Castable Refractory Products (VT600)

[david s]

This is normal because the firebricks often vary a little in size. You can use sand, but a 50/50 dry mix of sand and powdered clay is better.Some builders use it wet, but that makes removal much harder and the stuff will set so fast placement is difficult.

[TARibs]

So I was placing the firebricks and I have some raised edges on a few, I know I can grind them down but was wondering is there any issue in bedding the bricks in some sand to level them off?

Thanks
Tom

[david s]

Your calculations sound about right. The burn out fibres you need are polypropylene not PVA. You need way less than you’d think. For every 20 litres of dry castable a handful is plenty, but make sure they’re well mixed in the take a while to disperse completely.
Regarding your question re firebricks, it’s not easy to tell. Firebricks come in many different colours but are usually a creamy colour and not completely vitrified. They will not have a glossy appearance and should be more porous than a normal house brick. If out of a fireplace you’d expect them to be true firebrick.

[TARibs]

OK I think I have my calculations done for the amount of castable I'll need. Maybe someone can Check them?

Basing on a 32 inch dome. Calculating volume of sphere that's 4/3 Pi R Cubed. 4/3 x Pi (16 cubed) that gives me 17157 divide by 2 for half a sphere is 8578 Outer sphere is 4/3 x Pi (18 cubed) = 24429/2 = 12215
12215-8578 = 3637 inches cubed
Convert to Litres 3636/61.024 = 59.58 litres
Saw Dave S say 55lb bag coverts to 16.37 litres but my bags are 50 lbs which makes it 14.88 litres then reduce by 15% when mixed gives me 12.65 litres
59.58/12.65 = 4.7 bags Add the flue area and I need at least 5 bags maybe 6 to be safe

Next question is the stainless needles and PVA fibers how are those calculated based on these quantities. I thought I saw needles were 3% by volume so 5 bags x 50 = 250 x .03 would be 7 pounds of needles?
I have not been able to find a % for the fibers, any suggestions.

3:10 was what I saw you had posted somewhere else David so that was what I used on the Vermicrete.

Thanks
Tom

[david s]

The usual ratio of water to vermiculite is3:10 by volume, but if you are using the finer grade it requires more water. Troweling the surface brings more cement to the top to produce a harder surface. Too much water in the mix washes the cement off the grains resulting in an inconsistent mix.I find some powdered clay added to the mix makes it more workable. Too late for that now so trowels skim coat on the edges to protect them. You do want the stuff to dry out though so plenty of sun and wind on it for the time being is good. Keep the rain off it though.
Probably not a good idea to have exposed blanket in the entry. Weak vermicrete mix or ceramic rope would be a better choice or seal over the exposed blanket somehow.
Dome height to diameter is usually 2:1 (hemisphere) but some folk like it lower (Neopolitan dome) where the roof is closer to the food, for faster cooking pizzas. The ratio of oven door height to internal dome height is not unsurprisingly, the Golden Mean (1:1.618) but 2 or 3% higher or lower will make no difference.

[TARibs]

Thanks again Dave, those are great references.
I've placed the insulating Vermicrete now and am pretty happy with it, I'm a bit concerned that the corners are a bit crumbly so am planning to skim coat it with a Mortar layer around the edges, do you see any issue with that? Is it normal to be crumbly? I'm concerned I may not have used enough water and the mix is off? Right now I have it covered as it is supposed to rain today. Going to attach a picture now that I have all the forms off

If I understand correctly the thermal break is either blanket material or Vermicrete between the actual Dome and the Flue gallery?

Is there any type of height to diameter ration I should be looking for with the dome?

I also pulled a bunch of old bricks from an old fireplace, they certainly look like fire bricks but is there any way to tell for sure? Guess next step is to start cutting these.

thanks
Tom

[david s]

Tom,

Here's another build, using home-brew instead of castable refractory though.
#1
You can use just vermicrete for the insulation, but at least one layer of blanket is better as it also acts as an expansion joint. There is a problem of wet vermicrete swelling when it gets hot. A layer of blanket underneath it reduces this problem. You can stucco straight over the blanket, but it is more difficult because it is not a firm surface to work against. A vermicrete layer over the blanket results in an insulate layer that is also hard and rough enough to provide a good substrate to work against. In addition it allows you to get the form back to a nice hemisphere rather than the lumpiness of the blanket.
Here's an explanation of how I managed to reduce the depth of the entryway
#99
The thermal break is an effort to reduce conductivity losses from the dome to the flue gallery, but a small gap does little to prevent loss of heat by radiation. You wouldn't expect 1/2" of insulation over theme or under the floor to be of much use in reducing heat loss. It's more important function IMO is to act as an expansion joint, preventing stress on the cooler outer parts of the oven so they don't get cracked by the expanding inner over pieces. In my design I insulate both the oven chamber and flue gallery and have the expansion joint between the flue gallery and the outer decorative arch.
The vermicrete either under or over the dome needs to be sealed from the elements or it can absorb moisture.

