Author Topic: Stone box for grill  (Read 5660 times)

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Offline Engineered Ceramics

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Stone box for grill
« on: January 18, 2008, 05:49:57 PM »
Hi, my name is Bill Costa, and I work for a ceramics company.

To date, we have only sold part to the metal and glass melting industries, but I recently found this site and it seems like we could be a good fit, and we would be reasonably inexpensive.

What I've used at home is a ceramic "box" made from scrap ceramic at work.  I'd like to make one just for pizzas on grills, and maybe get one of you with a little more experience to try it out.

First however, I do have some design questions.

My basic design is a short, and wide ceramic box with one open side, which is where you would slide the pizza in.    How does this sound?

How thick should the bottom be?  3/4"?

How about walls ?  1/2"?

Top?  3/4"

I could also make the bottom a separate piece and surface grind it to a marble like finish, would this be of any value?

I'm thinking of using a Fused Silica material, which is far superior to cordierite.    With fused silica you should be able to take it out of a 800F oven and drop it into a bucket of cold water with any problem.  I'm  having R&D run some thermal cycling tests to verify this.

Let me know what you think.

Bill



Offline 2stone

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Re: Stone box for grill
« Reply #1 on: January 18, 2008, 07:23:32 PM »
Drop me a line Bill,

I'd be interested in talking to you,
wgustavsen@gmail.com

regards,
willard
2Stone blog: www.2stoneblog.com

Offline canadianbacon

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Re: Stone box for grill
« Reply #2 on: January 18, 2008, 08:39:20 PM »
Hi Bill,

what you describe sounds much like what many members here are doing with unglazed quarry tiles,
- basically building a box, which will reflect the heat, and give you nice results in your kitchen oven.

Do you have an image you can post here, so we can see what it looks like ?

Sounds interesting for sure. 

Does ceramic just natually absorb heat much better / faster than unglazed quarry tiles, and pizza stones ?

oh, and welcome to our community !  :chef:

Hi, my name is Bill Costa, and I work for a ceramics company.

What I've used at home is a ceramic "box" made from scrap ceramic at work.  I'd like to make one just for pizzas on grills, and maybe get one of you with a little more experience to try it out.



Pizzamaker, Rib Smoker, HomeBrewer, there's not enough time for a real job.

Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #3 on: January 19, 2008, 12:52:29 AM »

Does ceramic just naturally absorb heat much better / faster than unglazed quarry tiles, and pizza stones ?

It depends on a few things.

First, I'd consider unglazed quarry tiles, and pizza stones as ceramics as well, just with different properties.

The two that would seem to be the most important are heat capacity, and thermal conductivity. 

Heat capacity is mostly a function of density, and is the amount of heat you can store.  Thermal conductivity is a measurement of how fast (or slow) that heat is transfered.  Corderite and Fused silica have relatively low heat capacity and thermal conductivity.    Materials such as Mullite and Alumina have progressively higher heat capacity and thermal conductivity. 

Fused silica, Mullite and Alumina are all variations of the same creature, with different quantities of Silica (SiO2 ) and Alumina (Al203)

Fused Silica is typically   70% Silica   and   30% Alumina
Mullite is typically           35% Silica and 65%  Alumina
Alumina is typically no more than 15% Silica and 85% or more alumina.

As the Alumina content increases, the heat capacity and thermal conductivity increase, as well as chemical resistance, also, the maximum use temperature increases, which will approach 3200F.  BUT this come at a cost, both in terms of thermal shock resistance and dollars.

Typically I would recommend the material with the LEAST amount of Alumina that meets the rest of your needs.


Quarry tile and pizza stones (corderite I presume) contain silica and alumina, but also magnesium oxides, and higher levels of clay. 

Magnesium oxides are great for resisting attack from molten stainless steel, but really add nothing to pizza making environment. 

The more clay you have, the easier the part is to press (if your pressing parts), and handle in an unfired state, but it adds nothing to a part, in terns of thermal shock resistance or thermal conductivity.

