Baking the best pies of my life in a coal oven. Why is it different than wood?

[scott r]

I was fortunate enough to be asked to consult for a pizzeria with an earthstone coal fired oven last month.   I have worked with a few pizzerias in the past that bought wood fired ovens thinking they wanted to make neapolitan style pizza, only to realize that what they really wanted was a coal oven style bake (something closer to a 700 degree floor temp).   As soon as I pulled the first pie out of this coal oven I knew by the texture and the feel of the pizza that there was something different going on.   I know the difference was not in the dough formulation or mixing, because I have used the same exact methods and ingredients before with a WFO.  While the pizza was really good with wood (similar to a chris bianco style pizza)  I have never had this level of crispness before.   I was able to raise my hydration because of the drying aspects of the oven, creating a crust that was even more tender in the middle and more crispy on the outside than what I am accustomed to.   For me it was the ultimate coal style pizza...and go figure, it was baked with COAL!   

Im wondering if you guys can help me figure out why I noticed such a difference with coal.    Is it because coal gives off less moisture than wood?  All the wfo's I have used so far have been burning kiln dried oak, so I know the wood is about as dry as its going to get.   

Is it because these earthstone coal ovens are massive, with a super high dome, and the water vapor coming off of the fire is not contained to a smaller area? (so far I have only used low dome WFO's)

Interestingly the owners have fired the oven with wood, or a combination of wood and coal, and they claim that as soon as one or two logs are put on top of the coal pile the bake changes and the pies are not as crispy on the outside.     

Im looking for any thoughts, or discussion on the subject.  THANKS!!!

[The Dough Doctor]

Scott;
Think about it for a minute....how much moisture is in the wood that is being burned? Probably double didgit, and it can/will vary from lot to lot, if not piece to piece. Now switch over to coal, how much moisture in that lump of coal. For every pound of coal burned you get more BTU's of heat energy than you get from the same weight of wood, but without putting the moisture into the oven. We see a similar response when comparing gas heat (moist) to electric heat (dry). This might explain what you are seeing.
Tom Lehmann/The Dough Doctor

[scott123]

I think it's a combination of less moisture content and thermal mass, as well as air flow.

http://blog.forest2market.com/2012/02/14/wood-vs-coal/

Coal, like wood, does not have a moisture content of zero. Like different types of trees, types of coal have different moisture contents: the moisture content (MC) of anthracite coal is 2.8%-16.3% by weight, the MC of bituminous coal is 2.2%- 15.9%, and the MC of lignite coal is 39% or more.

Anthracite has historically been the choice for northeastern coal ovens, and, while 2.8% to 16.3% is a pretty huge fluctuation, I think it's safe to say that it most likely clocks in with less moisture than your typical kiln dried hardwood.

A thoroughly saturated massive brick oven that's hot, but not Neapolitan hot, will bake extremely evenly, and this even bake will translate into crispier exteriors.

Does this oven have a fan assist?  Even if it doesn't I'm wondering if the size/shape of the oven and the chimney configuration are creating a larger than normal amount of air flow. I found a noticeable improvement in crisp exterior when I used a convection oven for the first time.

[scott r]

Yes, there is a fan that you can turn on thats under the large bed of coals.   Its not needed or left on all the time.  Its only turned on from time to time when the oven looses temp and coals need to be stoked.    Coal doesn't seem to want to burn as easliy as wood.  I have asked a few friends to try coal in their WFO's (frank of vesta and widespread pizza) and so far the word is that without this fan under the deck the coals just wont pump out enough heat.  

[scott r]

heres a link to the oven  http://www.earthstoneovens.com/oven_pacb.shtml   I think its the larger of the two.    

The oven isn't as perfectly balanced as i'd like....its got too much bottom heat and that forces more babysitting of the pies than I would like, but what a great pizza this baby can make!  

The same ovens are used at anthony's and angelas coal fired pizza, which both kind of suck.   I think they are giving coal a bad name LOL!    

[scott r]

Im within a month of pulling the trigger on a WFO for my house.   The oven is going to sit outside the house, and just the mouth of it is going to poke into my kitchen.    Now, these coal burning ovens are way too expensive and massive for me to buy for personal use, but I cant stop thinking about how the coal makes for a superior bake.

Does anyone thing its a bad idea to try to retrofit something like the four grand-mere, or the blazingoven with a fan in the bottom of the deck so I can burn coal?  I am obviously going to need to find some type of specialty fan that wont melt LOL! Maybe earthstone will even sell me whatever they are using (fingers crossed).          

[scott123]

Does the fan have to be underneath the coals or could it be on the side?  If a side fan would work, you might be able to get away with running a few pipes into the mouth of the oven.

Or... maybe you could incorporate a grate into the floor that's connected to a pipe below. Once you get the pipe coming out of the oven/away from the heat, you can connect any kind of fan to it- even a hair drier or a mattress pump.

[scott r]

these are great ideas scott.    Thank you!   With that post you caught up exactly to my post count! 

[Jet_deck]

Does anyone thing its a bad idea to try to retrofit something like the four grand-mere, or the blazingoven with a fan in the bottom of the deck so I can burn coal?  I am obviously going to need to find some type of specialty fan that wont melt LOL! Maybe earthstone will even sell me whatever they are using (fingers crossed).          

