36" in Seattle, second oven

[kebwi]

Here's an odd proposal: Build a thin arch insert that converts an over-sized arch (perhaps .75X ceiling height) into a proper .63X arch. The insert would be left in place most of the time to optimize air/smoke flow and heat retention, but could be temporarily removed to fit a tall cooking item through. The insert could be either cast or steel (if casting a small piece like this that won't break apart is impossible).

Long version:

I would like to attempt a Neapolitan dome profile for my second oven as opposed to a rounder Pompeii profile, and regardless of whether a vertical wall alters the shape of the dome, it would still be a fairly short dome compared to a Pompeii (it is my understanding that Neapolitan domes aren't merely "segmental-arch" like in shape, with a flat, incomplete dome resting on vertical sidewalls, but are actually shorter overall at their apex than hemispherical Pompeii domes). Consequently, I still feel that 30" is about the lower limit for achieving a usable entry. Even at my current 30" design, the ceiling would be about 12" high. I can't imagine a Neapolitan 21" oven, which would verge on 8" or 9" high. Plus, how on Earth would one achieve the .63X entry height at that point. It just gets ridiculous. Even with my 30"x12" design, .63X prescribes an entry of a mere 7.5" high. Consequently, I'm probably going to significantly violate the .63X rule for my second oven. A 9" entry would be .75X of the 12" height and I'm not sure that's even tall enough (how tall is a turkey, sitting on a raised rack in a pan?).

Since I'm always coming up with crazy ideas, I've been thinking about building an arch insert, a 2" (final design undetermined, but about 2") tall semi-arch (tapering to perhaps 1" wide at the sides of the arch, crescent-shaped) that would sit within and under the arch, resting its 1" feet on pieces of brick at the sides of the arch such that the insert decreases the built-in arch height by about 2". It would either be refractory material (homebrew cast with fiber and maybe some very thin wire, to hold itself together despite its diminutive cross-section) or perhaps just a piece of stainless steel. It would almost aways be left in place, inserted into the cast-but-too-high 9" arch, dropping the arch down to about 7" to achieve the hallowed .63X. But on the rare occasion, I could pull the insert out, shove a turkey through the entry, and then put it back in place again -- and then reverse the process to get the turkey out. But for pizza, and probably even for bread, I would just leave this insert in place practically all the time.

I have no idea if this is a good idea, but I haven't figured out a better way to meet a set of conflicting goals:

1. Make a smaller diameter oven
2. With a Neapolitan profile and corresponding low ceiling
3. That still satisfies the .63X entry rule
4. But can still accommodate the occasional tall cooking item.

Thoughts?

[david s]

My 540mm oven has an internal height of 250mm and a door height of 160mm which puts the ratio at 64%
In old fashioned measurement that's 21",10" and 6.5"
I never have any problems fitting in the roast for the family, except when I once cooked a 4kg turkey which just scraped the top of the door when I put it in (was ok to pull it out though).

Your idea of a removable arch is interesting but would be a problem to move in and out when hot.

I think far too much importance has been placed on the 63% ratio rule. I say this because it was also my belief that the 63% rule was quite important until I modified my design for a build which entailed raising both the dome and the door height 50mm This produced a door to interior height ratio of 70% I was concerned with how it would fire and perform and was pleasantly surprised to find that there was no perceived difference. Obviously a small oven has a larger proportion of oven mouth to oven volume than a large oven does so there is more heat loss, although this disadvantage is well outweighed, in my view, by having better access and better fuel economy, as well as being lighter, cheaper to build and requiring less labour and time.

Read what James has to say regarding door height here

https://community.fornobravo.com/for...g-size#post764

[kebwi]

Sometimes poly (PVA) fibers are sized by "RECS", namely 7, 15, and 100. Any idea which one is best for our application? I believe it corresponds to size in some dimension, definitely length, but possible also diameter. Therefore, I would assume we want the lowest RECS possible to maximize its burnout. Can I get a confirmation on this?

Thanks.

[kebwi]

Thanks for your thoughts and the reference to James' post regarding entry height.

[david s]

Not PVA fibres, I think those you may be looking at are for strength. It is pp (polypropylene fibres you want. All the ones I've seen are 19mm long. The ones I get are by Radmix, who were bought out by Sika and they have macro polyolefin fibres 0.5mm x 48mm fibres for strength and micro polypropylene 19mm long finer than human hair fibres for reducing slump and shrinkage cracks in freshly place concrete. These finer fibres just happen to have the quality of melting at 160C which makes them suitable for our purpose. I do also use the macro fibres in standard concrete applications. Both fibres are alkaline resistant.

