40" pompeii, first time builder

[totalnewbie]

Hey all. Here's an interesting question. I've read many posts about 3:1:1:1 and how the lime and fireclay come into play to hold the material together when the Portland starts to break down at higher temperatures. That all makes sense.

Has anyone tried using standard mason's mortar from Lowes or Home Depot and then added fireclay. That seems like a pretty convenient and inexpensive way of getting home brew. What do you think?

For people who've worked with home-brew on a sand mold, does it feel and work like standard mason's mortar? I'm starting to wrap my mind around the idea of packing material around the mold.

[david s]

That is a possibility, but you won’t know what the proportions of lime and cement are in their mix. The homebrew is probably far richer in both cement and lime than a commercial mortar. Most mortars are designed to be weaker than the brick or block units they will hold together. The homebrew contains a high proportion of lime, clay and cement so if one fails the next one takes over. The proportion of lime, cement and clay in the 3:1:1:1 would be around 17% of each. The standard mortar mix is 4:1 which is 20% cement. I have no idea how much lime is added to those mortars containing lime, but the reason lime is added is to create some more workability and only a small amount is required to provide that result, but it's unlikely that a total of combined cementious material (cement and lime) would exceed 20%I. f you can find out the ratios of the ingredients of the commercial mortars, that contain lime, then you might have a chance of adjusting it. Probably far easier to start from scratch and simply follow the recipe by volume.

[totalnewbie]

Thanks David. That makes sense. I was hoping to confirm that it's best to cast the dome on top of the firebrick floor, not on the insulation. What is the logic there?

[WaterDog]

totalnewbie going back to your material volume calculations from post #48, you did make some errors that I think should be cleared up. As it turns out due to the nature of the mistake, the end result won’t vary too much.

First your inside diameter calc is fine except you’re building a hemisphere so you use 1/2 of that result. Second, since it looks like you’re building 2” thick, the outer diameter will be 40” not 38” as shown. Again, take half and subtract the inner volume. It gets you a hair over 2 c.f. Which is about where you landed with the roundup but that doesn’t account for any extra and may not be enough when you factor in your chimney gallery.

Save yourself some mental gymnastics next time https://www.omnicalculator.com/math/sphere-volume

Looking forward to your progress!

[david s]

totalnewbie going back to your material volume calculations from post #48, you did make some errors that I think should be cleared up. As it turns out due to the nature of the mistake, the end result won’t vary too much.

First your inside diameter calc is fine except you’re building a hemisphere so you use 1/2 of that result. Second, since it looks like you’re building 2” thick, the outer diameter will be 40” not 38” as shown. Again, take half and subtract the inner volume. It gets you a hair over 2 c.f. Which is about where you landed with the roundup but that doesn’t account for any extra and may not be enough when you factor in your chimney gallery.

Save yourself some mental gymnastics next time https://www.omnicalculator.com/math/sphere-volume

Looking forward to your progress!

Yes, you are correct. I did the calculation in metric which is far easier, but as you pointed out the result is much the same because the incorrect calculations cancelled each other out. The volume taken up by the oven mouth is usually almost exactly the same as the extra volume required around the oven mouth so the calculation of total oven volume casting can be assumed to be that of the volume of a hemisphere. The OP has included a separate calculation for the flue gallery. This varies according to the size and its thickness, so an estimate is appropriate.

[david s]

Thanks David. That makes sense. I was hoping to confirm that it's best to cast the dome on top of the firebrick floor, not on the insulation. What is the logic there?

This question pops up from time to time and the answer is that once the oven is built it really doesn't matter much because the interior form, wall and floor thickness are still the same.
Although there are some differences. If building on top of the floor (left drawing), you can cut the perimeter pretty roughly because the edges will be covered in insulation anyway. This can save a lot of time, especially if you don't have access to a wet saw, which you may not as you intend to cast the dome.
If casting around the floor (right drawing) you have the advantage of being able to remove a floor brick, should it need replacing, at the floor perimeter.
Balancing the two, I'd go for time saved trimming the brick edges because any floor bricks that require replacing, probably never and then only by your grandchildren, will always be in the middle of the floor.

Others may disagree.

[totalnewbie]

Excellent analysis as always!

[totalnewbie]

I have not completely given up on CalSil board (yet!). I have a vendor asking me if I want 150kg/m3 or 300kg/m3 density. I know there is also a choice of density for the blanket.

150kg/m3 is less expensive. Should I spend more and get 300kg/m3? Will I be able to tell?

[totalnewbie]

Well, that went away as fast as it came. The shipping was crazy.

Next question! I've started building my mold! I'm debating with myself between a 9x18 opening and a 10x18 opening. 9x18 is more authentic Pizza Napoletana and 10x18 is more practical. You can slide taller things in.

Any thoughts on that?

[david s]

The larger the opening the easier it is to work the oven, but it comes at a cost of greater heat loss, which in turn also increases fuel consumption. Both of these disadvantages are marginal, but should be considered. Oven access can also be dramatically improved by making the entry as shallow as possible. This does not affect fuel consumption and has the advantage of reducing thermal mass of the gallery thereby reducing its heat sink effect.

[totalnewbie]

This is one of those 6 of one, half a dozen of the other. It's going to be easier to build a hemisphere (9x18) than an ellipse (10x18), but will that extra inch come in handy?

[totalnewbie]

totalnewbie going back to your material volume calculations from post #48, you did make some errors that I think should be cleared up. As it turns out due to the nature of the mistake, the end result won’t vary too much.

First your inside diameter calc is fine except you’re building a hemisphere so you use 1/2 of that result. Second, since it looks like you’re building 2” thick, the outer diameter will be 40” not 38” as shown. Again, take half and subtract the inner volume. It gets you a hair over 2 c.f. Which is about where you landed with the roundup but that doesn’t account for any extra and may not be enough when you factor in your chimney gallery.

Save yourself some mental gymnastics next time https://www.omnicalculator.com/math/sphere-volume

Looking forward to your progress!

I just caught this. I was measuring on internal dimension, not external! Stupid. And thanks for catching that.

[totalnewbie]

I just realized I don't know how thick to allow for stucco. That matters because I realize the hearth size (built) and the dome internal size depends on how thick each layer is. Including the stucco,

Do I allow for 1" or 2"? I am planning on HomeDepot stucco and chicken wire directly on top of insulating blanket.

Thanks!

[rsandler]

I'd allow for extra room; I found on both my ovens that the insulation ended up being a bit bigger than I'd planned, and was crammed to some extent on the stucco/enclosure. On the second I sort of planned for it to not go to plan, and had enough room in the end, and was glad of it.

[totalnewbie]

Hey all. I'm ready to take the plunge and start casting. I have built the stand, the vericmulite concrete layer and the brick floor (dome on top). I have all the raw materials. Though I am still looking for the right fibers. Based on David's recommendations I am going with thin poly burnout fibers (handful per mixing trough) and AR fiberglass fibers (2%). Hoping to find them on Amazon -- I should have ordered them a long time ago.

I do have a question. There are a lot of posts on the right mix ratios and I'm not sure what the state of the art is. What is the best ratio?

5:1:1:1 (fine masons's sand, Portland, fireclay, hydrated lime)
5:1:1:0.5 (fine masons's sand, Portland, fireclay, hydrated lime)
3:1:1:1 (fine masons's sand, Portland, fireclay, hydrated lime)

I'm really going to do this.