In other fan assisted fires for ovens that I have seen, the fan is generally away (of course) from the fuel/combustion area.  The problem they run into is when negative pressure pushes the air backwards (back through the fan)  Most have a little flapper door that prevents (most of the time) this from happening.

This type of controller could be used to control the fan.  I have used a computer fan before.  http://www.auberins.com/index.php?main_page=product_info&cPath=8&products_id=191  and also the fan that comes with this unit.  A 5 cfm fan might not be enough for a "large" oven.  It might take 10 or 15 cfm to keep it hot.

My question is whether the thermal mass of this oven  would cause the controller to oveshoot the desired temp.  That is why I would suggest imbedding the temperature controller pretty close to where the direct heat is.

Just $.02

[scott123]

Scott, on the earthstone, would you happen to know if the fan is drawing fresh air from the outside or is it recirculating the air in the oven? If it's recirculating, then you're looking at greater complexity/far greater expense.

I haven't checked the patents yet, but a few years back I came up with the idea of an all steel/copper wire computer fan.  A fan like this would be safe to at least 1000 f and could be used to convert non convection ovens to convection for very little money.

[scott r]

Scott, on the earthstone, would you happen to know if the fan is drawing fresh air from the outside or is it recirculating the air in the oven? If it's recirculating, then you're looking at greater complexity/far greater expense.

im not sure about the fan.
im loving this input from everyone!    

[TXCraig1]

Scott, on the earthstone, would you happen to know if the fan is drawing fresh air from the outside or is it recirculating the air in the oven? If it's recirculating, then you're looking at greater complexity/far greater expense.

It draws in air from outside the oven - with the fuel density of coal, you need more O2.

A couple thoughts on why coal may produce a drier baking environment:
1) Wood has more hydrogen than anthracite coal (on the order of 2X as much).  I would guess there is at least 2X as much water as a byproduct of wood combustion as opposed to coal.
2) Perhaps the air induction of a coal oven forces out and replaces the water saturated air in the oven?

CL

[scott123]

It draws in air from outside the oven - with the fuel density of coal, you need more O2.

That's probably true, but one would think, with the chimney drawing smoke out, recycling the air in the oven would probably stoke the fire just fine.  It would also be more fuel efficient. 

[TXCraig1]

It would also be more fuel efficient. 

Actually not. Combustion efficiency will increase with increased air, until the heat loss in the excess air is larger than than the heat provided by more efficient combustion.

[TXCraig1]

Scott, also keep in mind that the combustion process for coal is different from wood. Coal combines with O2 and burns right on the surface. Much of the reaction is in the form C + O2 --> CO2

Wood on the other hand goes through an intermediate step where heat decomposes some of the wood into volatile gases. These gases then combine with O2 and burn (the flames). The reaction looks something like this: Hydrocarbons + Heat --> Char + CH20 (gas);   CH2O + O2 --> H2O + CO2 + CO + C (soot)

When all the volatile gases have been driven off, what is left (char) is pure carbon which burns much like coal. This is why a coal fire has very little flame. Flames convert fuel to heat much faster than char or coal as we know from our WFOs. There is much more surface area for the wood gases to mix with oxygen in open space than there is at the surface of the coal. You overcome this by increasing the available oxygen with a blower.

[scott123]

Actually not. Combustion efficiency will increase with increased air, until the heat loss in the excess air is larger than than the heat provided by more efficient combustion.

I think we might be getting our wires crossed on how we're defining 'fuel efficiency.' I'm not talking about complete/incomplete combustion of the coal, but, rather, heat loss up the chimney. The way I see it, a fresh air fan will push more air out the chimney and the door.  More air out = more heat out.  The more heat that escapes up the chimney, the less fuel efficiency.

[Matthew]

Im within a month of pulling the trigger on a WFO for my house.   The oven is going to sit outside the house, and just the mouth of it is going to poke into my kitchen.    Now, these coal burning ovens are way too expensive and massive for me to buy for personal use, but I cant stop thinking about how the coal makes for a superior bake.

Does anyone thing its a bad idea to try to retrofit something like the four grand-mere, or the blazingoven with a fan in the bottom of the deck so I can burn coal?  I am obviously going to need to find some type of specialty fan that wont melt LOL! Maybe earthstone will even sell me whatever they are using (fingers crossed).          

Scott,
Correct me if I'm wrong, but don't coal ovens have a hole in the bottom for the ash to drop out?

Matt

[dellavecchia]

I always thought that the old school coal ovens were more like the european bread ovens with the lateral/inferior throat firings - like a side chamber, and there was no direct fire where the pizza baked.

John

[David Deas]

Don't know about pizza but that (a giant offset) is certainly the best way to do BBQ.

Matt you can imagine there would have to be a way of clearing ash since a coal fire is harder to maintain and more easily smothered.  There shouldn't be too much ash to drop, though.  Coal is almost pure carbon.

[widespreadpizza]

John, IMO you are right pepes,  I believe and for sure patsys have a firebox under the oven,  off to the side,  completely different than using an easthstone like scott is describing.  I am familiar with Angelas ovens,  and as much as i hate to say it,  they make a pretty goood pizza,  albeit bromated.  Honestly,  I think it could be even better if they dropped about 100 degrees in temps.  Bottom line,  these newer ovens are much different than the old classics,  but they work well.  -Marc