Origen: 36" Pompeii Oven Build: Just getting started and have some questions....

[Origen]

I'm getting ready to start a 36" Pompeii style build. I have attached a few photos. As I was laying out the templates, I'm thinking my hearth might be a little narrow (that slab is 57"x67"). I know the Forno plans showed larger, but when I was scrambling to finish this in November, I got tired of handmixing concrete! On the width, I only have 5" after the floor and the first course (see photo); I'm planning on using 3" of blanket, so that leaves 2"! I was originally thinking that I was going to bolt angle iron to the sides so that I could brick around the dome--either with another dome, or vertical walls and put a gable roof upon the walls. I could also dowel into the existing slab and pour extensions; I suppose I could do that after the dome is built. The vent opening is 19"; the vent landing is approximately 14" long. The floor is 36" diameter; I'm planning a dome height of 18 or 19". So my first question is--what do you think? Is that slab size OK? Does the layout look OK?

Other questions that are I have: I'm planning on using 2" SuperIsol below the oven floor--do you mortar down the SuperIsol, or let it float? Do you use refractory mortar or fireclay as with the oven floor bricks? Just curious: would it be acceptable to let the floor bricks float, as opposed to fireclayed down? And speaking of 2" SuperIsol: is that enough? I reviewed the RandyJ build very closely, and I really liked the performance of his oven. I think he used 4" below the floor. I think he used 3" blanket, but also filled the house with vermiculite. So, does that killer oven performance come from the entire insulation package, or the insulation around the dome? I know a little about the diminishing returns of added insulation in areas other than a WFO, but, I would like some advice in this area.

Are medium duty firebricks OK? The bricks I have for the floor are medium duty and meet the specs outlined in FornoBravo's instructions. Is it a better dome with heavy duty bricks? I can easily get heavy duty bricks for the dome; the supplier even suggests that they are better for WFO's. What do you think?

I like the idea of cutting the "tops" of the first course bricks on an angle. I assume that the angle is the same as the angle on subsequent courses--is that correct? Will that angle be determined by the I.T. or does it need to be calculated? I do want to taper cut the bricks for the dome courses, and I'm not quite sure how to do that; I'm going to look at some data and see if I can figure that out.

Regarding cutting bricks: what type of blade? When I cut masonry or concrete, I usually use a segmented blade, but it seems to me these bricks maybe should be cut with a continuous turbo edge blade. I cut a few with a segmented blade and it seemed a little aggressive. I cut a few with a continuous rim diamond blade (not a turbo edge) and the cut was smooth, but slow and a little gentle--tentative.

Would it be best to keep the posts shorter than this? Seriously...

[gastagg]

Origen, 5" may be enough. It depends on how you decide to finish off the oven. I used stucco and had about 2" past the insulation, which was enough room. If you're going to build an enclosure, then you may want to make the oven a bit smaller. I"m not sure about extending the slab. I've got a 33" oven. What kind of use are you planning on?

Medium duty bricks are just fine for the dome.

Don't mortar the floor bricks down. The floor insulation just sits on the slab as well. It's not going anywhere. Put some plastic down between slab and insulation, and raise it off the slab a bit. I didn't, but that's the best practice at this point. I think Randy's build explains more detail on that.

Tapering the bricks is not necessary and creates a lot more work. Angle (cut from top view) and bevel (cut from front view) is what most builders do. I've attached a dome calculator, which is very helpful on getting started...only cut a few bricks at a time when you start the dome. Use a segmented blade for brick cuts. You'll wear the continuous rimmed blades down quickly. Keep the smooth part of the bricks toward the dome interior. Mortar will be the only thing on the other brick sides.

3" of dome insulation is enough. Floor insulation is key. 2" is minimum. If I can remember someone's tech analysis of insulation thickness using CalSil, I think that 1" more improved performance by 14 degrees.

[JRPizza]

Origen, when you attach pictures, if you use the camera icon in the upper left of the reply box the photo's will show up as thumbnails and make it easier to review your build. I think you probably used the file icon in the upper right, which is what you want to use to upload PDF's, cad files, etc.

[Origen]

Thanks JR. When I previewed the post, I couldn't figure out how to enter the photos properly. Thanks also gastagg; I will check out the dome calculator. I have to think about the insulation--if I want to add more to the floor. It would be a bit of a delay to get more calcium sil., but I might bite the bullet. Vermicrete is not as good right? Do you know how much vermiculite concrete equals 2" of CalSil board? I'm actually going to add a photo to this post to test my newfound photo loading skills! The photo was taken off one of the forums: it's how I want my dome to look!

[Origen]

Actually, I just thought I should test my newly learned photo uploading skills on uploading the original photos! A little slow on the draw there...Anyway, here are the originally incorrectly-posted photos.

[RandyJ]

I think that the calsil board is twice as efficient so 2" is at least as good as 4"of vermicreete. . Don't break the bank but you do want to do what you can afford. .