How this helps a little, and doesn't make things more confusing.

I'll have our cad guy draw up one on Monday, but I'm thinking a box that is about 5" high 16.5" deep, and 17" wide. 

Top and bottom will be 3/4" thick and the walls will be 1/2" thick.

This would give inside dimensions of 16" x 16" by 3.5"


One of the walls will be missing, so that pizza can be placed in the box to bake.

The bottom could be a separate piece for ease of cleaning, or to use as a baking stone for taller items.

To answer your original question, fused silica would have a far superior thermal shock resistance compared to quarry tile or corderite.

I will have some thermal cycling tests done next week, but I see no reason I wouldn't be able to heat these parts to 800F and then drop them in a bucket of cold water without having them break.  I've had corderite pizza stones crack on me in the past when I've added cold wet dough to a hot stone.

Short of beating them with a hammer or dropping them onto stone or concrete, fused silica parts should last indefinitely.




 

Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #4 on: January 19, 2008, 01:09:57 AM »
Here's a picture of the concept.

Ignore the vents in the sides, the cuts in the front, and the fact that the bottom plate is over sized.  These are just some scrap material I have laying around.

I'll have to look at some of the quarry tile designs, but this design has ceramic on 3 of the sides, in addition to the top and bottom.

Offline Villa Roma

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Re: Stone box for grill
« Reply #5 on: January 19, 2008, 01:16:56 AM »
Welcome EC.....This material seems to be ideal to adapt to pizza making. Can you let us know how much something like this would cost?

    Thanks, Villa Roma

Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #6 on: January 19, 2008, 07:49:05 AM »
Can you let us know how much something like this would cost?

I hesitate to give a price at this point, it depends on which material we use, the weight of the piece or pieces, and if we do any grinding.

To reduce weight, we could make it rounded in the back, and square in the front.  Is there any advantage to having it square?  You get more surface area, but if you don't use the back corners it's a waste.

Should there be any vents in the top to help steam escape?  I can add a 1/4" slot an inch from the back on the top piece if it would be useful.


Offline November

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Re: Stone box for grill
« Reply #7 on: January 19, 2008, 08:45:16 AM »
Bill,

Would you please provide here, or a link to somewhere, a list of physical and thermal properties of the refractory material you are offering?  I'm mainly interested in the thermal expansion coefficient, thermal conductivity, and heat capacity.  Also, were the ratios of silica and alumina you provided supposed to be molar quantities or standard mass quantities?  I've always known mullite to be composed of 3 mols of alumina and 2 mols of silica (3Al2O3.2SiO2) which would give it a mass distribution of approximately 71.8% alumina.  Obviously molar quantity distribution is 60% alumina.  Thank you.

Offline canadianbacon

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Re: Stone box for grill
« Reply #8 on: January 19, 2008, 10:44:40 AM »
Hi Bill,

Thanks for the info, and thanks for the picture.  Very interesting indeed.  It would interesting to see some
of your results with pizzas, and the times it took to cook them.

Very interesting concept Bill. 


Here's a picture of the concept.

Ignore the vents in the sides, the cuts in the front, and the fact that the bottom plate is over sized.  These are just some scrap material I have laying around.

I'll have to look at some of the quarry tile designs, but this design has ceramic on 3 of the sides, in addition to the top and bottom.
« Last Edit: January 19, 2008, 10:48:28 AM by canadianbacon »
Pizzamaker, Rib Smoker, HomeBrewer, there's not enough time for a real job.

Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #9 on: January 19, 2008, 11:34:29 AM »
Bill,

Would you please provide here, or a link to somewhere, a list of physical and thermal properties of the refractory material you are offering?  I'm mainly interested in the thermal expansion coefficient, thermal conductivity, and heat capacity.  Also, were the ratios of silica and alumina you provided supposed to be molar quantities or standard mass quantities?  I've always known mullite to be composed of 3 mols of alumina and 2 mols of silica (3Al2O3.2SiO2) which would give it a mass distribution of approximately 71.8% alumina.  Obviously molar quantity distribution is 60% alumina.  Thank you.

