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earthenware (longer rant); pete pinnell on clay body strength

updated tue 30 aug 05


Linda Arbuckle on mon 29 aug 05

Ron Roy said:
Date: Sun, 28 Aug 2005 15:09:13 -0500
From: Ron Roy
Subject: Re: earthenware in the microwave

It does need to be discussed - there is a lot of earthenware being made
sold that can actually be a hazard in a microwave oven.

If we talk about it - it can help those who choose to work at low
temperatures - how to make the ware safe to use or to mark it unsafe for
certain uses.

Anyone who does make their living selling earthenware should be very
interested in making others aware of the problems and the solutions.

I would be interested in how you prevent water absorption so that
do not heat up and become a hazard for instance.

In the end it's not about bashing anything - it's about making it


- - - -
What I object to is pillorying all earthenware as a 'bad' clay body.
High-fire ware is more subject to dunting and shivering from
crystobalite formation than earthenware. Do I hear people says, "Whoa,
only experts can make decent pottery using high-fire." Nope. And
crystoballite inversion takes place at about the temperature paper burns
- about 451 degrees F, so you can have ovenware problems (pieces
breaking apart) if you bake hot. Could also cause cups to crack when
boiling water is poured in. I haven't seen any warnings on stoneware
pots that this could be an issue. Porcelain is so dense it's often
brittle and chips easily and can easily be cracked in the microwave by
localized heating (e.g. those strips of fake bacon on a porcelain
plate). Does anyone gripe about people who use porcelain and tell them
it doesn't work? Haven't seen that. I have lovely sushi plates by a
famous pottery whose work I love that are vapor-glazed stoneware.
They're punky, the slip has popped off in places, and they're all
chipped. I have porcelain cups from a very good potter often on the
workshop circuit. They're all chipped from normal use (and I don't have
dishwasher and was everything by hand). I have a nice vapor-glazed
pitcher in stoneware that is glazed inside (lovely flashing outside),
again by a noted potter, and it seeps badly. I've had a stoneware cup
handle heat up in the microwave so much I needed a potholder to remove
it. Again, I don't hear anyone saying, gee, those "stoneware" potters
should make sure their work is safe.

All of pottery making is fraught w/technical difficulties. And the
answer to many questions is, "It all depends.... "

Earthenware does not generally vitrify. I.e. it stays somewhat
absorbent. This makes it a good insulator and it tends to be more
forgiving to heat shock. Water in a porous body may heat up during
microwaving, whether it's earthenware or stoneware. It is possible to
make vitrified stoneware and porcelain, but not all stoneware and
porcelain is compounded and fired to create a vitrified body. A
vitrified body will transmit heat more quickly due to the density, and I
would suspect is less able to take localized heating w/o breaking.

On my terracotta works, I use terra sigillata on exposed clay areas to
cut down absorption and create a better-looking finish than the bisque
look. Stan Andersen does not use sig, but his wares fare quite well in
the microwave. I make lunch on his plates and have made tea in his cups
in the microwave. I make tea in my cups, regularly, and have no problems
removing the cups by hand from 2 minutes in the microwave. So, yes,
terracotta can be a good tableware body. I think many people think of
earthenware as synonymous with very soft third-world, underfired ware.
Contemporary clay-body makers do a better job.

I cannot say that I've done an exhaustive research on what percentage of
studio earthenware works well for tableware (or exactly what % of
absorption makes something too hot for use in the microwave), but I do
know that I've had problems w/some works in earthenware getting hot in
the microwave, but ditto w/some stonewares. I can say that some
porcelains chip very easily, but my Stan Andersen majolica has been in
use for over 10 years and there are NO chips. So, I think people who
make sweeping generalizations should pay attention and be more
circumspect about the details and reality. There are technical issues
with all clay bodies, and ALL potters should work on making useful, safe
products. This is no less true for people using stoneware and porcelain
than earthenware (although the discussions have had finger-pointing that
earthenware is somehow more problematic). Below is a copy of a post of
testing on clay body strength from Pete Pinnell. In this case,
earthenware was strongest.

