Author Topic: Thermal Mass  (Read 2048 times)

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Offline fidelis55119

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Thermal Mass
« on: July 09, 2009, 12:20:04 PM »
I recently purchased my in-laws home.  My father-in-law is 2nd generation Italian and had a large outdoor woodburning bread/pizza oven built on the property.  Thanks to the vast amount of information people have shared on this site I am making pizza that everyone is wild about.

My biggest obstacle to turning out consistent pizza's is the oven's inability to maintain it's floor heat.  After burning oak for as long as four hours in the center of the oven I can get the floor heat around 750 degrees - at this point the interior of the oven is completely white/grey.  Even with the fire still burning and pushed to the back the floor heat drops considerably with each pizza.  The only reason I can come up with is that because the floor mass of the oven is so large the fire cannot sufficiently heat the floor to retain it's heat.  Below is a little bit about the construction of the oven. 

The oven:  The entire brick oven is 4' wide x 6' deep x 8' tall. The interior of the oven is approximately 24" wide x 30" deep with an arched roof which is approx. 24"tall in the center.  The overn interior is completely line with fire brick.  The floor of the oven is about 40" from the ground.  The first layer of the floor is fire brick.  Under the fire brick is then a concrete slab.  Under the slab is then filled with sand - roughly 36" deep of sand.

My instinct tells me the thermal mass of the sand which fills the brick base is too large to be sufficiently heated and retain heat.

Any thoughts would be greatly appreciated!



Offline Bill/SFNM

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Re: Thermal Mass
« Reply #1 on: July 09, 2009, 12:39:38 PM »
Have you ever tried running a large enough fire for long enough to see if your theory is correct. I'm firing my oven for 3-5 hours, depending on ambient temp and residual temp. For a while I had a punky batch of wood that took up to 6 hours to reach equilibrium. What are you using for fuel?

What is deck temp after pushing back the coals? Are you mopping deck with a wet towel (that wouldn't be a good idea in your case)? What is deck temp right before 2nd pie?



Offline fidelis55119

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Re: Thermal Mass
« Reply #2 on: July 09, 2009, 01:02:17 PM »
I am using oak which has been quartered.  I have used as many as a dozen logs over four hours to fire the oven.  As it burns down the entire floor will be covered in orange embers for 1-2 hours.  Though I would like to use a wet mop I have not because of the problem I am having so i have resorted to a wire broom.  I haven't recorded the laser temps after each pizza (because it is too depressing) but after making five 12" pies the floor temp will be closer to 500 degrees.

My gutt tells me the large sand base is draining the heat from the fire brick.  I am looking into opening the back of the oven, removing the top layers of sand and replacing it with insulation board.

I will post some photo's so this makes more sense.

Thanks for your help!

Offline BurntFingers

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Re: Thermal Mass
« Reply #3 on: July 09, 2009, 01:12:21 PM »
I too have a large mass to heat up.  There are times after a long disuse of the oven it takes an overly long time to get the hearth floor to keep its heat.  I have read about an old US Army field guide for cooks mentioning using "oven pine" to quickly raise the temperature.  I use oak and maple but find pine and fir cutoffs to flame up quickly and burn hot and faster than oak.  I also use what the boy scouts call squaw wood.  Twigs and branches that fall from the trees along with smaller pine strips to generate lots of heat fast.  The oak burns hot and slow.  It seems good to maintain the heat once things get to where I want it.  That seems to be my observation after 3 years of running my oven. 
We usually only fire up for large parties of 20 or more Pizza.  That uses at least one to two wheel barrow's of wood.  I start the fire in the morning and start cooking after 4 PM in the winter, a little less in the summer.  So that you don't think we are wasting all that heat.  We also make other baked goods,  Calzones, Stromboli, etc.
« Last Edit: July 09, 2009, 01:17:36 PM by BurntFingers »

Offline AZ-Buckeye

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Re: Thermal Mass
« Reply #4 on: July 09, 2009, 01:49:59 PM »
I am using oak which has been quartered.  I have used as many as a dozen logs over four hours to fire the oven.  As it burns down the entire floor will be covered in orange embers for 1-2 hours. 

I'm no expert, but a dozen quartered logs over 4 hours doesn't seem like much.  I use that much over 1 1/2 hours heating my oven.  The fire should be rolling over the top dome the whole time it is heating up, if that helps.

Offline BurntFingers

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Re: Thermal Mass
« Reply #5 on: July 13, 2009, 07:48:01 PM »
I agree a dozen logs is just kindling.  A good start.  Triple or quadruple that amount and you should have enough to get going. You mightn virtually have to fill the entire oven a few times after the fire gets going.  You might want to check The Bread Builders Hearth Loaves and Masonry Ovens by Daniel Wing and Alan Scott, pp 195-196 for some ideas about firing up. There are lots of other books on the topic like Your Brick Oven, Building it and Baking in it, by Russell Jeavons.  This forum is also a good source for advice.  Good luck.