Driving heat across the cooking floor

[james]

I have heard a number of people say that keep the cooking floor hot while cooking a number of pizzas in a row can be a challenge. We did an experiment with a Casa90 yesterday that was a good learning experience.

We fired the oven very well (over 90 minutes, and a pre-heat the night before), so it was hot and dry. pushed the fire to the side, and let the flame drop. The dome was well over 800F and the floor was around 750F. I then cooked two pizzas. The flame had almost completely gone down, though there was a big bed of coals.

After the two pizza, the floor had fallen to about 625F. I am guessing this is what a lot of people have seen. At this point, it would be increasingly difficult to make a great pizza.

I add two pieces of wanlut (about 4" diameter and 8" long). They caught fire very qucikly and burst back into flame, which reached about half way across the dome. Within a few minutes, the floor was back up to 750F. You could feel the fire driving heat across the floor.

The next pizzas were great.

While it is obvious that the fire is important for bouncing heat down on the top of the pizza, and recharging the heat in the dome -- it is even more important in keeping the cooking floor at good cooking temerature.

Salute.
James

[jayjay]

I guess you folks are speaking of logs and we are speaking of sticks.
I am wondering what size 'logs' you folks put into your oven when you are
firing it to make bread or pizza.
I have posted below regarding the size dementions of the oven and its
basic construction.
If your will give me information, I would greatly appreciate
what is sent. You can also email me at
pugoclaire@yahoo.com
if you would like to get or give a faster responce.
Thanks,
JJ
Philippines

[Alan]

I provide these comments without the benefit of direct experience - our first pizza is scheduled for tonight! - but with some knowledge of heat transfer...

"Recharge" of the floor should certainly be accelerated by the fire, but if the oven has reached equilibrium as shown in James' cartoon in the first post, the surface should also re-charge from heat spreading back from the depth of the floor. In other words, as soon as the surface cools due to heat transferred to the pizza, the surface will be cooler than the deeper portions of the floor and stored heat will move back toward the surface.

Anyone who has an infrared thermometer and a thermocouple at some depth into the floor should see the surface temp down a little after the pie is removed, and, of the fire is down, the surface temp recovering at the expense of a drop in temp at the "depths," followed by the two temperatures moving toward each other.

As others have discussed throughout the site in determining material choices, there's a balance between thermal mass that provides heat storage and thermal conductivity that allows stored heat to promptly become available for cooking.

[Xabia Jim]

We did a dozen pizzas last night.....roasting and breads today!

I moved the fire across the floor before pizza time so I'd have a good hot hearth. One large log (6-7 inch) was left and I used three smaller branches (2-3 inch) during the pizza's for the fire across the dome. The pizzas were done in 4 waves of 3 with pretty consistent results.

My oven is more of a bake oven with a large thermal mass so I think my hearth temp is more constant. I'll check with the gun next time as I rotate pizzas to check actual temp drop.

Just refiring this morning and the oven is still 350 degrees. In go the chickens!

[james]

Jim,

It's fun learning about a new oven. How thick do you think your cooking floor is?How hot is your oven when you are cooking your pizzas? And how long do they take to bake?

Inquiring minds want to know...
James

[Xabia Jim]

I started with a standard thickness floor (firebrick) that is about 6cm thick. After my first firing I noticed the bottom of the hearth was about 350 degrees. So I used some thin red construction roof? tiles for a shelf and stuffed in a layer of Arlite/Expanded clay balls. This gives me another 15 cm of mass...about 11 cm of the insulation layer and 4 cm of the tile. Now I do not see the underside of the hearth very hot until it has been firing a while.

After pizza night....We cooked the chickens yesterday morning, had the lamb/potatoes/tomatoes with friends and then heated it up a bit to make two rounds of bread...5 loaves and a bunch of rolls. (BTW the german caraway rye was so dense I couldn't cut it with a bread knife so I got out the slicer and thin sliced it...great texture a flavor.)

This morning the oven was still 200 degrees and I was wishing I had something to slow roast....now 10 or 11 hours later the oven is 160 degree hearth with a 175 degree dome.....could still put in that goat shoulder!!!!

[Xabia Jim]

About the pizza's james....I'm running pretty hard.

The dome goes white...digital is about 800 degrees.
...hearth is pretty hot....I moved the fire over before mopping the hearth

...take a few pictures...

Pizzas seem to go pretty quick...a few minutes....

I give them one turn usually...next ones go in within a few minutes....keep the fire licking the dome.....

....pizzas go quickly...both in the oven and after a short rest on the table, pretty much as soon as they're sliced

The last few straggler pieces are the post dinner snack in front of the fire...

I will take better notes next time...hearth temp throughout and time to cook...might have to bribe an assitant to take notes....

[james]

Hi Jim,
This all sounds good. It was good to add the insulation at the bottom, or you would have lost a lot of heat out the bottom of the floor. Are you able to keep the cooking floor hot the whole time you cook pizzas, or does it start to cool down after a while?

Does anyone have some nice "flame" photos. I am putting the cookbook together and collecting up photos.

Pizza prep photos would be good as well.
Thanks.
James

[beammeup]

I usually do about 6 pizzas per session. I find the first 2 are great the bottoms are perfect but after that I can not get the charred bottom. It is cooked but not as much as I would like. Perhaps after the first 2 I should replace the coles onto the cooking area for a few minuted to let it heat up again. I have 5" of vermiculite and 1" of sand under my fire bricks. I thought that would be enough.

[maver]

That insulation sounds adequate, if the rest of your oven is as well insulated, I wonder about whether your initial heat up time needs to be longer or whether you need to burn more wood (during heat up or while cooking pizza) to achieve a more thorough saturation of your hearth bricks. I have less perlite under my oven than yours and I can cook 14 or more pizzas with little trouble - remember to maintain a good fire while cooking the pizza, with the flames licking from one side to at least the dome apex.