[kebwi]

Oh. Ok. I knew the AR (fiber)glass was a different thing, but I got the incorrect impression that PVA and polypropylene were the same thing. So we have no use for PVA fibers in oven construction. We want the poly fibers to serve as -- as you point out, rather coincidental -- burnout fibers and we want AR glass for basic structural reinforcement, along with SS needles which are, to my understanding, serving more or less the same purpose as the AR fibers (while I realize the poly fibers are for a very different purpose, which I now realize is not the same thing as PVA, which we have no particular use for).

Thanks.

[david s]

Oh. Ok. I knew the AR (fiber)glass was a different thing, but I got the incorrect impression that PVA and polypropylene were the same thing. So we have no use for PVA fibers in oven construction. We want the poly fibers to serve as -- as you point out, rather coincidental -- burnout fibers and we want AR glass for basic structural reinforcement, along with SS needles which are, to my understanding, serving more or less the same purpose as the AR fibers (while I realize the poly fibers are for a very different purpose, which I now realize is not the same thing as PVA, which we have no particular use for).

Thanks.

There are many types of polymer plastics it's important to get the correct one for the designed application
eg
polyethylene
polyvinyl
polypropylene
polyolefin
polyester
and probably heaps more I've never heard of.

In our case it is the low melting point that is the desirable quality.

[kebwi]

Are the AF fibers and SS needles serving effectively the same purpose: physical structural support? I intend to use both (I've already acquired 2lbs of SS needles), but I'm curious if they are basically doing the same thing?

Would you recommend adding either to the ordinary concrete heart slab? I didn't do anything like that for my first oven and I don't get the impression many people on FB do, but I'm curious if it's advised for the spans and loads we like to support. I'm going to do a wide span, unlike your designs david s in order to (1) have more wood storage underneath, and (2) get vertical-to-ground support in he far corners of he hearth to support either enclosure framing or pergola posts. Since my first oven had a slightly wider span for a considerably heavier oven than the design for my second oven, I'm not worried about this issue, but I'm still curious about whether real masons would add some sort of fiber to these slabs we build use for these ovens (in addition to rebar of course).

Cheers!

[kebwi]

I was dry-stacking some concrete blocks today and (re)discovered that, of course, their nominal dimensions are overestimates. 6"x8"x16" are actually 5-5/8x7-5/8x15-5/8. So my stand will be slightly smaller in both dimensions than I realize, which is irrelevant. But of greater concern, I now realize that four stacked blocks does not reach 32" but only about 30" (maybe 30-1/2"). This will, of course, drop the oven floor by 2". With a 4" slab, 4" of insulation board, and 2.5" of brick, this drops the oven floor from my anticipated 42-1/2" to 40-1/2". My first oven was even lower I suppose. That oven's hearth came out to 3-1/2" by the time I packed the concrete after pouring it and I only used 3" of insulation board, so that oven floor was 1-1/2" lower than my second oven floor will be. I could add a fifth course of concrete blocks I suppose, which would put the oven floor at 48", but I think that's getting a little high. I don't want to build a small platform in front of the oven, not even a few inches, as it will be a tripping hazard.

Just pondering to myself here. I guess it doesn't matter too much.

[david s]

Large diameter steel is unsuitable for thin concrete slabs because of insufficient embedment and unsuitable for refractory for the same reason, as well as susceptibility to corrosion from heat and thermal expansion issues. Hence the use of stainless needles which reduce diameter, increase surface area and are corrosion resistant . AR glass fibres are good for thin concrete slabs and for refractory ovens if the temp does not exceed 900C, so no good for kilns. AR plastic fibres would melt, so not suitable for any refractory.
For the supporting slab, which does not see elevated temperature just use standard rebar.
Regarding the blocks, nominal dimensions include the thickness of the mortar joints, so if dry stacking them with masonry adhesive, the total of the joint thicknesses needs to be calculated. This can also sometimes cause problems if half blocks, or corner blocks are used.

[kebwi]

Well, I'm committed now. Once concrete gets poured, things become permanent really fast. After all my efforts to find or build a metal cart or frame, possibly with casters, that would facilitate either local relocation around the patio or distant relocation to a new site, and after considering timber framing to at least assist with eventual demolition (much easier to deconstruct that rebar-concrete), I just plain gave up and went with a concrete design instead. I'm not thrilled about the decision but I couldn't figure out another way to make it work.

