32" build in Calgary, AB - pompeii neapolitan

[david s]

ok, so while I already have 5 ceramic fibre boards 18lbs/cu ft each 12"x36"x2", I have bought to be delivered July 10th 8 Skamol calcium silicate boards each 58cm x 24cm x 4cm. I want to design my oven for certain conditions with 1000F inside and at ambient temp outside of lets say 100F, would those two layers of casi+cfb equivalent to about 3.5" thick be enough on that hot day and hot oven firing that concrete slab under them wont reach more than 125F? otherwise if I add a 2.5" 5:1 perlite cement under then do I need to worry about Calgary winter freeze(--37c in extremes I seen that average -25c easily ) that it would destroy the moisture soaked perelite-cement layer causing unstable base?

casi+cfb at 3.5"thick, what else do I add if any:
1) nothing, concrete slab gonna be very fine under them
2) 5:1 perlite cement layer 2.5" thick, how to deal with winter freeze as the temp wont reach boiling point on it to evaporate water in it
3) loose perlite in a container made of pavement concrete bricks mortared to base slab and tampered or crushed a bit, lets say about 3" thick
4) another 1" or 2" casi?
5) another 2" ceramic fibre board?

I am using 2 bags of Quikrete Sand/Topping Mix 25KG Model # 110326 to level the slab as well as green self levelling laser level on tripod. I guess most likely no need for the 2.5" perlite-cement layer

My feeling is that your best option is to go for number 1, given the following. Not sure what others would advise.

Insulation thickness wonders to the law of diminished returns.

The denser the material the poorer the insulation value.

Insulation ability to hold heat is a function of time.

Any moisture present drastically reduces insulation value because it is conductive.

Most insulating materials are very hygroscopic (except foam glass)

The best insulator is a vacuum.

[IH123]

I used today1.5 bags of "Quikrete Sand/Topping Mix 25KG Model # 110326" to level the top slab, only where oven foot print will be,~52"x~52" . I used my laser level to check everything. one back side of the layer was almost zero and front level was the thickest at about 1.5".

I will mostly proceed with the casi 40mm+CFB 2" insulation only. now waiting for everything to cure and keep thinking every day if I should cast a castable refractory doughnut under dome walls or lay it directly on top of CaSi board. if I was to not use doughnut support, it will be dome bricks==> CaSi==> Ceramic Fibre Board==> concrete slab with hearth will be same, as well as archway. if I was to cast a castable refractory doughnut then only the dome will be different that the dome bricks first course will set on a 2.75" thick doughnut 32" inner diameter 42" outer diameter(1" thick extra for edging protection and expansion of dome) and will skip first dome spring/soldier course to compensate for extra height.

after 4 hours, got a misting and covered with vapour barrier and tarp

[IH123]

my plan for the insulation and support stack as my oven design will have dome, hearth, and archway NOT connected , rather have ceramic fibre rope/paper between them to locks air and heat. that is a stand alone shell oven. while aluminum foil could trap moisture, it will also seal the fiber dust from the board as it allows the thin castable refractory moisture to not be absorbed by the board

[david s]

Aluminium is highly conductive. It only works to reflect radiant heat when surrounded by air. Just as it is useless if installed touching a steel roof. Apart from this issue when used anywhere in an oven it can also create a vapour barrier. This can be partly offset it the foil is perforated. Another example of this is the perforated pizza trays sometimes used in an effort to stop pizza bases being soggy. Best to cook them directly on the firebrick floor.

[IH123]

wondering if using hearth bricks as support for the dome bricks is sufficient such that hearth brick under a dome brick would be kept at 9" length shaped as a perfect slice with dome brick staggered placed over two of those hearth perimeter slice bricks. any settlement of insulation layer or hearth will propagate minimally to the dome bricks as its 9" long (vs 4.5" for dome) toward centre, and each dome brick is supported by two of those. This will require stainless steel banding around the hearth too (not just dome) so hearth bricks wont rotate sideways. wondering if this has been tried and if its as robust and stable as a solid castable refractory doughnut ring outside the hearth? I am sure its double the stability of placing the dome on CaSi board directly or even a herring bone hearth without banding

[david s]

Brick kilns sometimes have steel bracing for containment and which is adjustable to accommodate expansion and contraction. However with service temperatures often well in excess of double the temperatures we use for our ovens leads to over double the thermal expansion. For a WFO expect the thermal expansion to only reach around 1%, so it is debatable is a steel band, particularly if it is not adjustable, is of much use. Attention needs to be given to the much higher temperature of the floor-bricks in the centre which will be expanding well before heat has time to reach the steel band to allow for its expansion. The steel band at the base of the dome is usually only ever seen on ovens that depart from the hemisphere, namely those that use soldiers for the first course which creates a stability issue. The advantage of a soldier course is that it provides greater height at the base of the dome interior.

