Stress-test for ovenware:
A modest do-it-yourself first step for the low-tech
by Dick Lehman
Right before Christmas 1997 several baking dishes were returned to
my studio due to unexplainable/unexpected cracking (after 2-3 years
of use). Customers assured me that all appropriate care had been afforded
these pieces. I had no reason to believe that these regular customers
had not followed all the normal safeguards and my recommends for use
of ovenware pieces: 1) never going from freezer to oven, 2) never placing
in a preheated oven and 3) being sure to fill the pieces so that food
is in contact with all interior surfaces of the pot).
Additionally some mugs were returned to me which had multiple cracks
going from the rim, down the pot about one or two inches. These cracks
reportedly appeared through absolutely normal (or less than normal)
use: no microwave use, the liquid poured into these cups had been steeped
in a teapot before being poured into the mugs...the mugs, in some
cases, had been preheated with warm tap water.
I had a problem. What to do? The failures were so few, considering
the number of pots I sell each year, that I could have just replaced
the pieces and chalked it up to the cost of doing business. And it could
have been just a coincidence that these pieces had all come back to
me within a few weeks time perhaps I should
just ignore all this. On the other hand, it may have been that I was
on the front end of an avalanche of all the ovenware pieces I had made
within the last three years. That was an awful prospect!
Regardless of not knowing whether there would be more failures in my
ovenware, I thought it important to try to improve the clay body if
possible. And I hoped to do so without changing the clay body to such
an extent that it would alter the colors of all my glazes.
So I set out to attempt to create several new ovenware bodies. My knowledge
and abilities in ceramic engineering are limited, and I am embarrassed
to say that my attempts at altering/improving my ovenware recipe were
more intuitive than academic...and my reliance on anecdotal information
and common knowledge more pervasive than my acquaintance
with good science or accepted research. However, I did create/gather
six new ovenware clay bodies to test.
My deficiencies as a ceramic engineer notwithstanding, I needed to
develop a means of testing those resulting clay bodies to see
if any of the new formulations were actually an improvement over what
I was already using. (Assisting the small studio potter to develop a
dependable wide-spectrum-use high-fire ovenware is another article/discipline
all together, and should be reserved for someone who knows more than
I do about clay properties, thermodynamics, and ceramic engineering...and
I wish someone would write that article.)
Regardless of how we finally come to our ovenware recipes or
what those recipes are we need a methodology by which to make
thermal shock tests which are thorough, inclusive, severe, and comparative.
And these testing procedures need to be accessible, repeatable, and
to some extent, reliable. This testing procedure is what I set out to
I found no published recommendations for uniform thermal shock testing
or integrity testing of ovenware bodies...at least none which were geared
toward low-tech production potters. What follows are the methods and
approaches upon which I decided after talking with a variety of other
potters, and reflecting on the kinds of pot-failures we all had encountered.
Following each step of the shock test, the pots were each individually
tested for integrity. This integrity check is at best, perhaps, colloquial,
but it is a test which pays attention to both the sound of the piece
(the noise it makes when it is struck with a metal rod), and the look
of the piece (can I see any cracks?). The sound test method was
achieved by placing each pot on a metal stand, and then striking it
on the rim with a metal rod. (Most all of us recognize the difference
between the beautiful ring which a pot of integrity makes
when struck, and the dull thud/thunk which occurs when pots
have been cracked, overly stressed or damaged.) This striking was done
gently so as not to break the piece with the rod. Each piece
was struck with as similar a stroke as possible. Each piece was stuck
at the same position on the pot. While not each piece sounded the same
(due, at least in part, to small unavoidable variations in shape and
thickness) and while not each clay body sounded the same (due to recipe
differences) I did take note of the sound...the ringof each,
kept a description at hand, and tried (as the test progressed) to also
simply remember, and/or to compare to similar-recipe pots which had
not as yet been shock tested (control pieces).
The visual test involved putting water (heavily stained with
food coloring) into each piece for at least 10 minutes, following each
step of the test. I was looking for small cracks which might not be
visible to the naked eye (and which might not have revealed themselves
in the sound test). I assumed that any crack would collect,
through capillary action, some of the food coloring, and maintain some
color (in the crack) when the food-color-water was poured out of the
pot (which, by the way, did prove to be the case). Incidentally, all
the pots used for this test were glazed with a white glaze in order
to make the blue food coloring more obvious. Additionally I always checked
both the interior and exterior of each pot following each visual
test the food coloring almost always made its way through
the crack/failures in the pot and was visible on the bottom of the pots
as well as in the interiors; however in rare cases which involved really
small/beginning cracks, the failures were visible on the inside only.
