Stress-test for ovenware:
   A modest do-it-yourself first step for the low-tech studio potter

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[1]  — 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 create.

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 “ring”of 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” grade. 

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:

  1. 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.

  2. 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.

  3. 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 removing. 

    Multiple tests yielded no failures in any of the clay bodies, or in any of the shapes.

  4. 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.

  5. 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 rim.

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 cracks.

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: 


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:  

  1. 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 baking applications? 

  2. 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?

  3. 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? 

  4. What do we know about how our customers actually use the objects which we make:  how often and under what conditions? 

  5. 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? 

  6. 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? 

  7. 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?

  8. 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 potters, directly?)

  9. 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? 

  10. 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 face. 

  11. 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? 

  12. 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 realistic?

For those of us who use commercially produced ovenware bodies, there are  additional questions : 

  1. What do the commercial clay body developers know that we don’t know? 

  2. Should we be expected to rely upon commercial producers without expecting them to explain what’s 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? 

  3. 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? 

  4. Where is the information regarding industry standards with respect to ovenware located (assuming that they do exist), and how can one access this information? 

  5. 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 Goshen, Indiana.

This article is reprinted with expressed permission from Ceramics Monthly Magazine, PO Box 6102, Westerville OH 43086-6102, USA; www.ceramicsmonthly.org

© Dick Lehman, March 1998. All rights Reserved.



[1] 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.