I don't have data sheets at home, and they are not currently on our website (engineeredceramics.com) which is in desperate need of updating.

I'll post our data sheets Monday Morning.

As far as percentages go, it is done by weight.   

Academically speaking, you are correct, on your percentages, as mullite would be around 72%.  It's the narrow band in the phase diagram posted below.

Industrially, anything around 50% to 70% alumina (with the rest being silica) is considered mullite. 

You also have to remember that industrial ceramics such as ours are not made from super fine ceramic powders, but from fine to larger grains.  These can range in size from a fine powder to something as chunky as grape-nuts.  The distribution of sizes used to make a material is typically a closely guarded trade secret, as it can greatly influence physical strength and other properties.  It's easy enough to buy a competitors parts and run a chemical analysis to get the composition (it's also why I can post our data sheets).  It's basically impossible to take a fired piece of ceramic and determine the various sizes of the raw materials.

As I was saying, our ceramics are made from a range of grain sizes, so you can't look at the overall weight fraction to determine your mullite content.  This is because most of your mullite formation only occurs in the spaces between the grains.  In other words, the center of an alumina chunk will not react with any silica, even though it's part of the alumina weight fraction.

Hope I didn't make things more confusing.

Bill


Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #10 on: January 19, 2008, 11:46:13 AM »
It would interesting to see some of your results with pizzas, and the times it took to cook them.

Very interesting concept Bill. 


That's where I was looking for some help.   I would hate to have this product rely on my pizza making skills!

Once I get a design down, I'd like to send some of the experts here a sample to trial.

I could also add a grill pattern to the top of the top plate, or the bottom of the bottom plate and make it reversible, but I'm not sure if that would be useful for anything.

It would look like one of the parts in the lower right portion of the picture below.

Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #11 on: January 19, 2008, 11:51:40 AM »
Some data on our mullite mix:

Trade Name    MOR Shock GTS     (Mullite)
Al2O3 (wt%)    66-70    
SiO2 (wt%)    26-30    
Fe2O3 (wt%)    0.8-1.0    
Other (wt%)    2.1-2.3    
Bulk Density (g/cm3)    2.4-2.5    
Bulk Density (lbs/ft3)    147-153    
Apparent Porosity (%)    13-17    
Maximum Operating Temperature    2750F 1510C

The "Other" is basically ball clay, which is required for casting or pressing.  Without a little clay, you would never get a part out of a mold in one piece.  This is the same clay used to make non-engineered ceramics, such as dishes and flowerpots.


Some Generic Thermal conductivity data:

Corderite      3.0   W/m-K @ R.T.  (standard pizza stone)
Fused Silica  1.38  W/m-K    (edited)
Mullite          3.5   W/m-K @ R.T.

It should also be possible to add additional Fused silica to our mullite to hit a cordierite like thermal conductivity if thats even desired.

I think this will be the important data, differences in heat capacity can be compensated for by adjusting the plate thickness.
« Last Edit: January 19, 2008, 12:46:53 PM by Engineered Ceramics »

Offline November

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Re: Stone box for grill
« Reply #12 on: January 19, 2008, 12:23:17 PM »
Corderite      3.0   W/m-K @ R.T.  (standard pizza stone)
Fused Silica  1.38  BTUin/ft2hrF
Mullite          3.5   W/m-K @ R.T.

It should also be possible to add additional Fused silica to our mullite to hit a cordierite like thermal conductivity if thats even desired.

I'm already familiar with the generic data.  In fact, I've already posted cordierite's data elsewhere on this forum.  I was interested in your specific product's data because of statements like, "I'm thinking of using a Fused Silica material, which is far superior to cordierite."  Looking at the generic data, fused silica is not "far superior" to cordierite in terms of thermal conductivity.  What fused silica seems better suited for is housing, i.e. the walls of a pizza oven chamber, because the walls generally have to be nearer the source of heat and fused silica has great thermal expansion properties.