Perhaps I am a bit touchy after years of reading some of the major clay
figures who worked in only high fire say that nothing worthwhile can
happen with earthenware pottery, and that the materials are inherently
inferior. I do work in stoneware now and then at school. Stoneware
teapots on the Blue Spiral web site for the "Iced/Hot Tea" show.

p.s. Mel, thanks for the comments on the MFA post. You said, "with all
the high fire information around, program after program, how better for
a young potter to fill out your life study better than learning
earthenware." Interestingly enough, many of our pottery grads are not
doing earthenware. For example, we've had Kate Murray doing c10
reduction, McKenzie Smith doing various high-fire vapor glazing, Missy
McCormick doing cone 10 wood and soda, Nancy Barbour and Tara Wilson
doing wood, Kathy King, Shannon Nelson, and Valerie Duncan doing cone 6
oxidation. On the earthenware side, Wynne Wilbur (teaching at Arrowmont
this fall) did do majolica on terracotta, and recent MFA Tammy Marinuzzi
is working in earthenware w/slips and glazes. "It all depends" on what
the artist is interested in and what materials are the best arena for
each expression. We try to keep a diverse studio and embrace a wide view
of making art w/clay.

Linda Arbuckle
14716 SE 9th Terr
Micanopy, FL 32667
(352) 466-3520

From: Ceramic Arts Discussion List [CLAYART@LSV.CERAMICS.ORG] on behalf
of Pete Pinnell [ppinnell1@UNL.EDU]
Sent: Thursday, December 20, 2001 11:15 AM
Subject: clay body strength

For the final project in my Clay and Glaze class this semester, we mixed
about 50 clay bodies for testing, including red and white earthenwares,
stoneware, porcelain, and sculpture bodies. Besides other tests, we
extruded numerous bars of each body and broke them to measure MOR
(Modulus Of Rupture, which is a measure of the bending strength). There
are other strength tests that can be done (chipping tests, for
instance), but MOR is a quick and easy way to predict how well a body
will hold up to the bumps of everyday use.

Out of all these tests, there were a number of interesting trends:

1. Any amount of grog weakens clay bodies, especially in sculpture
bodies that are essentially underfired. Some of the sculpture clays were
so weak at cone 04 that we couldn't measure them- the bars broke at
initial contact before any stress was applied. Any texture in the clay
tended to have the same result, though the texture from using 50 mesh
fireclay seemed to have only a minimal effect. Really fine grogs- those
less than 80 mesh- also had little effect.

2. Glaze made a huge difference in strength. Crazed glazes lowered
results 50% or more from the strength of the same bar unglazed. Uncrazed
glazes raised the strength of the bars from 50 to 100 %. I had read this
before, and assumed that it was mostly related to the lack of surface
flaws on a smooth glaze (cracks like to start at a flaw- take away the
flaws and it's more difficult for a crack to start). What I found
interesting is that the amount of compression also mattered. We glazed
the porcelain bars with three different versions of my Pete's Clear
glaze, which ranged from mild compression for the original version to a
very low expansion version that places the clay in a very high
compression. Consistently, the higher compression versions produced
higher MOR results.

3. Clays have to be fired to maturity to get good strength. Even firing
porcelain bodies to cone 9 rather than 10 lowered strengths a good deal.
As an aside, I define maturity as the point at which a body achieves its
best strength and glaze fit, and no longer suffers from marked moisture
expansion. Absorption, in my opinion, is not a good indicator except
within one clay body group (such as "high fire porcelain"). Porcelains
may need to have less than 1% absorption to avoid moisture expansion
problems, while mature white earthenwares can have upwards of 20%
absorption (which is why those cheap white tiles on our shower walls
don't develop delayed crazing).

4. "Smooth" counts for more than "glassy", which seems to contradict one
bit of standard wisdom I've heard in the past.

5. Quartz seems to be a problem- at least in a minor way. Porcelain
bodies that used a combination of pyrophyllite and quartz were stronger
than those which used only quartz as a filler. It's a bit of a mixed
bag, though, because glazes on pyrophyllite bodies tended to craze more.

