48” Naples style oven in Edmonton

[beefborley]

I’ve got a supply of Heatstop 50, so I’m ready to resume my build. For those who’ve used this mortar would you recommend fully soaking the firebricks, or just dampening with a sponge?

[david s]

Somewhere in between. Do not lay the bricks with a wet surface. Just hose them a bit and wait until the water has penetrated.
Heatstop is not recommended for thick mortar joints. I think this is because it does not contain any burnout fibres that assist in lowering the risk of steam spalling. So either add some or leave the big gap on the outside to fill later, or just create a thick mortar joint with the stuff and ensure you do the drying fires really slowly.

[beefborley]

Somewhere in between. Do not lay the bricks with a wet surface. Just hose them a bit and wait until the water has penetrated.
Heatstop is not recommended for thick mortar joints. I think this is because it does not contain any burnout fibres that assist in lowering the risk of steam spalling. So either add some or leave the big gap on the outside to fill later, or just create a thick mortar joint with the stuff and ensure you do the drying fires really slowly.

What would you consider a thick joint? I don’t think I’ll have any joints on the dome greater than 1/2”. The joints on the opening arch will be closer to 3/4”.

For burnout fibres would you recommend the fibreglass fibres that are used for fibre reinforced concrete?

[david s]

https://community.fornobravo.com/for...9579#post69579
You would need to add some fine polypropylene fibres, not the fibreglass ones. You don’t need much but they need to be mixed in really well. You should then be able to make the joints as thick as you like. The fibres melt at 160C then leave a network of really fines pipes through which moisture can escape.

[david s]

Are you building a large radius low vault barrel oven? If so you will need to plan either buttressing or bracing of the low side walls to counter the outward thrust of the vault.
Otherwise it will not be structurally stable.

[UtahBeehiver]

Heat Stop install instructions show joints up to 1/2" that said, I have seen some builders using Heat Stop 50 having back joints as wide as 3/4 to 1 inch. I agree with David on buttressing of the vertical walls are necessary on a low dome build.

[beefborley]

Are you building a large radius low vault barrel oven? If so you will need to plan either buttressing or bracing of the low side walls to counter the outward thrust of the vault.
Otherwise it will not be structurally stable.

Yes, the dome height is 19” with a 48” diameter floor. As for buttressing, perhaps I should use 3” of reinforced insulating castable to buttress the soldier course and then just insulate the upper dome with ceramic blanket?

[UtahBeehiver]

I am going to defer to David S on whether insulating castable has the shear strength to act as a suitable buttressing. He is our cast expert. Some of the commercial low domes I have seen have a metal ring around the perimeter vertical wall with a gap that is filled with leca/concrete or p/vcrete insulation with the metal ring giving the shear strength.

[david s]

Yes, the dome height is 19” with a 48” diameter floor. As for buttressing, perhaps I should use 3” of reinforced insulating castable to buttress the soldier course and then just insulate the upper dome with ceramic blanket?

I’m not sure that insulating castable would have enough strength. The data sheet on all those products should give you their strength. Adding an insulating aggregate to any brew in order to make it more insulating drastically reduces strength. The attached table gives you an idea. As Utah says, most low domes use a steel flat bar running around and outside the base of the wall. The problem is then, what do you do about where it goes across the oven mouth. Maybe you could weld it to the steel stand.
I think Karangi dude’s low dome was supported by insulation held tightly in place by a pretty solid outer steel skin.

[beefborley]

I thought about a steel band that could act as a belt around the soldier course. At the opening you would have to tie it into the concrete slab somehow as the steel base is below that. Would you see conductivity issues with the steel brackets attaching to the concrete? Even on the outside of the firebrick I imagine they would get quite hot once the brick is saturated with heat.

[david s]

Yes, that's part of the price you have to pay for the advantages of having a large radius low dome or vault. As the band would be insulated you'd only get conduction where it joined the slab.

[beefborley]

I am planning to cast a 3" ring of insulating castable that will act as a buttress for the soldier course. I will reinforce this ring with vertical lengths of 15mm rebar anchored into the concrete hearth and two rings of 12mm rebar attached to these verticals at 6" spacing. For castable, I am planning to use either Kastolite 23 or Kastolite 50-25. (see attached Data Sheet). Can anyone provide information on how much CCS I should be aiming for for this buttress. And also, will this 3" provide adequate insulation at the bottom soldier course, or should I wrap that with ceramic blanket as well?

[UtahBeehiver]

To give you a K value (thermal conductivity) comparison, ceramic blanket has a K value of about 0.50-55 where as the Kastolite 50-25 K value is 2.23 meaning it will transmit heat over 4 times more than ceramic blanket. So it is your call. I do not know what the shear strength is on a insulating castable and if a insulatinf castable alone can act as a buttress without a structural backing. Perhaps David S can chime in on this.

[beefborley]

Thanks for the reply. I think the castable layer would end up being closer to 4” by the time I get all the rebar and forming in place. I think I will also form the exterior with sheet metal that I can leave in place to maintain the integrity of the castable.

[david s]

Good job, That looks like a pretty good solution.