36" in Seattle, second oven

[david s]

Going back to thin brick domes for a moment. I realize that the challenge is basically building a segmental dome, the dome version of a segmental arch, in which extreme lateral stresses are put on the lowest courses, such that the primary point of failure would be having the base of the dome slide outwards and with insufficiently thick walls that the mortar itself would not adequately glue the dome together against such slide-out forces (I get the impression that few ovens actually confine a catenary to their middle third in true self-supporting form, but rather are truly relying on mortar adhesive strength to overcome a lack of true self-support; hence the prescription for thick walls and wide mortar surfaces).

This of course leads to the proposal of buttresses when one is referring to segmental arches. Buttresses aren't a topic that come up so much with domes, but I do remember seeing one or two threads in which people poured a concrete ring around the base of their oven. Obviously I couldn't pour straight concrete as a buttress to the lower courses because that would be a heat sink, but this line of thought led me to the idea of pouring a vermicrete or perlcrete buttress ring, possibly with a ring of rebar or mesh embedded in it.

It would only need to come up as high as the lowest two courses, give or take, where those lateral forces are pushing the foot of the dome outward, and the ratio could probably be somewhere between that prescribed under a floor (5:1) and that which works over a dome (upwards of 10:1). I was wondering if I could pour a 6-in thick (or whatever thickness makes sense) perlcrete ring, in the 8:1 range (a compromise of buttressing strength and heat sink resistance), from the hearth, up past the insulating subfloor, past the brick floor, and up to the top of the bottom two (give or take) brick courses, i.e., to whatever height the angles are going to push outwards, about 12" high in total. Above this buttressing wall I would then resort to blanket insulation as per usual design, on the argument that the top of the oven both doesn't need buttressing but does need greater insulation since the top portion heats up so much more aggressively.

Would this design enable a 3-in thick brick dome to survive?

I suppose another idea is to forgo the blanket and literally encase the dome in perlcrete, effectively creating a fully top-loaded dome, which of course is a classic way of protecting non-caterinary arches going back to the Romans with them perfectly semicircular arches.

This of course would have some thermal implications, either with just a lower ring or with full encasement. The perlcrete layer, combined with the 3" brick, would represent an average thicker thermal mass then the same 3" wall wrapped in blanket, but the question is whether this design enables a thinner wall that heats up faster than a 4-1/2" wall but is still structurally sound due to a buttressing ring or full encasement.

Just curious.

The Pantheon in Rome, over 2000 years old is the world's largest unreinforced dome. It relies on its structural integrity by employing a couple of tricks. Firstly it uses the more stable semi-circular arch. Secondly it has massively thick walls at the base, 6 metres if I remember correctly. Thirdly in an effort to reduce weight on the dome it gets progressively thinner as it rises as well as the mix changing. They hauled tons and tons of pumice from the Pompeii region by bullock dray, hundreds of miles, to gradually replace the heavy aggregate in the mix with the pumice as the dome rises up.All pretty clever for a couple of thousand years ago.
The problem your idea has is that the mix needs to be strong enough not to compress so it will act as a suitable buttress , but still be insulating enough not to act as a heat sink. As heat rises by convection the top of the dome will get way hotter than the base which is thicker, so will introduce a big difference in thermal expansion (problems) as well as taking way longer for the base of the dome to heat sufficiently. The other method of supporting the dome near the base that the ovens in Naples used, was to have a steel band surrounding the lower courses. Steel of course introduces corrosion problems particularly where heat and moisture will accelerate a reaction to produce rust The solution that history has settled on, is to stick with the hemisphere for brick builds (hence the popularity of the Pompeii design), while cast ovens can deviate in the designed form also incorporating thinner walls.

I don't know if your proposed design would work, but I can bet it's been tried before and since you don't see any like that, there's a good chance it won't work. I'd like to see you give it a go so we could see the results. Table of strength:insulating value attached.

