven't found the answer to this in books yet. What would happen if a
> stoneware had enough feldspar so that it had closer to 0% absorption and
was
> used for functional ware, would it break?
> Paul B
David Hendley on sun 21 sep 03
There is no rule which says that stoneware should have an absorption
rate of about 2%.
In fact, there is not a clear dividing line between "stoneware" and
"porcelain". Some people even use the terms "porcelanous stoneware"
and "dirty porcelain" to describe a claybody that is on the cusp between
the two.
Myself, I aim for a stoneware with about 1% absorption.
The potential problem with formulating a very low absorption stoneware
is that you will over-flux it and experience the accompanying problems.
Finding a good absorption rate for your claybody is like the old
"The Price is Right" TV show. To win the prize on the show, the
contestants had to guess the price of a product. The winner was
the person who was closest to the actual price without going over
the price. If you guessed too high, you lost it all.
It's the same with stoneware; the closer you get to 0 absorption, the
better, but, if you go too far, you lose it all because you will have
slumping, bloating, or worse.
To accommodate variations in materials and firings, it is better to
build in a little leeway, to make darn sure you don't cross the
0% line.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
> Does anyone know why a porecelain body can be formulated to have close to
> zero absorption rate when fired, but a stoneware should have about 2%? I
> haven't found the answer to this in books yet. What would happen if a
> stoneware had enough feldspar so that it had closer to 0% absorption and
was
> used for functional ware, would it break?
iandol on mon 22 sep 03
Dear Paul,
Who says Stoneware should have an absorbency of two percent and why is =
this necessary?=20
First let's consider porcelain.
Most recipes (except Bone China)are based on a Kaolinite-Felspar-Quartz =
blend which gives a high volume of glass phase when fully mature. During =
maturation connecting free pathways between particles of mineral close, =
leaving gas inclusions where particles meet. The smaller of these gas =
inclusions diffuse at the expense of the larger ones, which grow (This =
function assists with the development of Translucency). If you were to =
measure the density of a series of samples you would find the values =
were not perfect because of the addition of the gas volume to the =
ceramic volume. It would be lower than the theoretical density. This can =
happen because of the high value of Felspar and Quartz in the mixture =
which forms a bonding glass.
Now consider a good Stoneware Body.
It is made from less refined clay. The raw clays used may contain =
sufficient melting materials such as Muscovite or Biotite mica, Arkose =
sands, Calcium carbonate and Haematite to assist fusion. Often =
refractory clays are added to counter adverse melting behaviour. But =
there is insufficient melting stuff to form a pervasive, all =
encompassing, glass phase. So gas inclusions do not break apart and =
retreat. They are retained on cooling as communicating pathways into the =
ceramic substance. This is where that water goes. If you were to measure =
the apparent density it would be lower than that of Porcelain, only =
marginally so, but enough to give you the difference between almost zero =
and two percent.
Now if that measured two percent adsorption is such that there are =
inter-communicating pathways right through the clay the ceramic will =
seep if it has to hold fluids. If it is surface porosity and the =
interior of the clay has inclusions which are bubbles and not tubes the =
it is acceptable. If within the interior of the ceramic it its tubes =
then it is not satisfactory.
So, what do you do if you get the latter case, something I have =
experienced. There are two things you may do which are common sense. The =
first is to raise the maturity temperature if the clay is not liable to =
Pyroplastic collapse and the glaze which is being used will not change =
its character. The second is to add a small proportion of Felspar and =
keep the maturity temperature constant. Just sufficient to help fusion =
of the crystal edges to each other with the fluid glassy phase.
I cannot see why the addition of Felspar to a ceramic body should give =
cause for concern.
I hope this gives you food for thought.
Best regards,
Ivor Lewis. Redhill, South Australia
Lee Love on mon 22 sep 03
----- Original Message -----
From: "David Hendley"
> There is no rule which says that stoneware should have an absorption
> rate of about 2%.
> In fact, there is not a clear dividing line between "stoneware" and
> "porcelain".
I　accept John Reeve's distinction, that a clay being white isn't
enough to call it porcelain. It also has to be translucent.
--
Lee In Mashiko, Japan Lee@Mashiko.org
"I pledge on the altar of God undying hostility to any
government restriction on the free minds of the people."
-Thomas Jefferson
Ababi on mon 22 sep 03
I use for Crystal glazing Limoge and I use Laguna's English porcelain.
I fire to ^6 and I do not care if they are "porecelained" enough. By the
way the Laguna one gave me 0.2% absorption in 1220C!
Ababi Sharon
Glaze addict
Kibbutz Shoval Israel
ababisha@shoval.org.il
http://members4.clubphoto.com/ababi306910
http://www.matrix2000.co.nz/Matrix%20Demo/Ababi.htm
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Lee Love
Sent: Monday, September 22, 2003 9:38 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: ABSORPTION OF PORCELAIN AND STONEWARE
----- Original Message -----
From: "David Hendley"
> There is no rule which says that stoneware should have an absorption
> rate of about 2%.
