We have been slowly curing our 42 inch Pompeii oven that my son and I built. On day 8 we are seeing some cracks on the outside of the dome. We have not yet put on our insulating layer of heat stop mortar which we will do this week followed by framing the dome in with durock and filling it with vermiculite.
Question is, how much to worry about cracks? I see them in a few spots either running around sections of the dome or up and down. I believe the dome would stand strong even without most of our mortar as we tapered each course of bricks for tighter fit.
Any comments would be appreciated.
Joe in Connecticut
Cracks, everybody has them.
Everybody has some cracks eventually Joe. Worry only if it leaks smoke or undermines stability, which is unlikely with tapered bricks. :)
Lburou, thanks for the quick response. Just made our first pizza's and they came out pretty good. Needing to get better tools with longer handles quickly! Your oven looks great. Wishing mine was so tight.
In reading some other posts, I fear I may have made a significant error in not putting any insulation between my concrete slab and firebrick floor. I had spoken to several masons and concrete folks in the area about insulations but none never heard of it.
Knowing that I will lose heat out of the bottom does anyone have any suggestions? Might I just need to add extra wood and wait longer for it to reach the needed temperature?
Also, when I frame out my dome and fill it with insulation vermiculite, do you recommend loose vermiculate inside a fiberboard frame or do I mix it with portland cement? I was thinking about 4 inches to insulate around the dome.
You're going to have real trouble getting and keeping pizza temperatures on that floor.
Sorry if I missed it but if you didn't mortar the floor bricks in place and the dome doesn't sit on the floor bricks you could add rigid insulation under your floor bricks. Yes its work but it could be done. The rigid insulation comes in thicknesses from 1/4 to 2.5 inches and even a 1/2 inch will make a difference. Your trade offs will be how much of the sacred 60 some percent door to interior dome height ratio you are willing to lose. You'll save money on the firewood and gain flexibility in oven use if you can make the change. Don't underestimate the flexibility of these ovens with regards to minor changes.
PS The 2 inch insulation under my dome cost $200 this was for a 4 by 6 foot sheet.
In your post #1 you mention "We have not yet put on our insulating layer of heat stop mortar........." Heat Stop is mortar, not insulation.
You will need a lot of vermiculite if you intend to fill the enclosure. Some of the places will eat up vermiculite without giving any insulation value. Corners and areas within the studs for example. I made an enclosure of aluminum window screen and fastened it to the slab around the dome then filled it with vermiculite. No need to fill useless voids with expensive insulation. One of my construction photos shows the screen and vermiculite.
Thanks all for the feedback. I figured I would need to burn more wood to keep it hot. Our first pizza only took 2 minutes but I can see we need to work on getting the floor a bit hotter as the cheese began to brown a bit too quick in relation to the crust getting that browned/ a bit burnt texture. Looking forward to practicing!
I like the idea of limiting the area around the dome for vermiculite. Thanks for that suggestion.
Unfortunately (or not) we mortared the firebrick floor to the slab and the dome also sits on the floor so we won't be removing them anytime soon to insulate underneath.
I will be interested to see how insulating the dome helps as well as how much more wood we will need especially in winter.
This project was a combination of my desire for an oven and my son's 7th grade school project! A bit big for a school project to say the least but we had a lot of fun and surprisingly, we did better than we may have anticipated. We didn't use any forms (risky yes, and not out of skill but simplicity(one less thing to figure out), and Italian stubborness). Hope to post some pictures soon.
Question about doors when heating up the oven.
Does anyone put the door in part way or just leave it off completely when heating up your oven? I haven't ordered one yet but want to learn how to use it well.
There are several things you can do with doors. One, put the door tight up against the inner arch to keep heat from piling out. BUT, do not do this while there is live flame in the oven. If you were to take the door off soon afterwards then fresh air rushing in could easily create back draft and a dangerous situation. Doors are often placed against the inner arch after embers are swept out, as for bread baking.
Two, place the door at the front of the oven in such a way that the heat can still vent up the chimney. Lots of reasons you might want to do that.
Third, you can make a "draft door" and place it at the front of the oven with the chimney between. Cut a hole or series of holes at the bottom of the door to allow air in. This will allow a live fire to benefit from a draft adjustment of sorts while allowing heat to go up the chimney. Others will chime in to describe their draft doors or you can search on the forum about them. Otherwise, don't plug the oven entrance if you have a live fire.
If you are considering insulation at all then there is a slight chance putting it against the bottom of the slab might help...a little. The slab is a huge heat sink but if it is hot under the slab then adding insulation could provide some benefit. Forum members may be divided about this but it is a thought for discussion.
I was thinking about your situation since reading about your uninsulated floor and the burned cheese. In your position, I might consider cooking the pizza dough on a pizza pan for a minute, removing from the oven, then cooking it again briefly with the toppings on it. I've read of someone doing that (for different reasons) but it might work around the larger than normal temperature difference you have between the dome and floor.
One other trick I've read about is to let the oven set 30-45 minutes, (with an insulated door in place over the inner arch), after getting it up to heat to equalize temps within the oven. Have you tried that yet?
Early versions of the fornobravo plans had insulation on the bottom of the hearth slab and they worked well enough....You are not much worse off than they were. :)
P.S. I would enjoy seeing some pictures of your oven.
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