Dave

[TARibs]

So this is the basic design I'm trying to follow as this post seems to have the best information on a complete build https://community.fornobravo.com/for...sted-over-sand, if anyone has another couple to suggest that would be helpful.

Right now I'm thinking 32-36 inch oven, 2 inches thick cast, with either a Thermal blanket and vermicrete cover or just the Vermicrete ( still trying to find references for the suggested thickness or these. If I go 32 (+2 inch cast x2) +(4 inches Vermicrete x2) that equals 42" and that leaves me about 8 inches for entryway in the front. If I eliminate the Vermicrete and instead use say 2" a blanket (I assume I then need a stucco layer over this?) I might be able to increase the size of the oven a tad. 36 + (2x2) + 2x2) = 44 so 6 inch entryway
I still have a few questions

Regarding the dome vs the entryway/door entrance and flue. There is discussion of a Thermal break, What is the purpose of this, I assume its between the two but how does one actually make it, Is some material placed between the dome and the entryway?
Are there suggested dimensions anywhere for the entryway as in how far should it extend (with a 6 inch flue it obviously has to be at least 6 inches, and how tall it should be?
For the Flue I see Formo bravo sells a transition to a stainless flue pipe, what would be a cheaper alternative?

Other questions: Is the Vermicrete under and perhaps over the cast dome waterproof or does it have to be protected from the elements in some way after the build (ie a stucco layer?)

Need to get me some Portland Cement now so I can get that Vermicrete mixed up.
thanks again

Tom

[david s]

The 5:1 vermicrete ratio is by volume, so for 5 buckets, cups or drums, or whatever, add one of cement.
Those needles are the correct material. Also the price is very good. I pay, after freight around 3x that. The burnout fibres are actually designed for another purpose. They are the very fine polypropylene fibres used in concrete reinforcing and are sold as a two part pack system, but they melt at 160C, so work quite well. They do need extended mixing to disperse properly though.I use Novomesh 950 but there are other brands. Just use the fine ones not the larger ones.

[TARibs]

Hi Again,

So the second 4 CF bag came today, How exactly does one measure out a CF of Portland cement? 5 gallon bucket? Also I've been trying to find a source for SS needles and Burnout fibres They are two different things correct? I found these Needles and from what I can see they are relatively inexpensive Are they the correct thing? http://www.newhorizonstore.com/Produ...l-needles.aspx As for the fibres could someone show me an example if not a link to a source? Thanks Again

[david s]

Sorry, what I should have said was get another 4 cu ft bag as one won’t be enough. The chart does not take into consideration the loss of volume on mixing. It is always better to have a little more than you require as it’s a pain not having quite enough.

[TARibs]

Thanks David I was going from this chart I found posted here on the site https://community.fornobravo.com/for...ratio-question 4x4x.33 =5.33 CF based on that chart I'd need maybe 6CF of vermiculite unless I'm reading it wrong or it doesn't take into consideration the loss in volume but the last column seems to give the yield? The top slab in the pictures I posted is well reinforced to support the oven.

[david s]

Vermicrete does not have sufficient strength to support a heavy oven. You must have a strong reinforced concrete slab on top of which you cast another vermicrete slab. The recommenced ratio is 5:1 vermiculite to cement and 4” thick. Vermiculite and perlite are interchangeable. I find that a mix of the two creates a more workable mix than either of them alone.You will need 8 cu ft of vermiculite for a 48x 48” x 4” vermicrete slab ( There is around 20% loss of volume when mixed with water and cement so account for this.

[TARibs]

And here it is with the top slab. Oh David you sent me a recipe with Vermiculite and Perlite. Could I just use Vermiculite? I have a 4 CuFt bag but no Perlite

[TARibs]

So after a long layoff I'm back at it again. Finally have the base built now ready for the actual oven to get done. I was getting ready to pour the Vermiculite slab and noticed 4 inches was recommended? Would 2 be of any help or should I do the 4? Right now I have it formed up for two so I guess I need new forms. Anyway here is look at where I'm at with the base which took forever to get where I wanted it.