> In fact, there is not a clear dividing line between "stoneware" and
> "porcelain".
I $B!! (Baccept John Reeve's distinction, that a clay being white
isn't
enough to call it porcelain. It also has to be translucent.
--
Lee In Mashiko, Japan Lee@Mashiko.org
"I pledge on the altar of God undying hostility to any
government restriction on the free minds of the people."
-Thomas Jefferson
________________________________________________________________________
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Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
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Earl Krueger on mon 22 sep 03
On Sep 22, 2003, Lee Love wrote:
> I $B!! (Baccept John Reeve's distinction, that a clay being white
> isn't
> enough to call it porcelain. It also has to be translucent.
Lee,
Just curious about nomenclature. If I had a true porcelain, by your
definition, and then added a little of something that took away the
translucency what would you call it?
What if it were translucent but not white?
Earl...
Bothell, WA, USA
Dave Finkelnburg on mon 22 sep 03
Earl,
Historically the most highly-prized porcelains have been translucent.
Some of us refer to white-firing, highly vitrified clay bodies as
"porcelainous stoneware." That label is, of course, arbitrary and open to
criticism. :-)
Good potting,
Dave Finkelnburg
www.idahopots.com
----- Original Message -----
From: "Earl Krueger"
> Lee,
> Just curious about nomenclature. If I had a true porcelain, by your
> definition, and then added a little of something that took away the
> translucency what would you call it?
>
> What if it were translucent but not white?
iandol on tue 23 sep 03
Dear Craig Martell,=20
How much Iron, and in what condition, are we talking about here? And =
what is about the iron content that causes embrittlement? Why does a low =
Iron content give ceramics with low absorption values?
Do you have any science or technology to back these ideas?
Best regards,
Ivor Lewis. Redhill, South Australia
iandol on tue 23 sep 03
Dear Cindi Anderson,
You tell us <<... According to the texts, there is a point as you =
approach zero absorption that the piece gets brittle....>>
Do the authors of such pronouncements give detailed physical, chemical =
or mechanical reasons why this should be so. I would like to know, since =
such a concept runs counter to other theories of mechanical integrity.
Best regards,
Ivor Lewis. Redhill, South Australia
iandol on tue 23 sep 03
Dear Earl Krueger,=20
I doubt if the confusion which exists about the naming of the products =
we make would arise if people were taught or learned the science and =
technology of the materials we use.
In a perfect World, all ceramic articles made from clay based bodies =
would be totally transparent. Understanding why this is not so is the =
key determining what we produce. It is the imperfections which create =
degrees of transparency, diffraction and reflectivity, be they additives =
or natural inclusions.
I am thankful this is the case. Without these errors of Nature or =
through our own intentions, Clayart gives infinite interest.
Best regards,
Ivor Lewis. Redhill, South Australia
David Beumee on wed 24 sep 03
How much iron? 10% R.I.O.
What condition? 99% pure
And what is (it) about the iron content that causes embrittlement?
Gimme a break Ivor. A reducing atmosphere causes oxidized red iron, Fe2O3,
to convert to FeO, reduced black iron. Fluid liquids form quickly, teaming up
with feldspathic melts, which dissolve mullite and quartz. When FeO-rich
liquids cool into glass, the glass has a brittle character. I know for a fact
that these words from Jim Robinson's Fear of Silica article are true, because
I've watched high iron stoneware bodies shatter when heated and dunked in
water. You want the exact temperature of the oven and the exact temperature of
the water? Get off our backs! Hopw many times do I have to explain this?
> Dear Craig Martell,
>
> How much Iron, and in what condition, are we talking about here? And what is
> about the iron content that causes embrittlement? Why does a low Iron content
> give ceramics with low absorption values?
>
> Do you have any science or technology to back these ideas?
>
> Best regards,
>
> Ivor Lewis. Redhill, South Australia
>
> ______________________________________________________________________________
> 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.
Craig Martell on wed 24 sep 03
Ivor axed:
>How much Iron
>what condition
>what is about the iron content that causes embrittlement?
>Why does a low Iron content give ceramics with low absorption values?
>Do you have any science or technology to back these ideas?
Hello Ivor:
Uh, ok, it this all or is there more? :^)
One source for this stuff is Hamer and Hamer, Breakdown, Breakdown Due to
Iron Oxide. The critical amount listed is in excess of 1.5%. The
condition is reduced iron. The cause is brittle glass formed by inclusion
of FeO with potassium and sodium silicates in high fired clays. Levels of
CaO and MgO can help alleviate this problem. He doesn't say at what level
these oxides help. So, having a lower iron content porcelain bodies fluxed
with potash and soda will not be as brittle as bodies with higher iron
content. From my own body experiments and tests, oxidized bodies, where
iron is an anti flux, the brittle problem is absent or not critical.
Other sources are W D Kingery and Jim Robinson. I don't have time at
present to dig this all out and post it to you or the list. I'd be glad to
do this when I get some breathing room.
regards, Craig Martell Hopewell, Oregon
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