Tejays Oven Build

[Tejaycar]

JRPizza that's awesome. I had not seen that one, but very similar to what I had in mind.

[david s]

If you've not cast your supporting slab yet, consider casting in some holes through it by inserting some plugs when you lay up slab. it's far easier than drilling holes later. These are to drain accumulated moisture from the insulation under the floor.

[Tejaycar]

Progress is being made.

I have a question on the insulation board. I hear >70 PSI at 5% is the standard recommendation here on compression strength. Can anyone explain why? The calculator has my 42 inch oven using 171 bricks in the dome. At 8 lbs each, that's 1,368lbs. let's say the insulating blankets and stucco finish double that and don't self-support at all. That has 2,736lbs of total weight on the outer ring of bricks.

Calculator also shows 34 bricks in the first ring at 4.5X4.5. So that's 4.5X4.5X34 = 688.5 sq inches (ignoring any mortar).

So we have 2,736lbs resting on 688.5 sq inches which means 3.97 lbs/sq inch (also known as PSI)

Am I missing something here? That suggests anything over 4PSI compressive strength should be adequate. Even with a 4X margin of error, we're looking at 16 PSI, not 70.

Where am I going wrong? Or are we just intentionally way over-engineering for the sake of safety margins?

[david s]

Progress is being made.

I have a question on the insulation board. I hear >70 PSI at 5% is the standard recommendation here on compression strength. Can anyone explain why? The calculator has my 42 inch oven using 171 bricks in the dome. At 8 lbs each, that's 1,368lbs. let's say the insulating blankets and stucco finish double that and don't self-support at all. That has 2,736lbs of total weight on the outer ring of bricks.

Calculator also shows 34 bricks in the first ring at 4.5X4.5. So that's 4.5X4.5X34 = 688.5 sq inches (ignoring any mortar).

So we have 2,736lbs resting on 688.5 sq inches which means 3.97 lbs/sq inch (also known as PSI)

Am I missing something here? That suggests anything over 4PSI compressive strength should be adequate. Even with a 4X margin of error, we're looking at 16 PSI, not 70.

Where am I going wrong? Or are we just intentionally way over-engineering for the sake of safety margins?

In practice builders have found a 10:1 vermicrete slab to have inadequate strength as an insulating slab. Its compressive strength is around 50-70 psi (see table)
I'd expect other forms of insulation to behave in a similar manner. I've recently done repairs on two ovens that used ceramic fibre blanket under the floor bricks which was totally inadequate(don't know what was under the dome). As both ovens had issues with water getting in the wet blanket was completely compressed and sodden. For vermicrete a 5:1 mix is a preferred mix with a psi of 175-22 5

[Tejaycar]

Alright, so here are my IT designs. The idea is to take a threaded rod with two coupling nuts, one on each end. Those nuts feed into the IT Bottom (https://www.tinkercad.com/things/jpiECP5FTRt) and IT Top (https://www.tinkercad.com/things/4YvC3KHijOs). There are then four attachments that can hook into the top. The wide (https://www.tinkercad.com/things/hjmqMlXLfut) and Narrow (https://www.tinkercad.com/things/imgjE4olUdb) brackets (each centered over the rod) and the short (https://www.tinkercad.com/things/9ZgTzO1z3Nh) and long (https://www.tinkercad.com/things/hv4qVkE2XeM) scribe to hold a sharpie marker at either the inner or outer edge of the dome.

WARNING: I have not printed any of these yet, so it's entirely possible they will fail badly. Use at your own risk.

[Tejaycar]

Alright, so a few updates on my plans.

• still trying to find a local source for calsil board. At this point I'm thinking I'll probably do a layer of foamglas and a layer of calcil for 4 inches total.
• Still struggling to get much input on fire bricks. See https://community.fornobravo.com/for...ng-a-firebrick. At this point I'm leaning toward the Jet DP heavy duty brick.
• I'm also planning to put thermocouples in a few places. Obviously most of these will only be used to learn about my oven. long term, only one or two of them are likely to provide lasting value. But I'll try to do a post about the data they give me during firing and cool-down. Tentatively:
  • middle floor under insulation
  • middle floor under bricks, but on top of insulation
  • near the top of the dome, almost all the way into the oven
  • near one side of the dome, almost all the way into the oven
  • near the top of the dome, between the bricks and the insulation
  • near the top of the dome, just outside all of the insulation
  • in the chimney flue

That's about it for now. I'm frustrated to be blocked at the moment while I source materials, but I'd rather get it right.

[DrScott]

I guess I have missed some conversations about Foamglas. Is it not acceptable as the only floor insulation?

[Tejaycar]

I guess I have missed some conversations about Foamglas. Is it not acceptable as the only floor insulation?