[kebwi]

Yes, I was aware of the metal band solution too. I saw a few (or perhaps one?) builds on the forum where someone had done something like that. I was considering something similar and then struck on the perlcrete buttress idea. And then I had another idea. Instead of a steel band (which I'm not sure I have the tools to shape), I considered make a two or three latitudinal circular chains, precisely the right circumference to rest just outside the lowest courses. The chain rings would then be held down (since the dome shape would allow them to "slide upwards") with longitudinal chain "legs" anchoring them to a few eye-bolts embedded in the hearth right next to the insulation layers under the brick floor. Picture a sort of chain spider web skirt around the lower portion of the oven, holding the lower courses in place. The bolts in the hearth would, of course, be a heath sink but the chains themselves would be pretty inefficient thermal conductors because chains have so little surface contact along their length. I mean, obviously some heat would transfer to the hearth, but maybe it wouldn't be very much -- or maybe it would. I just couldn't convince myself that such a design would be tight enough to hold the bricks in place enough to actually make any difference, from a structural support perspective, so I went off in the perlcrete buttress direction instead.

I think I should make a cast oven. I'm just frustrated that I gained good skills on my brick oven in 2009 and can't reused those skills this time around -- and I prefer the look of a brick oven (which is a ridiculous argument I realize).

I do want to make a lower, flatter oven than most I see on the forum, something more pizza oriented (tall enough fit bread an even turkeys in, but that doesn't require 16"). I'm surprised that the cast builds are often so hemispherical, not taking advantage of the fact that cast ovens can achieve a more Naples style shape than brick. Seems like a missed opportunity -- or perhaps it's better to make a more general purpose oven that will fit larger items and cook perfectly good pizzas in it anyway, instead of building an oven that only fits pizzas and very short items otherwise in a needlessly restrictive manner.

[kebwi]

Here's a new strange idea that popped into my head. I do seem to think outside the box on these kinds of things.

AR glass
I'm trying to come up the speed on the various structural mixins. I read about AR a little bit at first not even sure what it meant. I now realize it stands for alkaline resistance. And since it's usually in reference to glass, I've discovered it refers to alkaline resistant fiberglass, one can deduce that this is some form of fiberglass that isn't harmed by the high alkaline environment of concrete. While I generally associate fiberglass with insulation, it is my understanding that in this application they have no insulating purpose to serve (what would be the point of desiring insulation throughout a thermal mass product?), but rather are a physical matrix for the concrete to adhere to, to physically hold the concrete together against cracking, or potentially having whole sections fall out of the ceiling of the oven, so I get that. Also it looks like they start to fail at the upper end of pizza oven temperatures, a little over a1000F. I'm not sure what failure means in this context. Do they just not work to hold the concrete together, or do they actually disintegrate or suffer permanent damage if you run your oven too hot?

SS needles
Then there's the stainless steel needles, which is far as I can tell serve of the same purpose as the AR glass fibers, a form of skeletal substrate for the concrete materials to adhere to, but in a fashion less likely to suffer differential thermal expansion in the way that larger metallic support would, such as rebar or wire mesh, owing primarily to the needles' small cross-section which enables rapid heat dissipation and equalization. Since these serve the same purpose as the AR glass fibers, to my current early understanding which may be completely incorrect, I'm unsure whether any given oven project uses both. Is there a reason to use both in a single build? Is the goal of the stainless steel needles simply to offer support at the temperatures that the fiberglass starts to fail?

Burnout fibers
And I started reading about the burnout fibers and now understand the whole point of them. They are nylon if I read correctly and are deliberately intended to essentially vaporize or disintegrate as you heat up the oven the first few times, leaving a random latticework of microscopic empty tubes throughout the cast material, the purpose of which is to give steam a path to escape without cracking the material. It isn't entirely clear to me where the burnout fibers burn away too. If they are trapped within the concrete, it's not like they can vaporize away into the air. Don't they simply turn to ash within their confined space within the concrete? How do they then leave voids that steam could travel along? Yes I see descriptions of the fibers poking out of the surface after completion, so those few fibers that happen to protrude from the surface would leave escape paths, but surely that would be a minuscule fraction of a total number of fibers introduced to the mix, and it would only offer channels to the outside from sections of the concrete near the outside, namely within reach of the short and likely curled up fibers so it would not offer a channel to the outside from the inner regions of the cast. This seems like a very inefficient way to achieve this goal...which leads me to my weird proposal below.

