Tuesday, April 19, 2016

Flameware!

I don't know what started my interest in flameware.  Maybe it's the fact that pottery is frequently used with food, in the guise of coffee cups, serving bowls, trays, plates, and so forth!  What potter wouldn't want to take the next step and bake a nice loaf of bread in a ceramic loaf pan, or better yet, slip a nice crock of macaroni and cheese under the broiler to produce a wonderful cheesy delight?

Many potters who work with stoneware are already willing to use their ceramics in the oven, as long as they follow certain rules:  Don't put the pottery in a hot oven!  Don't heat empty pottery!  Never use the pottery on the stove or under the broiler!!  But, flameware clay is supposed to allow you to do all these things.  Flameware clay is designed to endure use over an open flame.  In this post I hope to enumerate the resources I have found on the Internet about flameware clay, as well as describe our first tests with a flameware clay body.

Flameware Resources

I feel very grateful to all the people that have been so willing to share information about flameware clay.  There are only a handful of resources online for this, but they are very detailed and give some very helpful guidance as to the approaches and considerations.  Here are a few of my favorites:

The Flameware Techno File article by David Pier in the May 2011 edition of Ceramics Monthly gives a technical overview of the challenges in using ceramics over open flame, and the approaches that flameware clay bodies tend to take.  It has an excellent level of technical detail that I feel is accessible to many people.

  • This article introduces the issue of cristobalite expansion in stoneware clay bodies, which goes through a significant nonlinear thermal expansion in the 350°-400°F range - a common temperature range for cooking.
  • There is an excellent chart showing the expansion rates for various materials and clay bodies.  The article shows how spodumene and other materials with very low thermal-expansion coefficients can be used to create a clay body that can endure use over open flame. 
  • The article also discusses how the porosity of a fired clay body can protect it against cracking under thermal stresses:  the pores interrupt the spread of cracks, protecting the pottery.  Of course, if the pottery isn't formulated to endure such treatment, cracks will continue to form over time, and the pottery will eventually fail.
  • Finally, it also includes some recipes for flameware clay and glazes!  (Keep in mind that the glazes aren't really formulated to have very low coefficients of thermal expansion; rather, they tend to be matte glazes that can be applied very thinly, and will probably hold up under significant thermal stresses when they are thin.  However, I think the best long-term approach will be to formulate glazes with low COE that will fit the flameware clay body very well.)
A Discussion of Flameware is a 2011 CeramicsTECHNICAL article by George Chechopoulos that goes into a highly technical discussion about flameware clays.  The article talks about the importance of glaze fit on the clay body.  Since most clay bodies and glazes expand and contract with temperature changes, standard glazes tend not to be suitable for use on flameware clay because they will either craze or shiver over time.

Flameware by Ron Propst is a 1974 article from the Studio Potter newsletter that describes earlier attempts to formulate flameware clays.  One of the highly entertaining anecdotes in this article relates to the fact that spodumene (a common ingredient in flameware clays) has an unusual characteristic in that it expands irreversibly in the 1700°F temperature range, which is typically during bisque firing.  I have observed this behavior!  It is definitely something to keep in mind.  Thankfully, this expansion completes during the bisque firing so that glaze firing proceeds as usual.

This article on Testing Ovenware by Dick Lehman has some great guidelines about how flameware should be constructed, as well as informal ways of testing it.  You really can't have confidence in your flameware until you really try to break it - if it can withstand the stress, you know you have a good clay body.

Finally, Cooking with Clay by Robbie Lobell is a great article from an artist's perspective.  This article includes a bunch of great recipes, including glazes of various colors.  Robbie Lobell has built a business on selling flameware clay; see her website Cook On Clay.

Flameware Tests

We decided to make up some flameware clay following Robbie Lobell's recipe:
  • 10% G-200 feldspar
  • 30% Spodumene
  • 10% Pyrax
  • 25% Hawthorne Fire Clay
  • 25% OM4 Ball Clay
  • (We left out the additional iron oxide, since we prefer a lighter clay body.)
The composition of ingredients vary, and both the Pyrax and the Spodumene have changed in minor ways over time.  (We acquired our Spodumene and Pyrax from Laguna.)  So, we wanted to be sure to thoroughly test our flameware body.  We made up a 1kg batch, and we threw a couple of vessels.  Here is one of them, a shallow dish about 7" across:



I was too lazy to make up flameware glazes, so we decided to give some of the studio glazes a go.  The inside is Watt's White, available from Aardvark Clay - I chose it because it's a very stable, reliable glaze; it would be fantastic for it to work out with flameware.  The outside glaze is Long Beach Blue, a favorite of mine.  Once we finished firing the piece, we began subjecting it to various tests, like heating it up on a hot plate and boiling water in it.


Of course, these tests invariably end up with letting all the water boil away, and then after the dish has sat on the heating element for a while, plunging it partway into a bucket of cold water to try to maximize the thermal stresses on the dish.

We have done this quite a few times by now, and the dish has survived!


Well, mostly.  After doing this 5-6 times, we started hearing a lot of very loud POPS from the dish when it was sitting empty on the hottest burner of our stove.  Turns out that the inside glaze was popping off of the dish:


The entire inside surface has crazing all over it.  Clearly we need to formulate some flameware glazes!  Flameware clay bodies have extremely low coefficients of thermal expansion; that's the only way they can survive the kind of treatment we have been dishing out.  The glazes used must also have a similar coefficient of thermal expansion, or else they will craze and shiver right off of the pottery.

So, it's clear we will need to keep testing!

(Note:  The interior glazes given in some of the above-referenced articles are not necessarily formulated for flameware.  They are typically applied very thinly to avoid these kinds of issues.  Specifically, the "Ann's Kaki" glaze recipe is virtually identical to Ohata Kaki, which has a very different computed COE from the flameware clay body.  It may simply be good at enduring the thermal stresses when applied very thin.)

We also made up a couple of 10cm bars to see how much flameware clay shrinks.  Currently we see this behavior:
  • 4.5% shrinkage of the raw clay from wet to bone dry.
  • During bisque firing, the clay expands 3% in size (i.e. 1.5% shrinkage from the original wet size).  This is due to the spodumene in the clay body.
  • During glaze firing (cone 10), the clay shrinks down 7.5% from the original wet clay size.


I also performed a simple test of the vitrification of the clay body.  I weighed the bars dry (105-106g), and then put them in hot water for about 2 hours.  At the end of this period I patted them dry with a paper towel, then weighed them again.  The results were again 105-106g, which indicates that the clay body is pretty well vitrified.  I can let the bars soak for longer, but I will probably need a better scale if I'm going to get a more precise measurement here.



You can see from the above picture that the clay body takes on a lovely toasty color when fired in reduction.

Next Steps

It was excellent to see the flameware clay hold up under the brutal treatment we were able to dish out on it... at least until the glaze started to fail.  So, the next steps clearly need to focus on formulating glazes that have a compatible COE with the flameware clay body.  I expect the solution will be to use spodumene in the glazes, since the Li2O has a very low COE.  I think it's time to throw a whole bunch of small test bowls and start formulating glazes...

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