Question on thermal mass

[rwiggim]

I read "The Bread builders" which made me want to build a brick oven. This site turned me in the right direction of the dome oven. One sticking point I am having and need some help with before I lay my oven floor.
In the Alan Scott model he mentioned it might be good to add an extra half layer of brick under the hearth floor as with heat rising the extra thermal mass would even out the heat in the chamber. In the plans from FB the hearth is firebrick laid on its flat side and then the dome is bricks cut in half. It seems like the dome would have and extra 1.75" of mass than the floor. Is that correct? That would seem to be good for pizza but maybe not bread. Would it be better (obviously more expensive) to lay the bricks on their edge and equal out the mass?

Thanks so much,

darren

[DrakeRemoray]

Re: Question on thermal mass

It does seem like evening out the mass would be a good idea, but I can say from experience that the biggest bread baking problem I have had is bread where the bottom crust is burnt. Now, this MAY be because I had to use medium duty fire brick, which I think in the Bread Builders book is described as giving up it's heat more quickly. In any case, I certainly have not had a problem with a too-cool hearth.

Drake

[rwiggim]

Re: Question on thermal mass

Thanks Drake,
All I need is a few people saying so and I am sold. As I am getting my bricks from Home depot they weren't very helpful in whether they will be medium or low.

[DrakeRemoray]

Re: Question on thermal mass

They should be able to come up with a spec sheet for the bricks...there is a "firebrick primer" around here somewhere...

here it is:
http://www.fornobravo.com/forum/f8/f...rebrick+primer

Drake

[Unofornaio]

Re: Question on thermal mass

It does seem like evening out the mass would be a good idea, but I can say from experience that the biggest bread baking problem I have had is bread where the bottom crust is burnt. Now, this MAY be because I had to use medium duty fire brick, which I think in the Bread Builders book is described as giving up it's heat more quickly. In any case, I certainly have not had a problem with a too-cool hearth.

Drake

As I have said before I haven't baked in one of these designs but I have quite extensively in my other ovens. One very important factor in doing bread in a mass oven is allowing the heat mass to equalize. I read somewhere Jim mentioning the same thing.
After you are satisfied you have the oven up to temp overall do a clean out put your door on and allow it to rest (equalize) for a while. If you have probes you will be able to tell how long if not you will have to approximate. After this check your hearth temp with a IR or another method which I find good for bread. throw a little bit of cornmeal on the hearth bricks, if it instantly burns so will your bread--too hot. You can temper the heat on the hearth by moping the floor waiting for the steam to escape and re closing the door. Depending on how HOT you have got the oven you may need to do this a couple of times. I find the cornmeal works very well but you can use straight flour as well (not the good stuff) this will burn faster but between the 2 you can learn the curve.
I'm sure Jim's explanation was more involved but I don't know where I saw it, perhaps you can find it. If you have already got the hang of it...never mind me.

[DrakeRemoray]

Re: Question on thermal mass

It has been a bit of trial and error for me on this, but it sure fun to try. Mostly my problem is that I fire the oven for pizza at 3 or 4 pm and by the time I am done cooking pizza and have scraped out the coals, and let the oven equalize, it is dark and I am impatient...

No thermocouples/probes on mine, I do have an IR, but will have to try the cornmeal trick. Thanks for the tip!

Drake

[edschmidt]

Re: Question on thermal mass

If your buying the firebrick from home depot I would make absolutely sure that firebrick is what they are selling you. To my knowledge they do not sell firebrick, and the employees tend to give you any answer that makes you go away. I would definately get a spec sheet, if not you will probably be building the oven from concrete pavers.

[El Puaco]

Re: Question on thermal mass

I agree. I haven't found firebrick at any Home Depot stores in the Seattle area. I found the best source was a masonry supplier. My experience is that they prefer to sell to contractors but if you tell them you're building a pizza oven their eyes get real big and suddenly you're their friend. Full size bricks sell for about $1.00 each.

[maver]

Re: Question on thermal mass

I'll add one more vote for single layer floor - yes it's thinner than the dome, but unless you are doing multiple batches (more than two) of bread you shouldn't have an issue. Remember that the dome heats more easily. Saturating the floor with heat to get the thermal differentials to even out is a bigger challenge than being able to bake a lot of bread. The more mass, the longer it will hold at the temp you want, but the longer it takes to get it there. The pompeii design really seems to hit the sweet spot here for a home oven.

Marc

[rwiggim]

Re: Question on thermal mass

Thanks Maver and others. In thinking about it a little longer it would make sense to want more mass on the hearth for multiple firings as the cold dough would cool the hearth bricks faster than the dome ones with no contact. Home depot doesn't have the fire bricks but they order them from a company called Mutual Materials. They are about 43 cents cheaper than the local mason company and can deliver a pallet right to my foundation slab via forklift. I will ask for a spec sheet, that is a good idea. I plan on putting 2 thermocouplers in but they only have 36" wires. Any suggestions on their placement?

[maver]

Re: Question on thermal mass

Mutual material's standard fire bricks are low duty fire bricks, exactly what you want, so it sounds like you are good there. I can't make any good suggestion on thermocouples as I only use IR.

[wlively]

Re: Question on thermal mass

Are the thermocouples (TC's) going to be your only source of temp? I ask, because that will matter. You were asking about increased mass for extended baking. Placing the TC's into the heart of that mass will tell you the temp saturation of the mass, which once you get to know the thermal charactertistics will give you a good gauge of baking time or energy remaining. If it will be your only source of temp then you will probably want one very near the surface of the brick to better gauge face temp.

I have 3, all the center of the bricks. They are in the floor, middle dome, and top center. I can tell you that the floor is the last part to heat up, as it lags behind the others by 250-300 degrees. So keep that in mind, that with high floor mass you are going to need a very long fire to get the floor mass saturated. If I only had two then I would go floor and 3/4 dome height.

As for length, only one of my TC's needed to be over 3 ft. You can buy extension wire, with plugs at each end, or could buy bare wire and connectors to create whatever length you want. If you are anal like me, you need to keep the lengths of each TC wire the same to prevent small difference in temp readings. Of course we are only talking 1-3 degrees.

[Kulas]

Re: Question on thermal mass

I'm not sure if this is the thread I should post this question on, but here goes: Can the pompei oven maintain its hearth temp with fire going if one would make pizza continously for an hour?

Just a curious question. I read a lot about heat retention with regards to the pompei oven used in baking bread but what about its hearth floor heat retention while making pizza?