Advice on mortarless brick oven design

[brianthelion]

Hey folks,  I'm planning on building a version of the mortarless ovens I've seen on this site. Mine will be round instead of square and I'm wondering what would be a good diameter for the interior to accomodate baking space and the coals. Also, what is a reasonable ceiling height to shoot for? Is there a thermometer available with multiple probes to track the heat in different parts of the oven? Any suggestions would be welcomed and I look forward to posting photos of the work in progress.

[buceriasdon]

Why not use mortar? I'm postive you will spend much more time fighting shifting, falling bricks than if you just did it right in the first place. IMO not a good idea.
http://www.fornobravo.com/pompeii_oven/pompeii_oven.html
Don

[brianthelion]

Actually, the guy at the masonry store was excited about the mortarless design. In his years of experience as a mason, he found that a perennial problem with mortar was the cracking due to expansion and contraction. The oven will be lined with a metal wall that will add stability. And in a few years when I retreat to Northern Vermont, I can take the oven with me.

[scott123]

Since I have a chipped tooth from a piece of mortar that ended up in a pizza, mortarless ovens have always appealed to me- especially steel plate domed ovens. I've scrutinized every mortarless oven I've seen, both on this site and elsewhere, and while some have some good ideas, many miss the mark.

First off, use firebrick for any brick exposed to fire. I've seen more than a few mortarless ovens use regular bricks, which is a really bad idea.

Second, insulate- both under the hearth and over the dome.  I've never seen a mortarless oven with insulation and insulation is critical for maintaining pizza baking temps.

Third. Weatherproof.   Rain on insulation is going to be both a mess and, with a very hot oven, possibly even dangerous.

Needless to say, what I'm describing is probably a lot more complicated and costly than what you're envisioning, but I think if you want WFO pizza, you've got to take the extra steps.

Another thing that appeals to me about a mortarless oven is the potential to save some money. Someone recently posted a link to a great deal on mineral wool blanket insulation (for the dome).  According to my calculations, I think one could insulate the dome for less than $100 with this. You can't put blanket insulation under the hearth, though, and board insulation can get really costly, so that's a bit of a wrench in my own personal oven building vision.  I might be able to fashion a perlcrete with sufficient compressive force, but I'm not sure. I'm still working on that.

Weatherproofing the whole unit with some sort of structure has been another tricky issue for me- especially some sort of inexpensive structure that could be disassembled and moved. It would be kind of silly to have a brick structure that could be moved, but a housing unit that couldn't.

One thing to consider when working with steel is that heat accelerates oxidation. That being said, you do see angle steel quite frequently in mortared oven plans (such as over the door) and I don't hear of any complaints about the steel components rusting out. If you are using steel, though, I would stick to steel components that are easily replaceable, such as angle steel and steel plate.

[shuboyje]

Scott,

Perlcrete has a bit more tensile strength then some give it credit for.  When I demo'd my old oven I actually was able to take the entire insulation layer(~5'x5' )out in one piece and purposely broke it into smaller pieces.  I think if you are sure to use agricultural grade perlite that is not coated in silicon and cast it into pieces 24" x 24" or so you would have no issue moving and re-using it.  Vermiculite, silicon coated vermiculite(around here sold as zonolite), and silicon coated perlite do not form nearly as strong of a product.  If I was building another oven I think I would d it this way anyway, perlcrete is very wet and takes forever to dry when cast in place, but like this it dries much faster in my tests.  I've even though about casting blocks of perlcrrete and then shaping them with an ax like the traditional builder do with tuff to insulate the dome in traditional fashion. 

As for the mortarless oven, If done right I think it would work fine. Scott has given lots of good advice already.  Depending on budget and how fancy you want to make it, I think you could make a really nice mortarless oven using castable refractoy.  If pizza is the goal you only need the oven 1.5" thick or so.  I'd make a round oven with walls 13" high and a flat ceiling cast into nice small pieces. 

[scott123]

Jeff, that's very helpful, thanks.

So if I'm hearing you correctly, traditional perlcrete can handle the weight when placed under the hearth? Are we talking about a slightly higher concrete ratio to perlite?  Also, what thickness would you suggest?

That 1.5" thickness suggestion- is that dome, walls and hearth?

