Perlite/Cement/Grog Dome

[tony s]

Hi
I'm building an outside wood oven with perlite-cement 5-1 for the dome. I have some refractory cement-Grog ! Would this work if I was to render the inside of the dome with this without problems.
Thanks
tony s

[Gulf]

Hi Tony,

Welcome to the forum. I moved your reply to a new topic. The thread that you replied to was about using perlite/cement as an insulating material. There are several threads on here about your type of build.. Someone recrently mentioned coating the inside of a light weight castable with homebrew. I'm not sure how it turned out. My fear is that it would turn loose and fal into the food. I think that I would add the grog to a home brew castable recipe and save the perlite for the insulation layer.

[tony s]

Thanks Gulf
The cement-racastable mix I have I guess is from crushed fire bricks as it has very small bits in it. Would this not hold up with about a 6 to10mm coating onto my perlite concrete I know you said that it may fall off onto the food but would that still happen if was this thickness of render ?

[Gulf]

I really don't have the experience to answer that question truthfully. I do know from experience that vermiculite/portland cement will swell when heated due to the moisture content. It takes a lot of time to drive all the water out of it. I'm sure that the perlite/portland cement will do the same. Imo that could cause a lot of cracking in the thin parge that you are suggesting. It just seems to me that this type of design is building a castable dome in reverse order.

Our castable guru is david s . He may be able to answer this better than I. As I mentioned above, I believe someone on here has tried a similar parge coat. I think that most of those threads are in the Other Oven Types sub forum. Take a look at some of those threads.

[david s]

Hi Tony,
Im not clear just what you are intending. The inner dome needs to be strong enough to withstand knocks and abrasions, if you are planning on casting the inner dome with a 5:1 perlite cement mixture it wont be very strong, youll be able to scratch it away with your fingernails. In addition the very high perlite content is very insulative which makes the casting virtually impossible to heat to any extent and with such low thermal mass will not store any heat. Generally the inner dome of a WFO needs to be dense and strong so it can store sufficient heat to even out temperature fluctuations as well as provide thermal mass for extended retained heat cooking. A perlite dome will work to cook pizzas provided a decent fire on the side is maintained. If you parge the inside of the casting in an effort to give the casting some strength and to prevent bits of the perlcrete mix from crumbling off, you are likely to find it will also flake off. A better method would be to create a dense layer against the mould, over which you could apply the perlcrete mix. However advice Ive seen says dont go any thinner than 2” with a castable or you can expect failure. It is also possible to increase strength by adding stainless steel needles which are the recommended reinforcing for refractories.
When you say you have refractory cement-Grog do you mean you have calcium aluminate cement as well as grog, or is it a proprietary castable refractory mix? Also there are many different types of grog (crushed sintered clay) For the temperatures we fire to a crushed fire brick grog is overkill IMO. A standard grog is sufficient and in practice sand is good enough. If you are parging with a Portland cement mix it must be done when the casting is still moist, say a day after demoulding and then cover it well for a week to allow the hydration process to do its magic. If a thin layer is allowed to dry too quickly or the substrate its applied to sucks too much moisture out of it then it will fail. Likewise if its too wet then it wont bond and is likely to flake off. If parging with a calcium aluminate cement based mix, the curing is done after 24- 48 hrs, so no need for extended damp curing.
There is a you tube out there about casting a perlite or vermiculite casting to create a pizza dome, but to my knowledge no reports on how they fare after around a year of use. I suspect that might be about their limit before beginning to crumble. If anyone reading this has built one of these please report. As already advised it is better to keep your perlite for insulation rather than for the hot face. There are lots of cast builds well documented on this forum. Do plenty of research before you commit or be prepared to do a rebuild down the track. That is probably the best way to go. Build something as cheap as you can fire and cook the hell out of it then from what you’ve learned build another, it’s fun.
Cheers,
Dave

[tony s]

Hi David s
Thanks for your reply, the mix I have is a castable refractory mix. I have already made my dome over over a gym ball with the perlcrete mix so I guess I'm stuck with what I've got. I have put a 2 " thick layer on so far then I plan to put a silicate-fire proof layer then wired and coated again with perlcrete to a thickness of about 4 to 5 inches. When it's all in place with the base in situ (made from perlcrete and fire bricks to about 5 inch thickness etc, then another coat of something to make it waterproof as I live in the UK and you know what our weather can be like lol. The final layer of 3/4" to 1" thick. What do you think ? at that thickness do you think it would hold the heat longer or is the thickness of no importance ? your thoughts would be most helpful.
Cheers
Tony s

[david s]

Hi Tony,
ok, so your mix is a castable refractory. Still not sure whether it is a dense castable and you’ve added perlite to the mix or if it is an insulating castable with the perlite already added. Look at the product data sheet and it should tell you the density. It should be in the region of 2000 kg/m3 (2 kg/litre) if so you could try troweling a layer of this dense mix over the inside at least 1/2” thick, bearing in mind what I said about moisture of the casting you are applying the mix onto.
I don’t understand what you mean by applying a “silicate fire proof layer” over the existing casting. What is the mix and how thick are you intending? A further perlcrete mix over that to act as insulation should be as lean as can be practically applied. As it will see less heat here a mix of Portland cement and perlite or vermiculite will suffice and is cheaper. I find 10:1 is about as lean as you can go but still being workable. By volume, 10 parts perlite or vermiculite to 1 part cement and 3 parts water with a little powdered clay to help hold the grains together is a pretty good insulative mix.
The outer shell is only a weather covering so no need to go so thick. I usually only make it around 12-15 mm thick and wrap it in cling wrap to hold the moisture for a week to enhance strength.