Carty Ellis on wed 19 oct 05
I fire both cone 04 and cone 6. Cone 6 pottery in my studio typically
doesn't leach moisture. Cone 04 does more often than not (not always). I
noticed a comment on a glaze recipe for cone 04 that it leaches moisture
(makes me think it is not expected).
For low fire I use Highwater Clay's Stan's Red - which is quite well
vitrified at cone 04. I realize that earthenware clays have a slightly
different body structure when fired than stoneware clay bodies. But I am
starting to wonder if seepage isn't a glaze issue, not necessarily a clay
body issue.
The more I contemplate this, the more I have to ask, are cone 04 glazes
often not fully melted? My maiolica pieces hold water, as do some using
Amaco (cone 04) Opalescent glazes. Most other cone 04 glazes (I can
understand the obvioulsy crazed ones leaking) let moisture pass.
Is seepage of moisture with earthenware an issue of fully melting or do
glazes at cone 04 have some physical reason to act almost like
semi-permiable membranes?
Just curious, and I haven't seen much other than anecdotal discussion on
this issue.
Carty
Ivor and Olive Lewis on thu 20 oct 05
Dear Carty Ellis,=20
Perhaps the problem is that clay which is fired to Cone 04 has =
insufficient material that will create a glass phase.
Perhaps Ron Roy might explain how the commercial people arrive at an =
earthenware composition.
In the mean time I suggest you make a slight modification to your clay =
for Cone 04 firings. Add Sodium Silicate or Borax to your clay. You may =
need about 2%. But be warned, this is a speculative suggestion. You may =
destroy its plastic qualities due to deflocculation.
Alternatively, soak you bisque ware in a dilute solution, say 2% in pure =
water and allow it to dry before you apply your glaze.
All you need do is test one or two pots.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ron Roy on sun 23 oct 05
Hi Carty,
Earthenware glazes are very rarely vitrified enough to not absorb water and
leak.
It takes so much flux to virtify at low temperatures that - if you try to
virtify the clay enough to stop leakage - the melting happens so fast you
will eventually have overfired clay.
The other problem is - if the clay can absorb water - the clay will
eventually rehydrate - expand and craze any glaze that is not already
crazed.
There are some clay bodies that will resist rehydration (high talc bodies
for instance) and that will help the delayed crazing problem but not solve
the the moisture absorption - which can lead to overheating in a microwave
oven.
Some potters claim that a water proof coating of say Terra Sig will stop
the absorption of water but I have not seen any tests that prove this. It
would mean the surface would have to be perfect.
Earthenware is easily chipable compared to stoneware - once chipped - water
can get in of course - no mater how perfect the waterproof surface.
Glazes would normally be water proof but because they do not cover the ware
completely most of the time - they cannot keep the water out
I think the main problem is when any microwaved pot becomes hot enough to
burn the cook - people will normally assume a mug handle will not get hot
enough to burn your hand for instance.
RR
>I fire both cone 04 and cone 6. Cone 6 pottery in my studio typically
>doesn't leach moisture. Cone 04 does more often than not (not always). I
>noticed a comment on a glaze recipe for cone 04 that it leaches moisture
>(makes me think it is not expected).
>
>For low fire I use Highwater Clay's Stan's Red - which is quite well
>vitrified at cone 04. I realize that earthenware clays have a slightly
>different body structure when fired than stoneware clay bodies. But I am
>starting to wonder if seepage isn't a glaze issue, not necessarily a clay
>body issue.
>
>The more I contemplate this, the more I have to ask, are cone 04 glazes
>often not fully melted? My maiolica pieces hold water, as do some using
>Amaco (cone 04) Opalescent glazes. Most other cone 04 glazes (I can
>understand the obvioulsy crazed ones leaking) let moisture pass.
>
>Is seepage of moisture with earthenware an issue of fully melting or do
>glazes at cone 04 have some physical reason to act almost like
>semi-permiable membranes?
>
>Just curious, and I haven't seen much other than anecdotal discussion on
>this issue.
>
>Carty
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Linda Arbuckle on mon 24 oct 05
Well, I think we've had this dance about earthenware before. Below, I
think, Ron, that's it's the earthenware BODY that isn't vitrified, not
the glaze as you wrote.
I think the problem as originally questioned (a glaze issue) is from
several things. One, earthenware bodies are not usually vitrified. They
often will absorb moisture if there is exposed clay. Using terra
sigillata does not waterproof the clay, but does cut down on absorption.