So that's three mixins so far. I suspect I've overlooked others that I would remember if I combed through various build archives. If I've overlooked any crucial mixins that are a fundamental part of cast ovens, then I need to figure that out quickly before I make some sort of mistake in materials obtainment.

All of this leads to today's strange idea and proposal.
Since the purpose of the burnout fibers is to create voided tubes throughout the concrete for steam to travel along, why not take your cast wall, immediately after packing it against the form, while it is still malleable and workable, and create countless tiny voids from the outside deep into the wall, by smacking or pressing some sort of brush against the walls, like a stiff steel brush of some sort. There might be some discussion to have about the best bristle diameter and spacing density of such a brush for this purpose. I don't know if It would be a better idea to use a brush with a dense arrangement of fine bristles, a looser arrangement of fine bristles, a looser arrangement of fatter bristles, or finally a looser arrangement of fine bristles. There is of course the concern that the concrete will settle back against the holes and basically fill them in, especially on the sides of the oven where gravity would work against this goal, but perhaps less so toward the apex were gravity would work along the length of the holes. Some hair brushes, instead of having a dense array of hundreds of fine bristles, as we usually picture hair brushes, and also essentially true of grill brushes and other steel and brass brushes, rather have a rather sparse array of stiff large plastic bristles. You can probably picture what I'm describing, they aren't that uncommon after all. One advantage this might have over burnout fibers is that the channels it creates all point in exactly the optimal direction, radially straight out of the wall so the steam can get the heck out of the wall as efficiently as possible instead of merely meandering around trapped inside the wall. Furthermore, by poking the holes from the outside, they are of course open-ended instead of trapped inside the concrete. I can even imagine this might benefit from a heterogeneous design. The lower walls may be more prone to gravitational settling across the cross section of these holes, which would compact them, and may benefit from larger more widely spaced holes, while toward the apex, the gravitational vector would fall more along the lengths of the holes and so would not necessarily crush them sideways and might therefore withstand finer and more closely spaced holes... If that even serves an advantage perhaps. After all, maybe the same wider arrangement of larger holes on the lower wall would work just as well at the top anyway and no heterogeneous design would be required. Such details are all open for discussion, but my question at this early point is simply whether this idea makes even the slightest bit of sense. For example, is the benefit of burnout fibers crucial at an extremely fine density throughout the clay, perhaps hundreds or thousands per inch. If voids of this nature are more widely spaced, has this brush technique would achieve, is that insufficient to allow the steam to work its way through the clay to such a void before the clay cracks? In other words, does the role of burnout fibers have to be essentially a microscopic structure of the concrete in order to actually serve its purpose? If so, then my proposal is completely off the table.

Thoughts?

[david s]

You are pretty much spot on with your understanding re the fibres.

AR glass: They are coated with zirconium to give the required protection. It is an expensive material so be wary of cheaper prices which could be a cheaper, thinner coating, like some cheaper cars paintwork. Best to just suck up the high price and access a reputable dealer rather than go for cheaper stuff on the net. Being glass their melting point is around double that which they'll see in an oven. I've tested this in my kiln and they begin to melt at around 900C, so quite suitable for an oven but not for a kiln. They are widely used by the concrete counter top builders so readily available. The best thing about them to use is that they are softer to handle than the stainless needles, a good length to provide strength and disperse readily in the mix.

