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

[Gulf]

Origen,

Great form work. Vcrete has great compression strength, but very little tensil strength. The weight of your floor and dome brick will be easily supported by the vcrete. It takes about seven days for the portland in the mix to set. It will take about 28 days for it to cure. It will take much longer for it to dry. No matter how long you wait, the corners of the pour will chip very easily. While laying the flloor brick and building the dome, you will come into contact with those corners many times. Leaving the forms in place will protect those corners.

Gypsum products are great for heat sheilding and protecting flammable materials like wooden framing. However, they do that by absorbing heat. Eventually, the gypsum will be heat saturated, and the heat will eventually spread to what ever it is touching. I think that I remember that two 5/8ths thickness of sheet rock ( on each side of wall) gave about 30 minutes of burn time for a fire to spread from one apartment to another. If it is absorbing heat, it is also robbing heat from where you want it to stay. You want as much heat as possible to stay in the oven.

[RandyJ]

Looks like you were busy today. I also see you came to the same conclusion that I did. It is just way easier to use the calsil than do the vermicreete. There is nothing wrong with it. Just a lot of work, and if you don't have the money it will do the job just fine. I think it cost me about $100 more to do 2 layers on my oven. I used 3 boxes for under the floor so $300. So for me it was worth the $100 to speed up the process.

I could be wrong but the dens glass that you are taking about I Think is like a exterior grade sheet rock. I don't think it would be suitable for what I think you are taking about. There have been a few people on the forum that have used it for there enclosure. That part is up to you. Cement board works great too.

Randy

[Origen]

I poured the vermiculite concrete today. It's easier to mix than conventional concrete, but I still wouldn't call it easy! I used a 5:1 mix--it's strange stuff, it's really bizarre how it behaves when water is added. I first put vermiculite in the tub, then dry portland. I blended it very thoroughly while dry. Then very slowly added water, then mixed, add water, then mix, etc. I mixed it like the mix used for mud shower bases (see photo) : it's dry, but when you squeeze it into a ball, it holds form. It's an entirely different mix, but it behaves very much like the mud base mix during placement. There is little, if any bleed water with the mud base, and I had the same condition with the vermiculite concrete. I've read that the mix tends to be a little "crumbly", and the forms probably should remain in place. I was wondering if that condition had anything to do with the mix: if you dry mix it and make sure the portland surrounds all the individual pieces of vermiculite, I wonder if that prevents that condition. I guess I'm going to find out in a few days! I used a tub for mixing, but I think a paddle mixer would be easier (at low rpm's). It would be slower maybe, but easier. About 75% of the way through the mixing and placing I said to myself: "You're a knucklehead, you should have just used CalSil." That is what I would recommend: unless cost is unusually prohibitive, use CalSil (I am putting 2" of CalSil on the Vermicrete, but I wish I just purchase 4".

I was also thinking that Roxul Rockboard 80 at 2" and then CalSil 2" would probably be fantastic. I was also wondering if you could use something else...gypsum based. Have you ever seen a wet hydronic system installed? Or, could you use something like DensGlas Gold? Not really as insulation, but for it's thermal resistance. I have no idea, but I was just wondering...

[Origen]

Ok, thanks Randy, I will keep it 5:1.

[RandyJ]

I did not do the vermicreete on mine but did use a little to seal the chimney to the dome on the outside. The vermiculite does compress some when mixed with the Portland and water. So you will likely need more than you think. As for a higher mix it starts to get soft and crumbly and could have trouble supporting weight. So you are better off sticking to the 5 to 1. That is tried and tested.I did 8 to 1 and it barely held together.

Randy

[Origen]

I haven't been able to pour the insulating concrete yet--monsoon rains! I posted a picture of the form above, but I was thinking: I really only want the insulation under the oven (that would encompass the oven floor and first course and the vent landing). The ceramic fiber blanket would then encompass the entire dome and floor, terminating at the concrete slab. I started to make this form (it is not yet "formed"), and I attached a photo. {Well, actually, I didn't attach a photo, it keeps getting rejected--why does that happen?} It's better to pour the round form rather than pouring all that extraneous material (that would be the original form above) right?

Also, I've never mixed vermiculite concrete (mixed plenty of the regular stuff), any hazards I should look for? I would like to mix more like 7:1 rather than the often mentioned 5:1. Any problem with that? Also, I've read some stuff on the forums regarding volume: I've calculated my volume conventionally, using cubic feet and it came up 3.68 cubic feet. Is 4 cubic feet of vermiculite and a bag of portland enough? The reason I ask is I have no reference point--is there shrinkage when it's mixed? A loss of volume? Has anyone ever poured a mud base shower floor using sand topping mix? I would think you mix the vermiculite concrete to the same consistency as a mud base. But again, no point of reference. Comments would be greatly appreciated.

Jumping ahead a little: do you put a moisture meter on it to determine when it's dry, or are there other obvious signs?

[UtahBeehiver]

I only looked at the rigid roxul 80 specs for under the floor only.

[Origen]

Great! Thanks very much for the information; I look forward to checking out your CAD thread. I'm a little ham-handed with design software, but I'm sure just looking at it will be a huge benefit. I did check out the Roxul forum information--I had previously seen that. I figured the Roxul would be fine for the dome, but I decided early on that I better use ceramic blanket. I kept hearing the chants: "There is no such thing as too much insulation....", so, I was not willing to deviate. I was really wondering if anyone was using the boards on the floor. That is an unbelievable product. Mineral wool made the way Roxul is made is non-carcinogenic as well.