Another factor is that some of the useful lowfire fluxes - sodium and
potassium (alkaline fluxes) - have a high coefficient of expansion. When
heated they expand a relatively large amount compared to other fluxes,
and upon cooling shrink a lot. If the glaze shrinks more than the body
in cooling, you have crazing. There are a number of beautiful "Water
Blue" alkaline glazes that are lovely turquoise w/copper + alkaline
fluxes. These glazes are usually crazed due to high sodium/potassium
fluxes that also provide the lovely turquoise color w/copper. Some dark
and/or opaque glazes are crazed, but it may be hard to see the crazing.
These would tend to let liquids seep.
Another factor for lowfire is cristobalite inversion. Check out Frank
Hamer's "Potter's Dictionary" for the complete explanation. Free silica
(i.e. not chemically bonded to something) in a clay body fired above
about 1950 deg F can be changed to cristoballite. This is about cone 03,
give or take. Cristobalite inverts (becomes about 2% smaller) at about
451 deg F (the temperature paper burns). This can put a glaze under
compression and counter crazing, if it happens in the right amount. If
there is a large amount of cristobalite formed, and this shrinkage
occurs (long after the glaze is rigid), this can cause dunting (cracking
in cooling). Dunting is usually more of problem w/high fire because it's
fired farther above the temp for cristobalite formation, and for a
longer time. But I think the phenomenon of cristobalite formation is the
reason that many earthenware potters fire to 03 rather than the 06 that
commercial glaze products use. A little bit of that compression is good
to counter crazing, and you'd have to fire to about 03 to reach that
threshold.
Majolica is a lowfire glaze, but unusually high in calcium flux for
lowfire. This tends to make for less crazing than a glaze that has more
sodium and potassium for fluxing, as the coefficient of expansion for Ca
is lower than sodium and potassium. High calcium tends to make a hard,
durable glaze.
Once more, I really challenge the blanket statement that lowfire bodies
chip easily. I have a lot of Stan Andersen dinnerware that I use daily,
and NONE of it is chipped, while the porcelain cups from a well-known
potter are all chipped. Give this one a rest. Not all earthenware bodies
are like third-world, underfired wares. Look up Pete Pinnell's tests on
clay body strength, which should be in the archives. He found
earthenware w/majolica glaze stronger than high fire bodies when doing
modulus of rupture tests.
Ditto the microwave indictment. Not all earthenware gets too hot to
touch in the microwave. I have personal experience, and have found that
some works very well in the microwave. Likewise, not all stoneware
bodies stay cool. Many are NOT vitrified and do absorb moisture. I have
a vapor-glazed stoneware pitcher that really seeps badly. As ever, it
all depends on the body composition, firing, and glaze.
Linda Arbuckle
14716 SE 9th Terr
Micanopy, FL 32667
(352) 466-3520
arbuck@bellsouth.net
http://www.arts.ufl.edu/artex02/html/ceramics/arbuckle.html
Date: Sun, 23 Oct 2005 12:34:44 -0500
From: Ron Roy
Subject: Re: glazed Earthenware leaching moisture
Hi Carty,
Earthenware glazes are very rarely vitrified enough to not absorb water
and
leak.
It takes so much flux to virtify at low temperatures that - if you try
to
virtify the clay enough to stop leakage - the melting happens so fast
you
will eventually have overfired clay.
The other problem is - if the clay can absorb water - the clay will
eventually rehydrate - expand and craze any glaze that is not already
crazed.
There are some clay bodies that will resist rehydration (high talc
bodies
for instance) and that will help the delayed crazing problem but not
solve
the the moisture absorption - which can lead to overheating in a
microwave
oven.
Some potters claim that a water proof coating of say Terra Sig will stop
the absorption of water but I have not seen any tests that prove this.
It
would mean the surface would have to be perfect.
Earthenware is easily chipable compared to stoneware - once chipped -
water
can get in of course - no mater how perfect the waterproof surface.
Glazes would normally be water proof but because they do not cover the
ware
completely most of the time - they cannot keep the water out
I think the main problem is when any microwaved pot becomes hot enough
to
burn the cook - people will normally assume a mug handle will not get
hot
enough to burn your hand for instance.
RR
>I fire both cone 04 and cone 6. Cone 6 pottery in my studio typically
>doesn't leach moisture. Cone 04 does more often than not (not always).
