Castable Refractory Products (VT600)

[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]

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]

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]

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]

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]

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]

Finally made serious some progress, I had plenty of wood but not nearly enough sand (plus no place to put it if I bought a lot more) so I started the dome with a nice stack of wood all arranged facing the door so I could get it out. I then covered it with compacted sand and created the front entryway, again not enough sand so I filled it with more wood and bricks. Was very nervous about how exactly I'd "pour" or place the castable without an out form and that part proved to be right. Was very nervous about getting the mix the right consistency so it wouldn't be too soft and just sag, I don't know how the home brew works but the stuff I had was not stiff enough to really stand on it own but maybe I was doing something wrong.

[TARibs]

So for anyone interested for future builds, what I did was go once around the bottom (about 6 inches high) the 1st day (took 3 hours to get aroundI leaned the bricks against pieces of wood to hold the Castable in place. I knew I would never get it all cast so I cut a groove in the lowest level to get a good key bond ( the next day) with the next level. On the next day I started a lot earlier at 9AM. I gradually went around and around vibrating as best I could ( working the layers together with my hands and a putty knife) to get pieces bonded together. I'm sure the inside is going to have grooves from all the pieces but I believe I got decent bonds between them all. Going over the top was probably the hardest to keep the correct thickness. My son kicked in helping around 5 and we finished up around 10 PM. I was nervous regarding wether I'd ever get the wood out of the interior but once I removed the door (which came off fairly easily) I was able to tap one of the two by 4s and their was enough space in the wood pile to allow movement so I was able to pull one piece out, once it was out the rest followed rather easily. Had a bit of an accident so wasn't able to clean all the sand out but what I can see looks good.

Its all covered and tented today as we're getting a lot of rain and I don't want that. Hopefully start drying air it out tomorrow.

[TARibs]

So next question the entry way and particularly the vent pipe. I think I know how I'll build the entry but what are options for vent pipes, what works what doesn't? Does clay pipe work, I believe stainless steel is best. Can it be single wall though, the double is pretty pricey. Been looking for transitions between the cast section and vent pipe but haven't seen too much. Any suggestions? Does the black pipe used for a wood stove work? Why not the galvanized (too hot for the pipe I'm guessing)

Thanks

Tom

[TARibs]

Getting ready to build flu gallery, I see some comments that it should be insulated? Is this necessary if wings are going to be brick? Present plan is to cast the upper section (maybe I should insulate that portion?) where the vent pipe will attach and use brick on the lower wings (sides) with a 1 inch layer of Castable inside the bricks. Hope that makes sense, Question is what to do between the transition of the flue gallery and the Oven Dome? Would inserting an inch of blanket there be acceptable? What about an inch of Vermicrete, would that be better? What other thermal break options are there?

I presently have a large pice of Styrofoam to cast over for the flue (see pic) I plan to build an out side form I can place over it in the hopes of applying some vibration and getting a nice smooth surface. Still working on the transition from the Cast to the SS vent pipe. may just put tabs into the cast (grooving the Foam) and inserting the Vent pipe into a slightly larger opening (Seen David suggest this on numerous threads). Thanks

Tom

[david s]

Going very well. Building an outer form for the gallery mould will be very difficult and hard to fill from the top. Just get the consistency right and work it over the form. I wouldn’t be bothering to have the brick sides faced with castable. Much easier just to have the cast section sitting on top of the bricks. The casting should be insulated but don’t worry about the brick sides, they don’t get particularly hot.

[TARibs]

Thanks Dave, what about some type of break between the Outer flue area and the dome, Any suggestions?

[david s]

Yes, you can do that, but remember that a small gap of 1/2” or so won’t do much to prevent heat. It may reduce heat by conduction, but do little to prevent heat travel by radiation. After all you wouldn’t expect 1/2” of insulation over your dome to be very effective. The primary function is to act as an expansion joint and to prevent expansion damage to the outer oven parts. In the case of my oven design the flue gallery is so light it hardly acts as a heat sink and is also insulated. I have an expansion joint between the flue gallery and the outer decorative arch which is in turn connected directly to the outer oven shell. That way the inner dome and gallery are independent and free to expand inside the outer shell and decorative arch

[TARibs]

Haven't posted much on progress but wanted to add to my little story. This is the Cast chimney construction, worked pretty well. Wood mold with styrofoam core, poured in refractory with SS needles, took form apart and then ground into a nicer shape. Chimney attachment was put in place with High Temp cement and surrounded by vericrete to make it tighter

[TARibs]

So due to the way I had to place the refractory cement (multiple small blobs) all done continuously you can see the inside result isn't perfect, See all the lines, I was originally concerned and tried smoothing it out with a thin layer of refractory cement, This looked good in a picture but over time it has spalled off, the oven is back to seeing all the lines but as yet has not developed any cracks