SS needles: If looking for them they're actual name is melt extract fibres. These are the industry standard for reinforcing refractory concretes. Because of their small size they have a large surface area to dissipate their heat to the refractory that surrounds them, unlike heavy rebar which will create thermal expansion issues. Being stainless gets around the rusting issue, but don't think stainless can't rust, it does, especially in environments where heat and moisture accelerate reactions, but way slower than mild steel. They're not called needles for nothing and create difficulties in placing the mix as well as sometimes leaving some sticking out of the edges of casts.I think this is the main reason most manufacturers don't use them as injuries are common. I usually get stuck at least once every time I use them.

Burnout fibres: These were originally developed to provide some strength to wet concrete and to reduce slump and shrinkage cracking. They do not provide strength to cured concrete. Being so fine they do not reduce the density of concretes so do not weaken the casts in the way perlite and vermiculite will.. Someone years ago decided to use them in concrete to provide a measure of fire protection. Or maybe they just discovered that by accident. If fire passes through a building expanding steam in the concrete wall has a tendency to create stem spalling, but if the concrete contains the fibres the steam is released. If the fire is really extreme the portland cement will die and the concrete is ruined, but in many cases the building structure is left intact. Being polypropylene mostly, although other materials have also been used, the fibres melt at 160C which is not far above the boiling point of water. I've tested these in an oven and yes they melt at almost exactly 160C. At that temperature they will be sucked into the refractory that surrounds them, then at a higher temperature simply burn away leaving a matrix of very fine pipes through which water vapour can pass. Regarding using them the greatest difficulty is their size because the smaller the fibre the greater the difficulty in getting it to disperse evenly into the mix.They tend to clump together. As a general rule they require around double the mixing time that you would normally use. I always pull them apart when adding them.

Other fibres: I also use basalt fibres which are around the same price as the AR fibres, but have a range of sizes in the supplied batch. They also are difficult to disperse, but provide slightly higher strength than the AR fibres below 400C, although slightly lower strength above 400C. The others I've been experimenting with are nano fibres. I've tried carbon nanotubes and graphene nano fibre additions, but there are problems with even dispersion and as you can't see them it is not possible to tell how well mixed in they are.

I don't think your idea of perforating the surface of the casting will work too well because you'd need to get the holes deep into the cast rather than just the surface. The other problem is that all those holes will weaken the casting, but you're thinking along the right lines.

[kebwi]

Other than AR glass, SS needles, and burnout, and then other than the weirder things you personally use, but which most other DIYers aren't using, such as basalt and graphene, have I overlooked any other structural or functional mixins to the concrete? I still don't feel that I've kept track of all the various things that are mixed into the cast concrete.

[kebwi]

I realize standard burnout fibers are extremely fine and produce microscopic pipes and tubes, but what would happen if you produced larger tubes? For one thing, I was thinking about breaking up angel hair pasta and mixing it in.

Alternatively, I was wondering why you couldn't shred essentially any form of conventional textile wool, or even just some sort of chopped up thread, into the mix. What about ordinary shredded wool or cotton balls or quilt matting or anything like that. Or perhaps you could buy paint brushes of some kind and chop the ends off at a chosen length until there's nothing left. Paint brushes come in a wide variety of materials, some natural, some synthetic. Although paint brushess can be expensive, they can also be extremely cheap and the point here is for something simply burns up. I would think cheaper would be fine here. Anything like that would obviously vaporize to ash at oven temperatures. Why do you need these special refractory substances for this task?

[kebwi]

I found a local supplier of FiberMesh 150 and 300. Its description online is obviously that of burnout fibers. It is intentionally supposed to vaporize in a fire to leave the structure behind that relieves steam pressure. Do you have a recommendation between the two sizes, 150 being the finer, smaller width of the two?

They also have Novomesh 850, which is a mix of poly and steel fibers. Any knowledge or thoughts about that?

[david s]

I realize standard burnout fibers are extremely fine and produce microscopic pipes and tubes, but what would happen if you produced larger tubes? For one thing, I was thinking about breaking up angel hair pasta and mixing it in.