I
>noticed a comment on a glaze recipe for cone 04 that it leaches
moisture
>(makes me think it is not expected).
>
>For low fire I use Highwater Clay's Stan's Red - which is quite well
>vitrified at cone 04. I realize that earthenware clays have a slightly
>different body structure when fired than stoneware clay bodies. But I
am
>starting to wonder if seepage isn't a glaze issue, not necessarily a
clay
>body issue.
>
>The more I contemplate this, the more I have to ask, are cone 04 glazes
>often not fully melted? My maiolica pieces hold water, as do some
using
>Amaco (cone 04) Opalescent glazes. Most other cone 04 glazes (I can
>understand the obvioulsy crazed ones leaking) let moisture pass.
>
>Is seepage of moisture with earthenware an issue of fully melting or do
>glazes at cone 04 have some physical reason to act almost like
>semi-permiable membranes?
>
>Just curious, and I haven't seen much other than anecdotal discussion
on
>this issue.
>
>Carty
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
skiasonaranthropos@FSMAIL.NET on mon 24 oct 05
Hello Linda,
Sorry for chipping in, sorry about the pun, but the alarm bells went off
reading the statement =93 ... earthenware w/majolica glaze stronger than
high fire bodies when doing modulus of rupture tests.=94
Can you elaborate as this is absolute counter to already recognised
learning. Whilst I would not argue that a properly formulated earthenware,
when appropriately fired with a suitable glaze can make practical cookware
a vitrified whiteware, say porcelain, is mechanically stronger than an
earthenware. Where a weakeness can be introduced to the former is if a
badly fitted glaze is used
Regards, Antony
John Hesselberth on mon 24 oct 05
On Monday, October 24, 2005, at 11:27 AM,=20
skiasonaranthropos@FSMAIL.NET wrote:
> Sorry for chipping in, sorry about the pun, but the alarm bells went=20=
> off
> reading the statement =93 ... earthenware w/majolica glaze stronger =
than
> high fire bodies when doing modulus of rupture tests.=94
Hi Antony,
I can help out here. Pete Pinnell reported this data on Clayart on Dec=20=
20, 2001. Search for "clay body strength" on that date and the full=20
post will appear. Below is part of that post. But also remember modulus=20=
of rupture does not necessarily correlate to chip resistance or other=20
properties we think of when we think about "how strong clay is". It is=20=
just one property of several that could be measured.
from Pete Pinnels post on 12/20/01:
> What were the strongest clays? This will surprise you- it certainly=20
> did me. The strongest clays, consistently, were (drum roll, please)=20
> red earthenware clays fired to a full cone 04.
>
> Yep, that=92s right. Plain old Redart based, smooth red earthenwares.=20=
> They were stronger than smooth, brown or gray stonewares, and even=20
> stronger (over all) than porcelain, which I had assumed would be best.
>
> Yes, it was very important to fire them to a full cone 04: cone 06=20
> didn=92t hack it. Surprisingly, taking them to cone 1 did not increase=20=
> MOR, though they certainly were denser and felt more solid and chip=20
> resistant.
Regards,
John
Ron Roy on wed 26 oct 05
Mullite is formed in clay beginning at about 1000C but it's presence cannot
be insured till about 1150C. Mullite crystals are long and thin and provide
a mechanical strength to clay bodies. It is the interlocking of these
needle like crystals that give stoneware the superior strength to resist
chipping better than earthenware.
There are some serious questions arising from Pete's experiment - as he
says - he was surprised - because the literature consistently says -
stoneware is stronger than earthenware. See the above explanation re
Mullite formation as the reason.
What we need to know about Pete's experiment in order to speculate about
why his results are inconsistent with reality are -
Were the bars glazed or not?
Was there any testing done on the stoneware and porcelain to determine
absorbency - in other words were they under or over fired?
Once we know the details it may be possible to find an explanation for his
unusual results.
Saying that not all stoneware is not properly vitrified to exclude water is
correct but the way - and it can be just as dangerous as earthenware in a
microwave.
I would be interested is learning of a way to make unvitrified clay not
absorb water - what is it about those earthenware pots - that don't get hot
in a microwave oven.
It would be a great help to all potters to find why some earthenware pots
function well in that situation.
In the meantime it would be wise for anyone making functional ware to find
out if their handles get hot because of absorbed moisture in a microwave
oven.
