The Ideal Pizza Stone?

[GSpots]

Hi Dave. My old stone is new, but I think you are correct that they are most likely the same.

The weight is in grams and the volume is in cubic centimeters. 1 cc=1 gm of water.

The test strips are around 60-80 cc and they can weigh around 140-180 gm depending the material.

Soapstone was 183 gm for 63 cc, or a specific gravity of 2.9.  Cordierite is around 1.9.-------------george

[Tampa]

Hi George,

Something is off -- maybe it is in my calculations.  Assuming you and I have the same “old stone”, here are my numbers.  The “old stone” is 41cm in diameter (20.5cm radius), about 1.27cm thick, and weighs 4165gm.

Density = gm/cm3 = weight/(stone area * thickness) = 4165/((20.5**2)*3.14)*1.27 = 2.5 gm/cm3

Does your Old Stone weigh about 9lbs (4200gm)?

Using similar reasoning, if your 20x20x5/8” stone weighs about 23.5lbs (9675 gm), we’re good.  Or if your 20x20x3/4” stone weights about 28.25lbs (11600 gm), we’re good.

I saw a comment above about using mullite.  If your stone contains mullite let us know because, depending on the concentration, it could be much more conductive than cordierite.

Dave

[GSpots]

Hi Dave, I am sorry about not being clear enough or having misstated something. My stone is a little over 16'x16" and about 1/2 " thick with legs and a bigger lump in the middle. I sawed a strip  off one end to match my other test strips so I could test its thermal expansion in my contraption. Then to find the specific gravity of the Old Stone material I took that same sample strip and weighed it on a gram scale and got 162.0 grams. Then I filled a 500cc graduated beaker to a known level and dropped the strip in. The water level rose by 85 cc. I then divided 162 by by 85 and found the specific gravity to be 1.9058823. That is why I say the specific gravity is 1.90.  My other calculations for soap stone and other known materials agree with what I can look up on line.

Pure cordierite (51.4 SiO2, 34.8 Al2O3, 13.8 MgO) has a text book specific gravity of 2.1, but most industrial cordierite products differ from the ideal formula as illustrated by the typical analysis of the recycled crushed cordierite grog I buy which is 53 Sio2, 36 Al2O3, 8.5 mgO, 1.2 Fe2O3. I have great respect for the Old Stone Oven product and think they sell a great product for the price. I am not trying to copy it, but using it as a standard to compare to. I am now testing some cordierite-mullite mixes to get greater density. I will also be testing some blends with pure alumina as Scott123 has suggested.
The big problem with all of this, is keeping the thermal expansion acceptably low. It needs to be much lower than soap stone. Just like trying to make your very best pizza, it is a matter of balancing a lot of different factors. A good stone for one person may not be good for everybody.

I just want to thank everyone for their participation. I have gotten a lot of very usefull information from you all and I will keep plugging along and see what comes out of the kiln next week.-----------george

[Tampa]

Thanks George.  I'm a fan of what you are doing.  Let us know what comes out next week.
Dave

[texmex]

This is all really interesting, and I applaud your efforts to create the ideal stone.

 I have been specifically searching for the right size stone to fit into my home oven.  I have been looking at cordierite 14 x 16 square stones made for the Weber grill (my oven is a narrow Kenmore in-wall unit that maxes out at 550 degrees and has an interior horizontal measurement of 17 x 17.5 inches).  Cannot accomodate anything larger, but the 12 inch round cordierite stone I borrowed from my mini black egg set-up is just too small for baking multiple loaves of bread, or when I want to bake one or two large pizzas. 

A friend loaned me a "Pampered Chef" 12 x 16 stone, even though I warned it might not be able to handle the heat (12 x 16 was barely large enough to accomodate 2 rustic loaves of bread) and it did crack halfway through the first bake; high moisture content being the obvious culprit. 

12 x 16 fit in my oven nicely, but a 14x 16 would leave recommended air circulation area on both sides of the stone, with maybe 1/2 inch to spare at front or back of oven.  So far, this 12 inch round cordierite kiln shelf has held up to wet doughs, and I don't doubt it will continue to do so. 

Hope these dimensions help you some way in your endeavors to help the home oven pizza enthusiast. 
Experimenting is awesome!

[GSpots]

Hi Texmex,

Every day I get closer to a really good material. Today I took some really interesting tests out of the kiln. I can make you a cordierite stone that will behave similarly to the one that is working for you in any size you want. A good cordierite stone won't crack even with wet dough.  I am not sure what a fair price for a small custom stone is but I will soon find out. Where the custom stone may make the most sense is where you need a special shape or thickness.

Another option for anyone wanting a special size stone is to find someone who has a hand held grinder with a diamond wheel on it. You can saw a cordierite stone to size easily. That is how I got my test strip off a commercial stone for testing. I have also had no trouble cutting soapstone.

If you want to persue a special stone, let me know.              george

[texmex]

Hi George,

The cordierite stone I use right now was purchased from a ceramics kiln furniture supplier.  It's a 1/2" thick round and it has handled direct flame (actually buffered with fire brick in the little black egg, but the flames do hit the stone directly as well as evidenced by the distinct flame marks on the bottom of it.) 
I've spilled room temp water on it when it was 500 degrees hot, baked high hydration bread on it at least 15 times and made close to 50 neapolitan super high heat pizzas with it (lost two pies with sauce and ingredients on it, making a total hot, wet mess) but still--the cordierite holds up! 

I would love to have a custom stone for my oven, as I think I know exactly what to expect from cordierite. 

It can handle all my screw-ups! ha. 

I wonder if the 14 x 16 Old Stone Oven brand I can already purchase would be the right size for my oven (taking into consideration the added circulation of air since it won't be a snug fit: is that a good thing or a bad thing?), or if I should take you up on the offer to make me one.  I just started researching larger stones today when I ran across this thread. 

I am all for supporting the "mom and pop franchise", and would much prefer to buy from an individual such as yourself, if the cost isn't prohibitive.  If we can come to a reasonable agreement I  may be interested in testing out your composite. I'll send you an internal message here. 

[GSpots]

Here I go again for those who are still reading this. The problem of the high density cordierite has been bugging me and I think I finally know the answer. Cordierite is named after a French geologist who discovered it in the 1800's. The gem stone is quartz like and often purple in color. There are no known large deposits, so it remains a gemstone like many others that formed as the earth's crust cooled. I have posted a photo so you can see what it looks like.
The mineral cordierite has a specific gravity of 2.6 so I think that is where the idea of dense cordierite comes from

The manufactured cordierite we use in kiln shelves and bake stones has a specific gravity of 1.6-2.1 depending on it's composition. That is why it is porous and does not hold a huge amount of heat. I am beginning to think from my testing that there is a range of cordierite and semi-cordierite compositions possible. When the necessary materials are finely ground and mixed together to give the chemical analysis:  51.4 SiO2, 34.8 Al2O3, 13.8 MgO, they will form nearly pure cordierite when fired to a temperature over 2300F for an extended period of time. The resulting cordierite has many industrial uses because of it's ability to withstand extreme thermal shock.

In my recent tests I have specific gravities as high as 2.48 with thermal expansion about a third that of soapstone. It seems that as the composition moves away from cordierite toward a shock resistant stoneware mixture the density and thermal conductivity go up. This causes a loss of porosity which is to be expected from a more dense material. This week I will be testing thermal conductivity and porosity of these new samples to look for the "sweet spot". The proof will be in the pies, as they say.         george