Randy

[JRPizza]

I went with 5 inches - would have been happy with 4, but two cases of 2.5" gave me two layers of board with very little left over. I didn't want to mess with perlcrete, but you can always supplement your board with a layer of insulating concrete - I think many of the builds, especially some of the older ones, were done that way.
Have you decided on what kind of an arch you are going to build yet? If you have not done it yet, you can use your IT (or a cad program) to make sure you have the fore-aft placement of your inner arch in the correct location. It's a good thing to do before you start mortaring.

[gastagg]

Origen, I've got 2" of CalSil and have no problem getting up to and holding heat, and after I saw the tech specs, 14 degrees is nothing, especially when you're at 550-700 degree floor temps. If it's going to cause delay and cost a lot more, I wouldn't worry about it. Once you get moisture out of the build, you'll be set.

[Origen]

I'm debating between pouring 4" of vermiculite concrete or grabbing more 2" CalSil; CalSil is painfully expensive! I also don't want to be stupid cheap so I have not decided what to do. I want the oven to be an engineering marvel--like you guys! I want it holding heat for a loooong time.

Why is a slip sheet recommended between the CalSil and the hearth? I can somewhat understand having an airspace--that would add insulation value in the right circumstance. So what would be used to shim the CalSil off the hearth?

I want to do a brick arch, but I'm not sure it's within the realm of my ability. I know how important this part of the oven is, so I certainly don't want to bollocks it. I started looking at the dome calculator and I started dusting off the part of my brain that learned geometry so many years ago, and I have looked at hundreds of outstanding forum photos...I think I need to get the insulation worked out and get the floor down so I can get my head wrapped around the dome and arch. Unfortunately, I was not blessed with excellent spatial imagination--I have noticed many builders here have that skill--it is a great gift. I am having a little trouble getting started: I know how I am--I don't fully understand this so it's paralyzing me a little!

One question I would like clarified: for a 36" oven, a 19" opening at around 12" in height would be fine, right? Also, the vent landing: 14" (dome opening to the front of the oven) is that OK? I have seen some info that suggests that a larger landing area might be more desirable.

[david s]

How about a compromise and lay a 1" vermicrete slab with 2" cal sil sitting on top of it. The cal sil will provide a nicer flat surface to lay your floor on to. Give the vermicrete a week to dry out before placing the cal sil on top of it though.

"
One question I would like clarified: for a 36" oven, a 19" opening at around 12" in height would be fine, right? Also, the vent landing: 14" (dome opening to the front of the oven) is that OK? I have seen some info that suggests that a larger landing area might be more desirable."
Not quite sure I understand this question correctly. The inner oven door should have a height approx 65% of internal oven height. Make the width whatever you want, but the wider it is the easier the oven will be to work, but the greater the heat loss will be.The outer decorative arch needs to be both wider and taller so an oven door will fit against the rebate of the inner oven mouth.

[Origen]

After honoring all the Mothers I know, I decided to pour 4" of vermicrete and put 2" of CalSil on top of that. I just built a form and photo'd it. If it doesn't rain tomorrow, I think I will mix and pour that. So Vermicrete takes a week to dry? That seems unusually long--can you explain that to me? It's not that big a deal, that delay-I can cut the floor bricks and make an arch template and start contemplating the arch I suppose. I have a window over the next 4 or 5 weeks where I will have more time to work on this than would normally be available, so I have to hit it hard. After that, it will be very sporadic!

[RandyJ]

I think it has to do with there being more water in it to help bind everything together. So it has to dry out some first. Not all if it is used up in the chemical reaction with the cement. I hope this helps or if I am someone who knows will chime in. Good luck getting started. If you have some serious time to put in to the build you can make quite a bit of progress. Good luck

Randy

[JRPizza]

Vermiculite can soak up and hold quite a bit of water, so you have to put as much water in the mix as would normally be required for the amount of portland you added, plus the amount of water the volume of vermiculite you added wants to absorb. After the reaction is done (the vermicrete has set), you have to give the water absorbed into the vermiculite structure time to exit. If you cover it with the CalSil too soon, it will take much longer for the vermicrete to dry.
Interestingly enough, looking at Wikipedia I see that vermiculite is used in the manufacture of CalSil board:
"Calcium silicate boards: exfoliated vermiculite is added to a calcium silicate slurry, this is then dewatered by pressing or by using one of the Fourdriner/Magnani/Hatschek processes to form a flat board which is then heat cured under pressure (typically 10-15 bar) for periods up to 24 hours."

[JRPizza]

I meant to ask - are you going with the flattened arch shown in the picture you posted above, or a hemispherical arch like this one.

[Yeager]

Another idea for you that may be simpler is put a layer of insulating brick under your calsil. It will have almost the same insulating properties as your vermiculite and is more tolerable of moisture (and elevates your calsil off the floor to avoid moisture wicking in). That's how I built my oven - 2.5" of insulating brick and 2" of Ceramic Fiber Board. The simulators show it dropping the temperature from a 800 degree oven floor to around 160 degrees under the insulated brick. The bonus is less work and you can continue building right away with the insulating brick vs vermiculite.

-Yeager!