The test is quite simple - make sure the ware has a chance to absorb some
water - they come out of the kiln with no water - so leaving them submerged
in water for a week should do it. The put a container with some water in it
(to save your oven) and the pot beside it. Microwave for 10 seconds - touch
the pot you are testing - is it getting warm? Keep repeating the test till
the water in the container boils - if the ware you are testing gets too hot
to touch - you should consider a label warning your customers to be
careful.
RR
>Once more, I really challenge the blanket statement that lowfire bodies
>chip easily. I have a lot of Stan Andersen dinnerware that I use daily,
>and NONE of it is chipped, while the porcelain cups from a well-known
>potter are all chipped. Give this one a rest. Not all earthenware bodies
>are like third-world, underfired wares. Look up Pete Pinnell's tests on
>clay body strength, which should be in the archives. He found
>earthenware w/majolica glaze stronger than high fire bodies when doing
>modulus of rupture tests.
>
>
>
>Ditto the microwave indictment. Not all earthenware gets too hot to
>touch in the microwave. I have personal experience, and have found that
>some works very well in the microwave. Likewise, not all stoneware
>bodies stay cool. Many are NOT vitrified and do absorb moisture. I have
>a vapor-glazed stoneware pitcher that really seeps badly. As ever, it
>all depends on the body composition, firing, and glaze.
>
>
>
>Linda Arbuckle
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
scott lykens on sun 30 oct 05
I must add, that i fire my earthenware in the cone 1-3 range.
It produces a much higher pitched ring than most commercially prepared cone
6 stonewares.
it also pruduces a much lower absorbtion %.
We have seen much discussion on the proper way to test for absorbtion and
semantics aside, the cone 3 red eathenware has produce absorbtion results
that prove tighter than some commercial cone 10 stonewares, and some
commercial porcelains.
When at Watershed, many of us overfired the local earthenware clay to cone
10 in woodkilns with resonable results, finding demorfaties to mimic closley
the stoneware issues.
to get a real good look at strength of clay body and absorbtion properties,
fire your favorite stoneware or porcelain to cone 04 with your favorite red
clay,
then do it again at cone 3.
>From: Linda Arbuckle
>Reply-To: Clayart
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: glazed Earthenware leaching moisture
>Date: Mon, 24 Oct 2005 01:20:08 -0400
>
>Well, I think we've had this dance about earthenware before. Below, I
>think, Ron, that's it's the earthenware BODY that isn't vitrified, not
>the glaze as you wrote.
>
>
>
>I think the problem as originally questioned (a glaze issue) is from
>several things. One, earthenware bodies are not usually vitrified. They
>often will absorb moisture if there is exposed clay. Using terra
>sigillata does not waterproof the clay, but does cut down on absorption.
>
>
>
>
>Another factor is that some of the useful lowfire fluxes - sodium and
>potassium (alkaline fluxes) - have a high coefficient of expansion. When
>heated they expand a relatively large amount compared to other fluxes,
>and upon cooling shrink a lot. If the glaze shrinks more than the body
>in cooling, you have crazing. There are a number of beautiful "Water
>Blue" alkaline glazes that are lovely turquoise w/copper + alkaline
>fluxes. These glazes are usually crazed due to high sodium/potassium
>fluxes that also provide the lovely turquoise color w/copper. Some dark
>and/or opaque glazes are crazed, but it may be hard to see the crazing.
>These would tend to let liquids seep.
>
>
>
>Another factor for lowfire is cristobalite inversion. Check out Frank
>Hamer's "Potter's Dictionary" for the complete explanation. Free silica
>(i.e. not chemically bonded to something) in a clay body fired above
>about 1950 deg F can be changed to cristoballite. This is about cone 03,
>give or take. Cristobalite inverts (becomes about 2% smaller) at about
>451 deg F (the temperature paper burns). This can put a glaze under
>compression and counter crazing, if it happens in the right amount. If
>there is a large amount of cristobalite formed, and this shrinkage
>occurs (long after the glaze is rigid), this can cause dunting (cracking
>in cooling). Dunting is usually more of problem w/high fire because it's
>fired farther above the temp for cristobalite formation, and for a
>longer time. But I think the phenomenon of cristobalite formation is the
>reason that many earthenware potters fire to 03 rather than the 06 that
>commercial glaze products use. A little bit of that compression is good
>to counter crazing, and you'd have to fire to about 03 to reach that
>threshold.