From what I've seen Foamglas has one advantage, and a few disadvantages

1. It does not absorb water, which is excellent. CalSil does, and looses most of it's efficiency until it dries out.
2. It is a bit more expensive (at least from what I've seen and personally had quoted)
3. It is slightly less insulating. Don't let the "range" of BTU/inch*hour fool you. Foamglas has a smaller range of operation, so if you just compare the thermal conductivity range between the two, foamglas looks better. But, if you actually compare thermal conductivity at any given temperature, you'll see that calsil is slightly better
4. foamglas has a lower operating temperature than calsil. Not much lower, but with a top end around 900-1000F, you're pushing up against the temperature of an oven. Moreover, I've seen some comments that foamglas may loose compressive strength as the temperature increases. Given that it's already pushing the supposed limits on compressive strength, that matters. Now, that said, if your oven interior is 1000F, I'm guessing the temp at the bottom of the floor bricks is already cooler (maybe 900??), so maybe this is not a big deal. CalSil is 1200, so not a lot higher, but enough to count.
5. Compressive strength is lower. Foamglas is between 60 and 90 PSI from what I can find, while CalSil is 120 PSI. The forums recommend at least 70 PSI. Now, that said, if my math is any good, the total weight of the oven spread across the area of the first course of bricks comes out to something under 10 PSI, so I'm not clear why the forum members recommend such a high PSI, but given they have WAY more experience with this stuff than I do, I'm not questioning it.

So, all that taken together has led me to my plan, which I've seen others do as well.

• One 2" layer of foamglas on the bottom. That will deal with any moisture that may leak in from rain.
• One 2" layer of CalSil above that, which will now stay dry, cost slightly less than the foamglas, insulate slightly better, and help to both reduce the temperature and spread the weight before it reaches the foamglas.

I also plan to make the insulation 1" wider than the outside of the bricks to ensure as firm of support as possible. With three inches of fiber blanket around the entire oven, that will give me 2 inches around the base still, so everything will remain very well insulated.

[Tejaycar]

Ok, time for an update.
I finally bought my bricks today! 200 Smithfield unbranded fire brick (made by alsey). They are "medium duty" but based on https://www.fornobravo.com/pompeii-oven/brick-primer/ they are closer to the light-duty specs. I went back and forth, as mentioned earlier, between these and the Jet DP high duty. Ultimately I'm relying on my own cheapness (they were $2.25 each vs $3.20 each for the Jet DP) and the fact that several retailers that have "pizza oven" sections on their websites are selling that same brick as their preferred brick for ovens. So, hopefully I don't regret it.


I've also made several test prints of my IT parts. I'm still adjusting the models as they were too tight of tolerances. I'm learning that I need at least .4mm of space between pieces to get things to fit snug while still sliding together well. The links posted previously should still work, and they'll always lead to my latest models. So, hopefully in the next week or two they'll be good enough for people to order from someplace like shapeways.


Finally, I took of my forms last week, which exposed some unpleasant, but not terrible, surprises. I thought I had vibrated it well, but I still have a lot of voids, including some pretty large ones in the weakest part of my hearth (right at the opening to the oven, with no cinder block under it. It's held up only by the concrete itself and a fair bit of rebar). I also realized that I really should have put some 4 mil plastic between the OSB and the concrete. Not only did the OSB soak up a lot of water, but now I have a funny texture all over the bottom of my hearth. Eventually I'll probably grind it smooth, but for now it's a reminder I messed up. Finally, the concrete was clearly too wet. That caused some streaks down my cinderblocks (no big deal as they'll get covered by stucco eventually), but also adds to my worry that the hearth will not ultimately be strong enough. Nonetheless, what is done is done. So all I can do now is hope for the best.


Lastly (I know, lastly following an finally is dumb. sorry) I have a quick question. I see lots of praise for "homebrew" as the mortar between bricks. I think the formula is 4:1:1:1 of Sand, hydrated lime, portland, and fireclay. What's the fireclay though? Everyone local I ask points to a pre-mixed product which the forums seem to say is a very bad idea. One place has Heat block 50, but I can't tell if that's fireclay or refractory mortar. Would that work in the homebrew mix, or is that effectively what the homebrew mix is trying to replace? Can anyone share a few name brands I might use as reference?

Thanks everyone!

[DrScott]

Thank you for the detailed answer. I planned on just foamglas but I will reconsider. There is a DI place not too far away I plan to pick up the board and blanket from.

I hope the hearth is strong enough, perhaps the air is limited to the edges and can be repaired. I.planned to build my base this weekend but SWMBO has different priorities....and my daughter's pending engagement party will require those to be done....soooooooooo......more delays.

Your IT looks awesome!! Mine will look like a roadside find most likely.

To my understanding Heatstop 50 is a refractory mortar which, as you thought, the homebrew replaces. I was planning on using the commercial product just because it seems easier for me. I'm certain the homebrew will work great. For the floor brick I planned to use brick dust from cutting firebrick in half....mixed with sand, of course.