[JRPizza]

Check out the thread below. The program I use is called FreeCad, and the thread is a link to my oven file (use the one on page 2). The program is free and quite powerful. It takes a bit of learning, but if you start with my file you can see what is going on pretty easy. Read the thread if you are interested. It is also the program I use to make models for the 3D printer. My model does not include details of the arch bricks taper - I have a spread sheet for that. I can help you work through the arch calculations if you want - just let me know. I'll need your brick dimensions if they aren't in an earlier post.

https://community.fornobravo.com/for...en-build/page2

[UtahBeehiver]

I just looked at the roxul rigid board 80 cut sheet and the specs for thermal transmission and compressive strength are better than the thermogold 12 I used plus it is water resistant.

[Origen]

You know JR, your exposition of my inability to be creative enough to think of searching Forno and Roxul via Google is why I'm all cramped up with psychological paralysis with getting things really going! And thanks, I will check that Roxul info out. Are CAD files the .dwg format? I previously had a reader for that format and I am sure I can get it again. I also have a software that I think converts .dwg to .pdf (but I'm not sure about that). I would love to take a look at the drawings. If that is acceptable, let me know how I go about getting them.

Finally, UtahBeehiver, thanks for the wise advice: nothing good is ever forced or rushed, it all comes in the fullness of time.

[UtahBeehiver]

Some how my long winded post got lost in the electron world but has since been covered by JR. In a nutshell, thermal specs, roughly is .06 CaSi,.12 insulating fire brick,v/p-crete varies (david s has a chart somewhere in his thread) W/M c. So choose what works best for time and budget. But don't hurry the process, patience in building these projects really pays off.

[JRPizza]

Origen, I did a search using a trick I found for Roxul on the FB forum (using google, not the native forum search), and got quite a few hits (67). You might want to give some of them a read.

Yes, UtahBeehiver's oven is the gold (er copper?) standard for Igloo's - all I could hope is for mine to look kinda like his at about a hundred yards away , but building an oven gives us all a chance for our creativity to come out somewhere. My printed fixture worked well - it has the angle I wanted for my tapered arch, and stops at each side. I indexed the bricks against one side and did all the cuts fat side to skinny (part way thru), then did a 180 turn of the fixture, indexed the bricks against the other edge and lined up the blade. The only problem is some of my bricks did not have a nice 90 degree "face" so I had to screen a bunch of them with a square and use the squarest ones for the arch.
I've got my oven all laid out in a cad program, so if you want any sketches (like for arch placement) let me know.

[Origen]

JR, I went and checked out UtahBeehiver's photo album--Holy Smokes! That is...I don't even have a word for it at the ready! I saw some photos on gastagg's thread with copper ovens and thought that would be beautiful once the copper patinated (I like that word)--but to bend your own shingles and apply to a dome--WOW! I did a shed roof once where we bent square interlocking shingle, but diamonds...on a dome? After checking your two photos above, I went to your build as well Really great! That 4" CalSil makes me salivate--it looks good, that's gravitas! That outer arch is slick--serpentine indeed I do think I am going to do a hemispherical arch; I prefer the visual and dome intersecting dome makes sense to me! I am really impressed with the fixture generated by the 3-D Printer--that's great. Before I got started I was thinking that a 3-D printer might be helpful in this endeavor--but I didn't think about it for jig prototypes! I have been thinking about brick cutting jigs and testing a few things. I'm using an Imer saw (Combi 250V--the older model): the cutting head rides on a rail and the cutting motion is "pulling". I have a 10" conventional tile saw, but the Imer is a beast and figured it would be better for bricks. The rail can also be tilted, so, with the addition of a jig or manipulation of the material to be cut, I guess it would be capable of compound cuts. Have you ever seen a tool called a "taper jig"? It is usually used on a table saw against the fence--it's very simple: two aluminum rectangles with a guage and a wingnut. I've been testing that, but I don't know if it's the best approach. By the way, on UtahBeehiver's build, he used Pittsburgh-Cornings Foamglas below the CalSil. From what I understand, that product has been available in Europe for a long time, and is relatively new to the US market. I tried to find a source and pricing, but was not able. Based on how it's made and where it's used, I suspect it is expensive, but maybe not as much as CalSil. Now I was thinking a while back, before I purchased 2" CalSil, that Roxul's rigid insulation board could be used on the oven floor. The material I'm talking about is super heat resistant (same as CalSil) and I think it has similar compressive strength--but it is nowhere near the cost. I looked around the forums and could not find anyone using it, so I abandoned the thought. I was interested to know if anyone had any thoughts on that. That will be the last whining I do about the cost of CalSil, I'm kind of getting tired of listening to myself!

[JRPizza]

I just thought the hemispherical arch looked better. When I was getting started, two of the builds I was drawn to were Gulf's and UtahBeehiver's, which both used the hemispherical design. It was easier for me to wrap my head around the integration with the dome, and seemed like the stresses were more balanced and would not require buttressing. If you build a doghouse buttressing the arches is not a big deal, but not needing to with an igloo makes for better lines, at least to my eye. I believe you lose a little room at the upper edges of your opening vs a flattened arch, but with my oven size that was not an issue. I also really liked the serpentine (I like using that term too) vent that goes well with the hemispherical arch. I did four tapered bricks at the bottom of the arch, then four more bricks with the same taper but turned 180 degrees to make my opening. I also attached a pic of how I cut my arch bricks - I cut them all part way through, then reset my jig and completed the cuts through the other side. Printed the fixture on my son's 3D printer.