What were the strongest clays? This will surprise you- it certainly did
me. The strongest clays, consistently, were (drum roll, please) red
earthenware clays fired to a full cone 04.

Yep, that's right. Plain old Redart based, smooth red earthenwares. They
were stronger than smooth, brown or gray stonewares, and even 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
didn't hack it. Surprisingly, taking them to cone 1 did not increase
MOR, though they certainly were denser and felt more solid and chip
resistant. Within red earthenwares, we got consistently higher strength
from those using wollastonite as a secondary flux (5 to 10%), rather
than talc. It seemed best to use red clay in amounts of 50 to 70%, and
while Redart alone (for the red clay portion of the body) gave the best
strength, we got much better workability (and only a tiny bit less
strength) by using a mixture of red clays, such as Redart mixed with
Ranger Red (from Texas) and Apache Red (from Colorado).

As with porcelain, the clay was made much stronger with glazes that fit,
and higher compression glazes were strongest of all. Our all-time
champion (for strength, NOT workability) was the following recipe,
glazed with Linda Arbuckle's Majolica and fired to a full cone 04.

Redart, 60%
KT 1-4 Ball Clay, 30%
Wollastonite, 10%

I thought you might find this interesting. I only teach a Clay and Glaze
class one semester every three years, so while I plan to do some follow
up tests (these tests raised as many questions as they answered), don't
look for those results any time soon!

Pete Pinnell
University of Nebraska at Lincoln

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Steve Slatin on mon 29 aug 05

Linda --

With all respect ...

The issue of imperfect surfaces, weak surfaces,
limitations of unglazed surfaces, etc. in mid-fire and
high-fire stoneware *is* a hot subject here. Just the
last two weeks there was another round of it over the
issue of crazed surfaces. (One prominent potter
opined that if you don't even know what causes pinging
you've got no business trying to teach pottery; a few
dozen people replied that they loved their crazed
ware, completely missing the point ...)

Folks who take a well-sealed surface and
low-absorbency body as essential to food-contact
surfaces and folks who do not dish out, and take,
quite a bit of 'tude from each other. Ron isn't
treating earthenware any differently than he does
mid-fire or high-fire ... nor should he. Or anyone

I didn't see anyone pillorying all earthenware here
(nor, as you point out, should they) what I saw was
people warning about things that are widespread
problems with earthenware and speculating about the
problems users might have.

I find, as a vendor of mid-fire ware, many people will
walk up to my display and say "oh, is this pottery?"
or "is this ceramics?" and when I assent (after all,
what else could I say?) they make a complaint about a
piece they bought that died in a microwave or cracked
after being through the dishwasher or something of the
sort, and then walks off in disgust. We all suffer
when poor-quality* materials are sold.

I end up having to parrot over and over again that I
test each clay and glaze combo for microwavability,
dishwasher safety, and leaching ... one of my market
managers likes to walk up while I'm with a customer
and, in a piercing tone, say "Well, sir, are these
pieces microwave safe? Are they DISHWASHER safe?"
{In his defense, it *is* funny.}

And thanks very much for quoting the P. Pinnell post,
which I had not previously read. His test technique
was interesting, and his result -- a 50% difference in
strength for the uncrazed surface -- was more in line
with ware durability than the 500% elsewhere quoted.

Best wishes -- Steve Slatin

*And let's not get into another round of snot-o-grams
from people who love and those who hate their crazed,
or unglazed, or too-rough-to-wash surfaces -- they are
suitable for admiring and for very careful handling
and admiration, but they're not inherently utilitarian
and shouldn't be sold to most buyers or used in most
families. If they are sold as such they lead to a
wide range of dissatisfied users, and reduces the
range of future potential buyers. If that's good for
any potter, I'd like to here how.
As utilitarian ware, leaching, crazed, unglazed, and
absorbent pieces are simply inferior, no matter how
beautiful they may be.

--- Linda Arbuckle wrote:

Steve Slatin --

Drove downtown in the rain
9:30 on a Tuesday night
Just to check out the
Late night record shop

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