As you can see, I placed plastic on the patio under the blocks before stacking and filling the cores so they won't bond to (or even mess up) the patio (I'll cut all the plastic away at the end, leaving only the footprint under the walls, separating the walls from the patio). The two long walls aren't interlocked or in other way bonded to one another at the corner, which might vaguely assist demolition someday, but ultimately, this structure is going to be pretty stuck for a long time now. Obviously, it would have been stronger to interlock them at the corner, and even stronger to fill the corner core interlocked, but I am trying to make this thing somewhat deconstructable someday. The hearth, with the rebar pins shown, should provide protection against sheering failures in either direction. I suppose I could put some corner braces on it if I feel like it.

As shown, I followed david s 's design of projecting rebar "pins" straight up out of the cores that the poured hearth will anchor against. I used 3/8" rebar since the 1/2" rebar I used for my first oven was unnecessarily difficult to work with. I will use 3/8" rebar for the hearth as well. Not shown in the photos is that each filled core actually has two pieces of rebar in it, the second one not long enough to protrude out the top of the core.

I will cover the top of the wall in plastic before pouring the hearth so they don't bond to each other. Likewise, I will wrap the rebar pins in corrugated cardboard or thin, flexible sheet-like styrofoam, such that the pins don't permanently lock to the poured hearth. Again, the intent is that in the far flung future, one will be able to palette-jack, forklift, or crane the hearth (with oven) straight up off the walls, either for transport to a new site or just for easier final demolition.

It turns out that nine cores of six-inch concrete block, four blocks tall, uses precisely six 60lb bags of concrete. That was approximately what I had calculated in advance, but it was still neat to hit the mark so perfectly.

[kebwi]

Good grief. Literally the second I start posting actual construction photos, my thread gets moved out of the photos forum. I can't catch a break.

[kebwi]

Big day today. With the help of a friend, I poured the hearth in near-80F weather under a beating sun. For this reason, I mixed the concrete pretty wet and occasionally misted the ongoing job while I was mixing the next batch (always either two or three 60lb bags at a time). The form/frame design is flush with the concrete walls on the rear and left sides, where the oven is cornered in by two retaining walls, but has a chamfered overhang on the front and right sides. I hope it doesn't chip, crumble, or otherwise fail along the 1.5" thick front edge (it's obviousliy 4" thick everywhere else, as per the standard design). The entire form is lined with plastic except the sloped counter overhangs. I definitely wanted plastic over the concrete walls so the hearth wouldn't bond to the walls, to facilitate eventual deconstruction (the hearth can theoretically be lifted straight off the walls; even the various rebar "pins" protruding from the walls that will anchor the hearth in place are wrapped in thin foam so they won't bond to the hearth, again to enable lifting the hearth off the walls; likewise, the two walls are neither interlocking at the corner nor bonded together in any other way). I chose not to line the counter overhangs with plastic to avoid crease-lines in the concrete, but went ahead and lined the plywood in the middle to insure the rougher-textured plywood wouldn't bond to the concrete; the counter wood is very smooth and less likely to bond, although I confess I didn't oil it or anything. I hope I didn't make a mistake in that regard. I don't have a vibrating tool. I considered using my sander, but instead chose to pack it down with a 4x6 part way through the job. There are five weep holes in the form of 1/2" dowels surrounded by 5/8" thin black irrigation hose (PVC seemed needlessly thick, which would have required a larger concrete void at the outer diameter, and also cost a little bit more). I'll knock out (or drill out, if required) the dowels, leaving the plastic behind.

[kebwi]

I'm happy to leave plastic over the curing hearth for the rest of the week. I'm aware of the recommendation to trap the humidity in for several days. But should I take the forms off at some point, or should I just leave everything as is for most of a week before taking anything apart?

[Giovanni Rossi]

My slab is cantilevered on all sides with an integrated lintel supporting the front, so I left the forms on for 3 weeks and continued the wet cure for a 4th bc. I had the time plus my slab is a lot thicker than yours. The forms popped off without any problem, but I had oiled them pretty well. If you didn't use a release agent on the wood not covered by plastic, you may not want to wait so long. A lot of the Forum members have tons of concrete experience. There's probably a sweet spot for the time that someone could help you with.

If you are worried about the forms sticking, it looks like you could remove just the boards on the edge. You could do that after 2-3 days and leave the bottom supports in place for a while. I wouldn't put a lot of weight on those edges for at least a week.