Offline November

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Re: Stone box for grill
« Reply #13 on: January 19, 2008, 12:26:41 PM »
Bill,

By the way, where are you getting your data for fused silica's thermal conductivity?

EDIT:
Converting your fused silica "k" numbers to SI units: 0.19890137865 W/m-K
« Last Edit: January 19, 2008, 12:29:06 PM by November »

Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #14 on: January 19, 2008, 12:44:52 PM »
I'm already familiar with the generic data.  In fact, I've already posted cordierite's data elsewhere on this forum.  I was interested in your specific product's data because of statements like, "I'm thinking of using a Fused Silica material, which is far superior to cordierite."  Looking at the generic data, fused silica is not "far superior" to cordierite in terms of thermal conductivity.  What fused silica seems better suited for is housing, i.e. the walls of a pizza oven chamber, because the walls generally have to be nearer the source of heat and fused silica has great thermal expansion properties.

Your right, cordierite has higher conductivity.  I don't think much in terns of thermal conductivity.  Most of our products are used above 2000F, where more heat is transfered by radiation than conduction.

Where I was thinking it was superior was in thermal shock resistance, which it is.  I've broken several cordierite pizza stones in the past from placing cold food on them.  This should never happen with fused silica, but I'm running some trials to check.

You should also remember that thermal conductivity changes with temperature, but I think most of those occur over 900F.

As I've said, I'm not a pizza expert, and this might not work at all, that's why I would like to trial different materials.

As far as my Fused Silica data, I just pulled it off google somewhere.  oops my bad, That is SI units.

ON EDIT: Do you want more thermal conductivity? Is that a good thing?

Bill 



« Last Edit: January 19, 2008, 12:49:54 PM by Engineered Ceramics »

Offline November

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Re: Stone box for grill
« Reply #15 on: January 19, 2008, 12:52:26 PM »
As far as my Fused Silica data, I just pulled it off google somewhere.  oops my bad, That is SI units.

That's what I suspected, but I wanted to give you the chance to look at your source again.

I'm not sure there's any need to do any testing with fused silica for pizza baking surface purposes.  The added resistance to thermal shock is a moot point when it takes over twice as long to transfer thermal energy.  That means the pizzas that take 3-4 minutes in my 600F oven could take up to 8 minutes instead.  That's unacceptably long for a hearth style bake.

If you have mullite to offer, you should run with that ball.  My only concern would be the cost.

Offline November

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Re: Stone box for grill
« Reply #16 on: January 19, 2008, 01:02:32 PM »
ON EDIT: Do you want more thermal conductivity? Is that a good thing?

It's generally a good thing, but the goal for a baking surface should include a thermal conductivity at least as high as traditional baking surfaces.  It isn't that the conductivity should be through the roof; just not worse than what's already out there.

Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #17 on: January 19, 2008, 01:08:55 PM »
That's what I suspected, but I wanted to give you the chance to look at your source again.

I'm not sure there's any need to do any testing with fused silica for pizza baking surface purposes.  The added resistance to thermal shock is a moot point when it takes over twice as long to transfer thermal energy.  That means the pizzas that take 3-4 minutes in my 600F oven could take up to 8 minutes instead.  That's unacceptably long for a hearth style bake.

If you have mullite to offer, you should run with that ball.  My only concern would be the cost.

Mullite is the cheaper of the two products for us, as our fused silica uses a very high purity raw material.

We could also add silicon carbide to dial up the thermal conductivity even higher.


Offline November

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Re: Stone box for grill
« Reply #18 on: January 19, 2008, 01:10:50 PM »
Mullite is the cheaper of the two products for us, as our fused silica uses a very high purity raw material.

We could also add silicon carbide to dial up the thermal conductivity even higher.

Now it's getting interesting.

Offline Engineered Ceramics

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Re: Stone box for grill
« Reply #19 on: January 19, 2008, 01:10:58 PM »
Another question, is there any benefit to having a polished marble like surface, or is a quarry tile like finish better?