I decided to test both mugs and baking dishes. The mugs were of a
variety of shapes, and utilized all 6 of the different clay bodies I
was testing. The baking utensils were of two kinds: flat-bottomed baking
dishes (two sizes), and bowl-shaped bakers.
The mugs went through two stages of testing. The steps were designed
to provide progressively more thermal shock. The first test was simply
to fill each room-temperature mug half-full of boiling water. By filling
only half-full, I reasoned that there would be more dissonance within
the piece itself: not only did the piece suddenly go from ambient temperature
to boiling temperature...but only part of the piece would make this
transition, thereby causing the piece, within itself, to have both ambient
and hot surfaces...increasing the interior thermal stress. This first
test yielded no failures.
The second step was to freeze the mugs at -10 degrees Fahrenheit for
45 minutes. The mugs were then brought out of the freezer and immediately
half-filled with boiling water. (One purely anecdotal observation which
seemed to confirm the presence of noticeably different temperatures
within one piece: one and a half minutes after a frozen
mug was filled with boiling water, its bottom side was too hot for me
to comfortably pick up, but there was a frozen bead of ice, still intact
on the rim of the mug.) This second test revealed no failures.
The freeze/boil test was repeated once again for all the mugs. Again,
no failures resulted.
With no failures in hand after these tests, I reasoned that the mugs
had experienced far more dramatic stress than normal use would render. So
I discontinued the test, with all 6 clay bodies receiving a passing
In retrospect, I believe that continuing the testing (perhaps hundreds
of cycles) until at least some of the mugs failed....(or a test designed
to be even more severe and which ultimately would have caused at least
some of the mugs to fail) would have produced more usable information. As
it was, with all clays passing, I knew little more about
the maximum stress capacity of these 6 clay bodies at the end of the
mug test than I did when I started. I had no mean of first failure
for these clay bodies.
I moved on to testing the baking dishes. I tested multiple pieces of
each of the six recipes. I tested two basic shapes: 1) flat-bottomed
baking dishes with sharp right-angle corners where the bottoms
of the pots met the side walls; these I made in two sizes: 11 1/2 inches
in diameter, and 8 inches in diameter, and 2) baking dishes with rounded
bowl-like contours (one size only: 10 inches in diameter). These tests
I devised with progressively more dramatic shock intended/designed into
each subsequent step. I determined that I would continue testing until
at least some (if not all) the ovenware bodies failed. The order in
which these pots were shock-tested is as follows:
Baking dishes, at room temperature, were placed into the preheated
500 degree Fahrenheit oven. They were allowed to remain there
for 10 minutes. The pieces were then pulled out, allowed to cool
naturally, then tested for integrity. (I included this fairly
benign test since I have always cautioned my customers never to
place the baking dishes into a preheated oven.
This test was repeated 3 times for each piece, for each of the
six clay bodies. No failures were observed in any of the clay
bodies, or any of the shapes.
Reasoning that sometimes my customers may put food-filled baking
dishes into already-hot ovens, I tested each piece several times
by placing an ambient temperature piece, half-filled (and not
fully filled as I recommend to my customers) with water, into
the 500 degree oven (having the pieces half-filled created the
potential for exacerbating the shock within each individual piece).
Repeated tests yielded no failures for any of the six clay bodies.
I next froze empty baking dishes in the freezer for 45 minutes.
Then I took pots directly from the freezer to the preheated 500
degree oven...and left them in the oven for 10 minutes before
Multiple tests yielded no failures in any of the clay bodies,
or in any of the shapes.
Next I froze a half inch of water in the bottom of the baking
dishes before taking them directly to the preheated 500 degree
oven. The pieces remained in the oven until the ice had melted
and the water began to boil.
This is the first point at which I experienced failures. Two
of the clay bodies failed. However, only the flat-bottomed pieces
failed. But both sizes of flat-bottomed bakers failed in these
two recipes. The cracks were clear and obvious.
I had predicted that the frozen water test would crack all the
pieces. That not being the case, I moved forward to a more dramatic
shock test: I froze all the pieces (empty), placed them in the
preheated oven for 15 minutes, then took them directly to the
sink where 1 inch of cold water was poured into them.
