Author Topic: I'm a new member  (Read 727 times)

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Offline GSpots

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I'm a new member
« on: June 10, 2011, 12:59:52 PM »
Hello everyone, I am George Scatchard, an old but still active potter living in Vermont. Recently I have become obsessed with the thermal expansion properties of high temperature pottery. It all started with mugs cracking from bad clay composition. I have built a Rube Goldburg contraption to measure the thermal expansion properties of various clay mixtures and now am making experimental pizza stones from a modified cordierite clay body. Because I fire hotter and in a reduction atmosphere I am able to do things that can't be done at lower temperatures in oxidation the way most cordierite stones are made. I have read a lot on line about the various claims of manufacturers and the preferences of many pizza chefs. I assume no one material would please everybody, but I am looking for guidance from you experts as to what I should shoot for. Some of the considerations are: denser than regular cordierite? still porous? thicker or thinner? glazed or unglazed?

I am hoping to come up with a really good product within the next few months, so I will appreciate any help I can get. I don't think I will really make money on this project because the materials and firing are expensive and I am limited to hand methods of production. That means I can't sell them in wholesale quantities.













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scott123

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Re: I'm a new member
« Reply #1 on: June 10, 2011, 04:04:54 PM »
Hi George, welcome to the forum.  The work you're doing sounds fascinating.

As I'm sure you've found in your research, cordierite can vary pretty widely from manufacturer to manufacturer.  From what I've learned, cordierite can be extruded or pressed.  I'm not exactly certain which of the popular cordierite brands (old stone, pampered chef, williams sonoma, etc.) are extruded or pressed, but I do know, from studying ceramics in college, that clay molecules are flat and that when you press them, they sandwich together and become stronger- a lot stronger.  This is one reason why slipware (clay molecules that dry facing every possible direction) is so much weaker than hand built or thrown pottery.

I have a working theory that extruded cordierite, due to moisture content of the extruded clay, is most likely less dense than pressed. If this is the case, then one could look at the weight/volume/density of commercial stones and determine which are extruded and which are pressed, but I haven't gotten that far with my testing.

The strength obtained by pressing is flexural (bend strength).  Flexural strength (and thermal conductivity) are directly related to thermal shock resistance.  I've seen videos of pressed high alumina cordierite stones going from red hot (more than 1000 deg. f.) into ice water without any issue. I highly doubt any extruded material could achieve this.

Lack of porosity/air pockets is critical to a good pizza stone.  Air is an insulator/slows the transfer of heat/conductivity.  When heat doesn't transfer quickly, you will have hot, expanded areas next to cooler unexpanded areas, resulting in structural failure. Air pockets also allow a stone to absorb water, not so much during baking, but more during cleaning and/or moisture from humid air.  Water plus heat equals massively expanding steam. Lastly, and most obviously, air pockets decrease strength.  Air is the kiss of death for pizza stones *cough* fibrament *cough*

Conductivity is also critical for good stones, especially for the home baker.  It's essential, as discussed, for thermal shock resistance, but materials with greater thermal conductivity transfer energy at a faster rate, allowing lower temp home ovens to bake pizzas faster, resulting in pizzeria quality results. From what I've read, cordierite is silica and alumina, and, because alumina is more conductive, it's ideal to maximize the alumina content.

The other side of the heat transfer coin is thermal mass. Dense, highly conductive materials such as steel can make great pizza at thicknesses as low as 1/2".  Cordierite, in order to bake pizzeria quality pizza in an average 550 deg. home oven should really be at least 1".  Thickness gets tricky because, as you go thicker, the stone will weigh considerably more, which, in turn, can potentially challenge the structural integrity of the oven shelf.

Offline GSpots

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Re: I'm a new member
« Reply #2 on: June 10, 2011, 05:10:18 PM »
Hello Scott, Thanks a lot for responding. I have been able to test one of the most popular pressed cordierite brands by sawing off a strip to test in my device, and I am consistently able to get lower thermal expansion. I hadn't given much thought to the forming process, but I have thrown and slip cast the stones I have made so far. With my cone 10 reduction firing, iron acts as a flux, and I have tested up to 4 % without any problem. I am hoping the iron will raise the specific gravity some. I am also experimenting with a lot of texture on the back side to double the heat absorption rate which should help. I will be doing specific gravity calculations soon to see where I stand there.
 

Offline tinroofrusted

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Re: I'm a new member
« Reply #3 on: June 10, 2011, 06:34:32 PM »
Super interesting reading.  Since this is a topic that most people will be interested in, it might be helpful if it were moved or copied from the new member area to the pizza tools section.  George, I suspect that if you develop a better mousetrap you will have lots of customers from this forum! 

Best regards,

Tin Roof 

Offline GSpots

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Re: I'm a new member
« Reply #4 on: June 10, 2011, 06:48:41 PM »
Thanks Tin, This is my first day so I am just learning my way around here. As my work develops I will keep posting results, and some day try to figure out a fair way to have some stones tested by members who have experience with several stones.

scott123

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Re: I'm a new member
« Reply #5 on: June 11, 2011, 04:07:56 PM »
George, while I think that expansion testing is a good indicator, as you begin to finalize your formula, I think some quench testing is order.  Extreme quenches are what separate the manly pizza stones from the boy-ish ones and are rock solid proof that they'll stand up to the thermal rigors of pizza baking.

Offline GSpots

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Re: I'm a new member
« Reply #6 on: June 11, 2011, 08:26:04 PM »
Thanks again scott, that is something I have not done yet, but it will be easy. In my testing device, I heat 10" strips of the fired sample clay to over 1600 F. to measure total expansion. I will pull out a sample and dump it in water
to see what happens. I just ordered a graduated cylinder so I can figure out the specific gravity of some of the best
of the samples. I am trying to get a denser variation of cordierite that will transmit heat faster and still not crack. I have been working for months on this in between trying to make a living. I think I am close to a usable product, but
I keep thinking of new ideas that I have to test. I will be firing my big kiln again later this month and hope to have a bunch of new stones in it to see how they work.

scott123

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Re: I'm a new member
« Reply #7 on: June 11, 2011, 10:04:11 PM »
Sounds good, George.

And I'm sure this goes without saying, but be careful with the quench, as there's a chance the cordierite sample might explode when it gets wet. This is more than just safety goggles, but real distance between you and water.  Perhaps some more Rube Goldbergian inspiration might help.
« Last Edit: June 12, 2011, 08:38:23 AM by scott123 »


 

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