[IH123]

can 1/8" ceramic fibre paper be put under dome brick only between dome brick and hearth deck under them without mortar (keeping the dome brick on the slip ceramic fibre paper sheet)? would the dome slip and collapse? - assuming I am using stainless steel banding to hold dome outward thrust- or they must be mortared?

[david s]

The weight of the dome on that junction is unlikely to allow for any movement there. It is the gaps between the loosely laid floor bricks which don’t have any weight on top of them, which does the job that allows for the expansion. Both the perimeter floor bricks and the bottom dome bricks sitting on top of them will receive the same rate of temperature increase and hence the same rate of thermal expansion, making a slip layer unnecessary in that position in my opinion. Others may disagree

[IH123]

my dome base would be of angle: 168.91° instead of 180° because its a sphere cap not full hemisphere , as the flame rolls in the low high dome it will heat the dome first and then bricks that are not under the logs (burning wood) will get heat by radiation and convection from gasses however they will be eventually be at lower rate of heating than the dome (or could) that a dome brick will have slightly more expansion rate than a hearth brick (or at least because its closer to the hottest area in the path) therefore creating a tiny displacement as it expands faster that if not allowed could cause a crack. as bricks in sphere of dome will become slightly larger by heat expansion so is the dome radius and because dome has more bricks(mass) than hearth and because contact surface or way of moving heat from dome to hearth is not necessary one-to-one ratio then dome may always be hotter than hearth transiently and thus expands more than hearth (or expands first); with herring bone pattern, the displacement is not always outwards to the void so that's where a slip layer may help -- I guess. however someone could argue because dome bricks are not mortared to the hearth bricks then they would still over come the friction and slip because the force to over come the friction of an existing full crack (represented by two different bricks contacting each other) is always less than the force needed to crack the brick as its restrained by the friction (assuming a single half brick can withstand the force the dome creates as it expands (really up to the flame randomness)) . a slip layer (when it lasts) might be peace of mind because it will protect a not very hard brick from cracking if dome weight is immense that friction is immense more than a half brick could take. of course that produce the dome slipping outward risk however with stainless steel banding around my dome first course it might be ok. what do you think? thanks

[david s]

Your steel band has around the same thermal expansion as brick, but because it's on the outside will become tight early in the firing as the brick temperature rises well over that of the band. During heat up the temperature rises way faster than it drops on cool down. After the oven has reached service temperature and held at that point for some time we call it "heat saturation" At this point the band should also be of a similar temperature and therefore having expanded, exerts less force to contain the bricks. As the oven cools the reverse occurs and the steel band contracts at much the same rate as the bricks. For kilns with adjustable steel bracing it is usually loosened as temperatures climb and then tightened on cool down. I presume you won't have this facility, but as I previously posted the expansion is only in the region of 1% so the loosely laid floor bricks should be adequate to deal with that.
When you get to firing your oven an IR thermometer is quite instructive to show you the very large temperature differences (and their corresponding thermal expansion) at different points of the oven. The temperature at the bottom of the dome of both floor and dome bricks is always way lower than the floor centre or upper dome during fire up, until after around two hours when the temperature reaches saturation and evens out.

[IH123]

CaSi boards arrived today, will be removing the workform of the concrete slab on the weekend and let the slab dry out for about a week and then will start the construction of the oven

[IH123]

what the most effective way to seal the insulation boards hazardous dust and fibre when having CaSi 40mm + Ceramic Fiber board 2" on each other ? I don't want to breath or eat any of that . can you please tell me?

would this work:
Ceramic fiber board 2"==> Casi board 40mm ==>1/16" refractory mortar skim coat ==>1/8" pure play sand ==> hearth bricks laid tight as herring bone pattern
?