>
>
>
>Majolica is a lowfire glaze, but unusually high in calcium flux for
>lowfire. This tends to make for less crazing than a glaze that has more
>sodium and potassium for fluxing, as the coefficient of expansion for Ca
>is lower than sodium and potassium. High calcium tends to make a hard,
>durable glaze.
>
>
>
>Once more, I really challenge the blanket statement that lowfire bodies
>chip easily. I have a lot of Stan Andersen dinnerware that I use daily,
>and NONE of it is chipped, while the porcelain cups from a well-known
>potter are all chipped. Give this one a rest. Not all earthenware bodies
>are like third-world, underfired wares. Look up Pete Pinnell's tests on
>clay body strength, which should be in the archives. He found
>earthenware w/majolica glaze stronger than high fire bodies when doing
>modulus of rupture tests.
>
>
>
>Ditto the microwave indictment. Not all earthenware gets too hot to
>touch in the microwave. I have personal experience, and have found that
>some works very well in the microwave. Likewise, not all stoneware
>bodies stay cool. Many are NOT vitrified and do absorb moisture. I have
>a vapor-glazed stoneware pitcher that really seeps badly. As ever, it
>all depends on the body composition, firing, and glaze.
>
>
>
>Linda Arbuckle
>
>14716 SE 9th Terr
>
>Micanopy, FL 32667
>
>(352) 466-3520
>
>arbuck@bellsouth.net
>
>http://www.arts.ufl.edu/artex02/html/ceramics/arbuckle.html
>
>
>
>
>Date: Sun, 23 Oct 2005 12:34:44 -0500
>
>From: Ron Roy
>
>Subject: Re: glazed Earthenware leaching moisture
>
>
>
>Hi Carty,
>
>
>
>Earthenware glazes are very rarely vitrified enough to not absorb water
>and
>
>leak.
>
>
>
>It takes so much flux to virtify at low temperatures that - if you try
>to
>
>virtify the clay enough to stop leakage - the melting happens so fast
>you
>
>will eventually have overfired clay.
>
>
>
>The other problem is - if the clay can absorb water - the clay will
>
>eventually rehydrate - expand and craze any glaze that is not already
>
>crazed.
>
>
>
>There are some clay bodies that will resist rehydration (high talc
>bodies
>
>for instance) and that will help the delayed crazing problem but not
>solve
>
>the the moisture absorption - which can lead to overheating in a
>microwave
>
>oven.
>
>
>
>Some potters claim that a water proof coating of say Terra Sig will stop
>
>the absorption of water but I have not seen any tests that prove this.
>It
>
>would mean the surface would have to be perfect.
>
>
>
>Earthenware is easily chipable compared to stoneware - once chipped -
>water
>
>can get in of course - no mater how perfect the waterproof surface.
>
>
>
>Glazes would normally be water proof but because they do not cover the
>ware
>
>completely most of the time - they cannot keep the water out
>
>
>
>I think the main problem is when any microwaved pot becomes hot enough
>to
>
>burn the cook - people will normally assume a mug handle will not get
>hot
>
>enough to burn your hand for instance.
>
>
>
>RR
>
>
>
> >I fire both cone 04 and cone 6. Cone 6 pottery in my studio typically
>
> >doesn't leach moisture. Cone 04 does more often than not (not always).
>I
>
> >noticed a comment on a glaze recipe for cone 04 that it leaches
>moisture
>
> >(makes me think it is not expected).
>
> >
>
> >For low fire I use Highwater Clay's Stan's Red - which is quite well
>
> >vitrified at cone 04. I realize that earthenware clays have a slightly
>
> >different body structure when fired than stoneware clay bodies. But I
>am
>
> >starting to wonder if seepage isn't a glaze issue, not necessarily a
>clay
>
> >body issue.
>
> >
>
> >The more I contemplate this, the more I have to ask, are cone 04 glazes
>
> >often not fully melted? My maiolica pieces hold water, as do some
>using
>
> >Amaco (cone 04) Opalescent glazes. Most other cone 04 glazes (I can
>
> >understand the obvioulsy crazed ones leaking) let moisture pass.
>
> >
>
> >Is seepage of moisture with earthenware an issue of fully melting or do
>
> >glazes at cone 04 have some physical reason to act almost like
>
> >semi-permiable membranes?
>
> >
>
> >Just curious, and I haven't seen much other than anecdotal discussion
>on
>
> >this issue.
>
> >
>
> >Carty
>
>
>
>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
>
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>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
>melpots@pclink.com.
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