At this point all 6 clay bodies failed in at least one case. But there
were some interesting observations about the ways in which these different
recipes failed: two of the recipes (the same two which failed in
step #4) failed in quite noticeable fashion with large cracks,
and with some audible noise as they cracked. (However none of these
pieces actually broke apart into multiple pieces.) The cracks began
in the bottom interior of the pieces and progressed directly up to the
Two other bodies failed less noticeably: the cracks were all shorter
and thinner than the previously mention two clays. But like the first
two mentioned, the cracks began in the bottom interior of the piece
and progressed in one single line, up to the rim.
The fifth clay recipe showed relative strength compared to the other
bodies previously mentioned. In the flat-bottomed baking forms, all
the cracks were contained within the bottom and did not proceed up the
side of the piece. The cracks were not obvious, and the sound
test did not reveal failure only with the visual
test were the cracks visible, thanks to the food coloring in the
And regarding the sixth clay body: only the largest of the flat-bottomed
bakers failed in the final stress test. Even then, there were no cracks
visible to the naked eye. The sound test appeared to indicate
a pot of integrity. The visual test, however, did reveal
the smallest of cracks in the interior of the bottom of the piece. They
were the smallest/shortest cracks of any test piece.
The smaller flat-bottomed baker (made of the sixth clay body) survived.
In fact it was cycled (again!) through the freezer-with-1-inch-of
-ice/500-degree-oven test three times with no failure...then
twice through the freezer/500-degree-oven/cold water test. Again
it survived and passed all the integrity tests. We are now using it
in our kitchen.
And finally a word about the bowl-shaped bakers. Five of the six clay
bodies in bowl-shaped forms also failed (the same five clay bodies which
failed in the flat-bottomed-shaped pieces). However it should be noted
that it required additional applications of the most severe test in
order to cause them to fail. The soft line of the curvilinear form seemed
to distribute and withstand more stress before failing. The bowl-shaped
form made from the sixth clay body did not fail after repeated testing
under the harshest tests.
CONCLUSIONS AND SEVERAL ADDITIONAL LEARNINGS:
It seems clear that one clay body was the most durable in these tests.
What may need to be asked is whether these tests in any way relate
to reality....to real use, etc. While I suspect that my observations
and conclusions would be considered only anecdotal in a
thoroughly academic/professional testing environment, I do believe that
my conclusions are not totally without merit, nor should these learnings
be totally dismissed. However I realize these tests do not give conclusive
evidence about how a particular ovenware body is going to fare after
3-10 years of regular use (or, sometimes, abuse). But the tests do give
immediate comparative evidence regarding how different ovenware clay
bodies respond to a series of increasingly severe thermal shock tests.
I believe that for those of us who make pots for use for baking
and serving and washing, and continued use there needs to be
an affordable (the cost of this test for me was my time, and the broken
pieces), accessible and immediate testing method for the ovenware bodies
we develop. Without the aid of some affordable, relatively quick and
thorough testing method, we as potters are left with only several options
none of which may serve us well: 1) use a proven industrial or premixed
ovenware clay body. (This option may not satisfy our individual taste
with regard to texture, color, glaze compatibility, or workability.
It may or may not be truly a quality ovenware body under the conditions
by which any particular individual makes and fires. And additionally,
to make such a choice determines that one will be fully subject to the
quality control methods in mixing and composition, which someone other
than ourselves utilizes and enforces.); 2) use and test our own products
for two of three years before marketing them (who among us can afford
this?); 3) go about blithely making and selling our ovenware, only to
perhaps make a costly discovery two or three years later, that we have
two or three years worth of pots which must be replaced for unhappy
and unsatisfied customers.
The method which I developed and used is not foolproof or even nearly-fully
proven. It relies too much, perhaps, on anecdotal information. It may
or may not relate exactly and in a calibrated way to how clay bodies
stand up to regular use over the course of many years. But it does give
those of us who wish to mix our own ovenware clay bodies, a methodology
for uniform comparison of clay bodies in the light of severe and multidimensional
thermal shock. It will provide for us at least some helpful information
upon which to make some comparative, informed decisions.
In the process of performing these tests, several other observations
and conclusions surfaced for me. Some may be self-obvious....others
may be merely colloquial. All may be worth considering as you make your
next ovenware forms:
The larger the piece is, the more likely it is to fail in ovenware
The broader and flatter the bottom of the pot is, the more likely
it is to fail.
Pots with soft, rounded corners, curves, and lines seem to survive
better than ones with sharp corners and direction changes...no matter
Filling the entire exposed inner surface of a baking form with
food (thermal mass) will lessen the thermal shock and prolong the
life of the piece (our customers need to know this).