Alternatively, I was wondering why you couldn't shred essentially any form of conventional textile wool, or even just some sort of chopped up thread, into the mix. What about ordinary shredded wool or cotton balls or quilt matting or anything like that. Or perhaps you could buy paint brushes of some kind and chop the ends off at a chosen length until there's nothing left. Paint brushes come in a wide variety of materials, some natural, some synthetic. Although paint brushess can be expensive, they can also be extremely cheap and the point here is for something simply burns up. I would think cheaper would be fine here. Anything like that would obviously vaporize to ash at oven temperatures. Why do you need these special refractory substances for this task?

You are certainly thinking along the right lines. The reason I use them is because they work. Any alternative fibre should be tested. You can do this easily in a conventional oven. I’ve tested larger polypropylene fibres using cut strands of poly rope and while they do melt being a larger diameter they melt at a higher temperature than the really fine ones. My experience with firing pottery demonstrates that steam spalling occurs at around 250-280C where sudden steam expansion within clay walls is sufficient to blow the pottery apart from the middle of the wall in the wares. Thicker sculptural works are more vulnerable and require more drying and more careful firing. Since an oven wall is some 2” thick it falls into this category.
The larger the fibre diameter the greater the volume of air left behind in the casting. This reduction in density results in a weaker casting so the thinner the fibre the better. Human or animal hair is another alternative, but if testing it in your kitchen oven make sure you do it when your wire is not home. Burning hair really stinks.

It's the Sika Fibermesh 150 you want. The ones for reducing steam spalling, shrinkage and slump crack resistance. The 300 fibres are an order of magnitude thicker, graded and are for strength enhancement.
The Novomesh 850 use the larger graded poly fibres combined with mild steel fibres. Ok for strengthening standard concrete, but neither fibres suited to refractory. The pony fibres too big and the steel fibres not stainless.

[kebwi]

So a brick oven is, by volume and surface area, primarily brick with a little bit of mortar. And a cast oven is -- in effect -- all mortar (or concrete, but follow along with me). One can imagine a spectrum between those two extremes. In fact, the furthest extreme in the brick direction would be a dry stack with no joints at all. Ok, so we have this spectrum, where, moving from the brick end to the cast end, the mortar joints get bigger and bigger and the bricks shrink in size (at least on the two surface area dimensions, although perhaps maintaining their size in the radial wall thickness dimension). So, for example, we can envision a dome at the halfway point along this spectrum where the brick width and the "mortar gap" width are the same, such that each brick is separated from its neighbors by a brick width's worth of mortar. But we can envision any point along this spectrum. For example, the mortar gap could be half or a quarter the width of a brick, just really wide mortar gaps in an otherwise brick oven. Or the mortar could be even wider, such that the dome is essentially a cast oven but has isolated narrow bricks polka-dotting it at interspersed intervals. The bricks could be perfect rectangular solids, but they could also be very slightly wedge-shaped to alleviate the fear of falling through...and of course they needn't be square or rectangular as viewed radially. Since they are isolated from one another they could be literally any shape at all on the two surface area dimensions.

Since all of this refractory material is more or less the same composition, I presume differential thermal expansion would not be a huge problem.

My question is -- wildly speculatively pondering -- would there be any benefit or point to building a dome in this fashion at any point along the spectrum? Sure, the "wide mortar" end might benefit from fibers and other support like that, but I'm just wondering if it could be useful in some fashion. Would peppering a primarily cast oven with evenly distributed, but still not touching, individual bricks, serve any purpose whatsoever? It would obviously be a piece of cake to actually build. You could just include bricks into the cast wall as you build it up and over to the top. I'm just not sure there's any point, other than the novelty of it.

