A new brick bread oven, a year later
There are several things I have learned from building my brick bread oven that may benefit someone planning to build one.
It's a compromise. Too big and it takes a forest to heat up, but can bake large quantities of bread in one bake. Too small, and the oven won't maintain it's heat. The advise I got was to build it with a hearth that is 30" x 36". I thought that would be too large, but it turned out to be about right. I can bake up to 8 loaves of bread at a time, but it only takes a couple of hours to heat up the oven. One layer of firebricks laid on edge is sufficient with lots of insulation. There is no need for extra mortar over the barrel top.
There is no need for expensive insulation designed for insulating boilers in steam plants. Rockwool insulation from your local lumber yard does the trick perfectly. It's rated for 2000*F which (by my estimate) give a 3 to 1 safety margin. For the outside of the firebricks to get over 650*F, the inside of the oven would be a way to hot for even pizza. I wrapped the firebricks with one layer of 6 inch insulation, plus an addition layer of 4" insulation for a total of 10 inches of Rockwool insulation.
I wrapped industrial grade aluminum foil (available at Costco) over the fire bricks to reflect heat back into the brick and minimize drafts. I also wrapped foil over the outside of the bat insulation.
It's true that you can't have too much insulation. If I was doing it again, I would probably install an additional layer of insulation. Lot's of insulation along with adequate thermal mass equates to a longer baking window. If the oven is up to temperature, but the dough needs another half hour to rise ... no problem if the oven is built right.
Installing a thermal couple was the best piece of advice I got. I drilled a hole from the outside of the firebrick about half way into the brick, half way up the wall. When heating the oven, this is the temperature gauge I watch. You can get a thermal couple on ebay for just a few dollars.
Monitoring the temperature of the inside of the oven while the fire is on, is meaningless. The oven walls will get up to 600*-700* very quickly, but if you raked the fire out at this stage, the oven would cool down very quickly. I ignore the inside oven temperature until the thermal couple, measuring the temperature of the firebricks in their outer half, is up to 400*F. At this point the inside temperature will still be about 650*. I then rake out the fire and seal the door. An hour later the inside of the oven will be about 450* AND the temperature of the WHOLE firebricks will be close to the same. In other words, I have a thermal mass of over 1500 lbs of brick at 450* Putting 20 lbs of bread dough in the oven for 40 minutes does not lower the temperature much. And with all that Rockwool insulation (and a well fitting insulated door), the oven will stay hot for days.
If I bake once a week, the oven will still be warm (about 80*) before I light the new fire. If I don't bake for several weeks then the firebrick and insulation will have absorbed moisture from the humidity in the air. It will then take longer to get the oven up to temperature because it needs to heat up this moisture, and 'boil' it off.
It's easy to get complicated with steam injection but there is no need. A good garden hose sprayer on the mist setting does the trick. After the bread goes in the oven, I just spray in a mist of water before sealing the door.
Extending the roof to provide some dry counter space by the door of the oven, is well worth the effort.