This test provides no information about the affects of glaze
fit on a particular clay body, or the implications of glaze
fit upon thermal shock failure.
This test is not informed by any of the existing normative standards
which (I assume) exist within industrial ceramics related to mean
of first failure the average number of times a piece
must be shock tested before it will fail.
Even with all the limitations of my methodology, we can take heart
that these severe tests indicate that some clay bodies (at least
in the short run) will perform far above the normal expectations
and requirements which most studio potters would hold, with respect
to the environments in which they might expect their ovenware clay
bodies to satisfactorily function.
The conclusions and observations which this testing implies are
not related to how microwave-worthy these ovenware bodies
might be. All tests were made in conventional ovens. No microwave
tests were attempted. (That is a separate testing procedure.)
Some additional musings:
For those of us who make our own clay bodies this testing procedure
may beg the question on a number of fronts:
What do we as potters know about our primary materials? What do
we know about each of the clays/ingredients in our clay bodies...what
they do individually when fired, and how they interact with each
other, how the cumulative body of ingredients respond in normal
Are there demonstrable correlations between our anecdotal observations
and our common knowledge about what makes for a good
ovenware body, and the standards proven in industry?
Should not a really significant base of ceramic engineering and
materials-awareness be a baseline expectation for all who graduate
with a bachelors degree in ceramics?
What do we know about how our customers actually use the objects
which we make: how often and under what conditions?
What are our expectations about how well and for how long
our ovenware will hold up under normal use?....and what
does normal use mean, explicitly?
What are our customers expectations about how well and for how
long our ovenware will hold up under normal use?....and
how do our expectations and the expectations of our customers compare?
Do we expect that as studio potters our works will be able to/should
be able to compete favorably (with respect to durability and versatility)
alongside the engineering expertise invested in, for example, Corning
ware? And is the durability and versatility of a Corning
the standard after which we seek? With what part of industrially-made
ceramics (if any) would we expect our works to compare favorably?
Might there not be some way to create a better/more productive
link between the experts in industrial ceramics and the educators
who teach most of us what we know about clay?....(or with us, as
Might there not be a way of establishing friendlier ties between
commercial clay producers and those of us who mix our own clays.....ties
which would better equip us to solve our own clay body problems
without overstepping the proprietary domain upon which commercial
clay producers depend (at least to some extent) in order for them
to stay in business and remain economically viable?
Would we not benefit from clearer avenues between potters and the
clay-experts in industry? After all, industrial ceramics has already
solved almost all the questions and problems which we will ever
Should not every production potter have the tools and know-how
to confidently and competently produce an ovenware clay body which
will live up to some generally-agreed-upon performance expectations?
It is reasonable for a production potter to expect that a single
clay body can serve all the needs of a wide product line? A potter
who creates a wide range of products with a wide range of product
application needs (conventional oven use, microwavability, liquid
storage including long-term oil storage in oil candles, sculptural
strength), and expects one clay body to do it all, is placing really
high expectations upon a single clay body. Are such expectations
For those of us who use commercially produced ovenware bodies, there
are additional questions :
What do the commercial clay body developers know that we dont
Should we be expected to rely upon commercial producers without
expecting them to explain whats in their clay formulas, and
how those component ingredients work? Could there not be more helpful
conversation between potters and clay producers, without the producers
giving away all their secrets?
Might there be more than blind following by potters
who utilize the products of clay producers....might the applications,
and needs, and problems and discoveries of the practitioners play
a greater part in informing the decisions which clay producers make
while formulating their commercial bodies?
Where is the information regarding industry standards with respect
to ovenware located (assuming that they do exist), and how can one
access this information?
Within industrial ceramics, there must be some generally accepted/understood
technical and thermodynamic and ceramic engineering understandings
about what makes a good ovenware body. Surely some of
this information is not proprietary or secret. If that is the case,
where can one find this information?....and how best might we production
potters understand and utilize it?
Dick Lehman is a frequent contributor to ceramics periodicals
throughout the world. He maintains a full-time studio and gallery in
This article is reprinted with expressed permission from
Ceramics Monthly Magazine, PO Box 6102, Westerville OH 43086-6102, USA;
© Dick Lehman, March 1998. All rights Reserved.
 As a point of interest, I am happy
to report that these pieces, which were all returned within a several
week time frame, were the only ovenware failures to be returned to
me during the entire next 12 -month (and counting) period.