[david s]

I have often pondered the same idea. My thinking would be using clay pavers or one inch thick firebrick tiles laid flat against a sand dome mould, with the pavers having chamfered edges (easy job with a wet saw) then a castable refractory (or homebrew) worked over the surface to increase the wall thickness for desired strength and thermal mass.
The only way to test this idea is to build one and see how it stands the test of time.
there could be problems of the two layers wanting to de-laminate, but I would lean towards it working pretty well. golf pro is using the same technique in his build using full thickness bricks. Cleaning up the inside surface after sand removal would be a pain.
The large restaurant oven I recently built(Well known Italian manufacturer) used large 8mm graded firebrick aggregate in their castings.(I use max 5mm, my proprietary castable refractory uses 3mm graded high temp aggregate.
Whatever you settle on the addition of the burnout fibres is prudent even for homebrew mortar that fills in the large wedge shaped gaps in a conventional Pompeii build.

[kebwi]

I can't search the forum effectively for "golf pro". What is the exact member name? I'd love to see it.

[david s]

I can't search the forum effectively for "golf pro". What is the exact member name? I'd love to see it.

He hasn't updated his thread for a while (I've been PM ing him) but he's progressing well.
Start here
https://community.fornobravo.com/for...ild#post452416

[kebwi]

Again, I acknowledge that the answers to my questions might be buried on page 13 of various individual build threads, but it's easier to just ask again because this information hasn't been consolidated anywhere (no criticism intended; I just feel bad pestering you with questions that have surely been asking hundreds of times before). So, question: How much AR fiber, poly fiber, and SS needles do I need? Obviously, this is a function of oven size. I'm currently leaning toward a 30" design, although the next post presents my concerns about oven size considerations.

I think the right way to do this is:

1. Calculate cast volume
2. Convert volume to weight (since fiber additions are done by weight)
3. Calculate proportional weight of necessary mixins

Is that the approach everyone else takes? Estimate the concrete volume of the cast using spherical volume subtraction isn't quite right for me since I would like a lower, Naples-like, profile, so a full hemispherical calculation using the floor radius would over-estimate and using the Naples ceiling height would under-estimate, but I could start with that. So, is the trick to calculate the volume of the casting, convert the volume to weight (since fiber additions are done by weight) and then calculate the corresponding ratio relative to the presumed weight of casting?

A 30" hemisphere comes out to 1.9 cubic feet of concrete, disregarding the absence of concrete in the arch entryway and the addition of extra concrete to build the arches and gallery:

OD_volume_"^3 - ID_volume_"^3 = shell_volume_"^3 / 2 = half_shell_volume_"^3 / 1728in-per-cuft = half_shell_volume_'^3
20580"^3 - 14137"^3 = 6443"^3 / 2 = 3222"^3 / 1728in-per-ft = 1.9'^3

So, a 30" oven uses around two cubic feet of concrete, assuming the missing entry section and the added gallery section approximately offset one another.

Concrete tends to come in 60lb and 80lb bags, which fill .45 and .6 cubic feet each. Therefore, 2 cubic feet is 267lbs of concrete (an overestimate for an oven with a lowered ceiling).

Each mixin (AR fiber, poly burnout fiber, SS needles) has some prescribed ratio by weight, somewhere in the couple-of-percent range. I don't recall off the top of my head what the prescriptions are for each of those three mixins. I need to come up to speed on that, but I think one of the three was supposed to be added at 1%-2%. So, continuing the calculations above, that would imply I need 2.7-5.3 pounds of that mixin...and so on for each of the three mixin types.

Is that how one is supposed to calculate the amount of mixins needed?

This all leads to my next question however, for which I will start a new post...

[kebwi]

I'm leaning toward a 30" oven at the moment. My first oven was 36" and I started this project thinking I would replicate it (in fact, when I originated this second oven as a brick design, I intended to perfectly replicate my first oven's brick layout except to remove one of the two stretcher courses at the bottom to drop the 16" ceiling down to 13.5" in the hopes of a more Naples-style oven). But now that I'm thinking about a cast oven, I'm also thinking about shrinking the whole thing down to 30".