[shuboyje]

For clarity you would need a structural layer of some sort under the perlcrete, but yes the perlcrete supports the compression load of the oven no problem.  Under the oven I use a 6:1 ratio of Perlite to Portland by volume.  Mix the dry ingredients with gloved hands then gradually add water and hand mix until you can squeeze a ball in your hand that will hold together.  The perlite shrinks a ton once wet ,so I figure 2 times the needed volume of perlite.  For instance for 1 cubic foot of finished perlcrete I would figure 2 cubic feet of perlite.  4" would be plenty for a pizza oven for home use.

I would do the dome and walls 1.5" thick because that is the minimum thickness recommended for most castable refractories I have looked into.  This would be plenty of mass for pizza.  I would probably do the floor in standard thickness firebrick.   

[scott123]

Thanks, Jeff.

[Michael130207]

I have almost finished building an oven similar to what you describe although square. Entirely of firebrick. I used standard firebrick about 95 and 30 half thickness fire brick. It sits on ground on 4 16inch pavers. The inside dimensions are 27 inches square, the outside is 32 inch square. It is similar in design to the mortarless ovens seen in this forum .  The ceiling is made with angle iron and standard firebrick. The half thickness brick is laid on top of the ceiling going the opposite direction of the ceiling. On top of that I put durock and then poured a 2 inch layer of perlcrete. Under the oven floor I put a 4.5 inch layer of perlcrete. Just fired it up tonight for the first time and got pretty good results. Will try to post pictures. It kept the heat pretty well. Still giving off a fair amount of steam from the water left over in the perlcrete which was poured a few days ago. Once everything is good and dry I will stucco the top perlcrete. Pizza in picture took about 2:30. Whole project cost about $250 so far.

Mike

[scott123]

Mike, that's a nice looking pizza. Did the top brown slower than the bottom or did they finish about the same time?

I can't tell from the photo- are you using insulating bricks on the outer walls? If not, don't forget about the need for insulation- on all sides.

[Michael130207]

Thanks,  It all browned at the same rate.  Floor temp was about 675, and as I had put a log on the fire about 15 minutes before cooking the pizza the ceiling was too hot to measure with my thermometer.

The sides are not insulated. I just decided to build an oven a month ago. My first try was basically this oven with no insulation under the floor or on the ceiling. It worked surprisingly well but was frustrating as I couldn't keep it hot enough for more than one pizza without adding wood and playing with the fire, totally unsatisfactory but addictive. So I poured an insulation pad under the floor and put one on the ceiling. I lowered the ceiling to 11 inches and narrowed the opening. My next move is to pour perlcrete insulation pads for the sides and back and keep them all in place with a wire cage made from an old tomato cage. I would like to keep it able to be disassembled. I keep a grill cover on it to keep it dry when not in use.

Mike

[scott123]

Sounds good, Mike, I like the tomato cage idea.

Once it's all insulated and you've dialed in the ceiling height, I'd be very curious to see what kind of bake times you can get with traditional Neapolitan doughs using Caputo flour. If you build a sub $300 mortarless oven that can reliably do Neapolitan pizza, that would be quite a coup.

[Michael130207]

I finally completed the next step in building this oven. Insulating slabs formed from perlcrete and attached to the sides and back of oven. I had to scrap the tomato cage idea and shell out $20 for wire fencing. I fired it up today and baked 8 pies over 3 hours. It takes about 90 minutes to heat up to 700+ floor and  900F ceiling. I was surprised that insulating the sides as opposed to just the ceiling and floor didn't seem to make a huge difference. It did help but not as much as I thought it would. That may be because they are not sealed to each other or the bricks. Air may infiltrate too much. Although I wanted to make this oven easy to disassemble I will probably stucco it together as I like it. If I have to  in the future I can cut apart all the walls with a edge grinder and cutting wheel.

It was fairly easy to maintain a 700F degree floor and walls and 900F plus ceiling but did require a constant fire. I added a log about every 30 minutes while baking pies.

I am attaching pics of the oven and some of the pies. Most pies took about 2 minutes to cook, some a little longer and one made it in 90 secs. I think I could consistently get 90s if I made my dough balls smaller. I used 325g for 13 inch pies, way too big. I did not use Caputo flour though. I use gold medal bread flour, I haven't gotten that purist yet.

The cost of making it has gone up a bit. I figure I have spent about $400 now. Working at ground level is a little bit of a pain but I wasn't willing to invest the time and money in making a pedestal for it, maybe next time when I do a proper oven. Overall for the money I am really happy with it.