However, the problem with going smaller in diameter is that any desire for a Naples-style profile, which would involve a height that is substantially lower than the radius, becomes problematic. A fully hemispherical 30" oven would have a 15" ceiling -- but my previous 36" oven, which had a very slight vertical drop (18" radius but 16" height) was still 1" taller than a perfectly hemispherical 30" oven with a 15" ceiling.

And I don't want a hemispherical oven anyway, or even the very slight diminishment of Pompeii designs, like my previous 18"R/16"H oven. I want something notably lower and flatter than my first oven, a pizza oven. But at a 30" diameter, this preference implies an oven in the 10"-12" height range. I don't want to go any lower than that so I at least have the have the option of using the oven for the occasional bird (I don't see how a low height impacts making bread since bread wouldn't have trouble fitting into such heights anyway, unless being too close to the ceiling singes the top of bread in problematic ways).

BUT wait! Oh my goodness, the hallowed .63 arch ratio kills this design. Even if I push the height up to 12" (not really the low ratio I want anyway), the entry ratio then prescribes a 7.5" arch height. Even if I round that up to 8", that's hopeless for getting anything other than pizza and bread in and out. It's almost as if Naples-style ovens have to have nearly full-height entries since they have already lost all the height they can afford in the oven ceiling's profile.

What am I misunderstanding here? Do people simply never use lower-dome pizza ovens for any other purpose (maybe some bread)? Or put differently, do home-DIYers tend toward taller Pompeii designs, not only for the easier structural support of a rounder dome, but also to obtain a more general-purpose oven that can be used for taller items, and then just hope and assume they'll get perfectly good pizza out of such taller, rounder ovens anyway -- which obviously is the case; they make fine pizza, implying this is a bit of a nonissue: The solution is simply to abandon a Naples oven, especially at smaller floor diameters, and go ahead and build a tall, round oven that can be used for a wider range of cooking? BUT, pulling this thread a little farther, if Pompeii profiles are perfectly good pizza ovens anyway (of course they are I concede) then what benefit do Naples ovens offer in the first place? I thought low, flat ovens are supposed to be better pizza ovens.

By shrinking my intended oven diameter from 36" to 30", I have backed myself into a corner with my inclination to also build a lower, flatter ceiling. These seem like mutually exclusive design goals.

Unrelatedly, I wish anyone other than david would participate in this thread (thanks for a few others I admit, but my point stands). This question isn't about cast oven building at all. It's about design and personal project goals. I wish I was allowed to start new threads for questions that fit into very different topics, with a relevant subject line for those question threads, so each thread would be categorized (and searchable and attention-grabbing in the thread-list) by its relevant subject line. I don't approve of the new way the forum has been organized frankly.

[david s]

Just work out the volume of the hemisphere of your desired internal diameter and dome thickness. Although I the form you want may deviate from a hemisphere the difference will be so slight that it doesn’t matter much. The extra required to make the inner oven mout is about the same as the void that is made for the mouth itself, so just calculate on a full hemisphere without an oven opening.
Regarding a calculation of materials required, someone did do a calculation using volume, densities and mass, but as the homebrew is so cheap it’s hardly wort doing. Sorry I can’t remember who it was, but it’s on one of the cast build threads.
regarding the fibre addition, without delving into my tech data I can’t remember the addition by weight because it’s far simpler to work on volume once you’ve calculated the amount by weight. For the AR fibres add around 500 ml for every 10L wet castable, or 500ml of ss needles, or a combination. For the burnout pp fibres you need way less because being so fine they distribute more widely, only around 250ml, but they must be thoroughly dispersed. Because it’s so fast and cheap you’ll have the whole thing done in a couple of weeks rather than the months it takes for a brick build, although you do need to take extra time damp curing, drying and slow water elimination, of which You would be aware. Apart from the blanket and floor brick floor, it costs peanuts.
Most cast manufacturer’s designs,FB included produce ovens with a low dome and high walls at the base. This is something that’s really easy to do using the sand castle method.