New to this oven game Introductions Discuss New to this oven game in the Good Background Information forums; I have been reading intently through this forum as I construct my oven. I have finished mineand it looks great. ...

[mulga]

I have been reading intently through this forum as I construct my oven. I have finished mineand it looks great. My problem is when I fired it properly for the first time I just could not retain the heat, it lost most of it through the roof.
I am wondering if this is because I use normal concrete motar and on the roof used a fair bit of it when I finished off the top of the dome
I am wondering if a couple more layers of perlite and alumina cement render will help to retain the heat? I AM HOPING SO.
I know someone who installed an insulating blanked then covered it with chicken wire a few years ago but now has to re-render it as the chicken wire started to rust and the old render has broken up.
Apart from that and the need to extend my brick chimney with a length flue it looks great.

[pacoast]

Well you should have used a refractory mortar. I don't know how durable common mortar will be in a high heat application. With a little luck your the mortar just needs to cure.

It is normal after only one or two firings to not see high temperatures. The mortar is still curing and there is a lot of trapped water in there. After everything cures, ovens will usually hold much higher temperatures.

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[mulga]

Thanks for that. WA does that mean you are a Sandgroper? Any thoughts on the perlite alumina cement render? cheers.

[ThisOldGarageNJ]

did you insulate your oven at all ? show us some pics...

Welcome to the addiction...

Mark

[dmun]

Any thoughts on the perlite alumina cement render?

If you tell us what it is, we'll give you our thoughts.

A common oven insulation is perlite, or vermiculite, mixed with portland cement, and applied at least four inches thick. It has no alumina content. Is this what you're talking about?

"Render" is UK/AUS speak for what we call stucco. It usually covers the insulation, and serves to keep water out of the oven and insulation.

We need more information to give you any useful feedback.

[pacoast]

Originally Posted by mulga

WA does that mean you are a Sandgroper? Any thoughts on the perlite alumina cement render? cheers.

Afraid not. In my case it means Washington State. I'd surely love to visit your part of the world sometime though.

I'm not familiar with perlite alumina cement. Is this to apply directly over the bricks or as a final outside coat? Alumina render is what we call refractory mortar in North America and is intended to increase the thermal capacity (hold heat). Perlite is an insulator and is usually added to cement to prevent heat loss under the hearth floor or as the last coat before the render (stucco). I haven't seen a product that has both alumina & perlite in it before. Do you have a brand name or link to the product?

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[Rastys]

Hi Mulga and welcome.
You are the 20th Adelaidian on my list with a few also in the Adelaide Hills.
many have completed and enjoying their builds, as I am. I have a 40" Pompeii and frequently do pizzas, breads and then roasts so the oven holds the heat for several days. I used a super wool thermal blanket and 3" vermiculite cement. It is imperative to insulate unless you own a wood lot.
Drop me an email with your contact details and we can discuss it easier than typing here unless you want to share your views with the other members and visitors.
Pictures and specific build details are what we ned to give you worthwhile advice to solve your dilemahs.

Cheres.

Rastys

[mulga]

From what I can gather the perlite , 5 parts, is mixed with the alumina cement, 1 part, and brickies sand, 2.5 parts, then applied as a thermal render (stucco). That is the theory anyway. The bricks in the dome are half and quarter bricks on end then motared together, normal motar. I have no insulation. I have rendered over them a plasteres sand render with red oxide colouring. On the base, hearth, I have normal red clay bricks with 100mm (4'') concrete for a base. Hmmm how does it sound guys? How the hell do you upload pictures? Oh yes I have only given it 1 half decent firing.

[pacoast]

Sounds like the perlite alumina render is mostly an insulator, so it would go on last. If the render is to be exposed to the weather (rain) double check that they have some kind of waterproofing material in the mix to keep water out. As long as it waterproof it should work great.

It's strongly recommended that you have some kind of insulation otherwise your oven will lose a lot of heat & consume a lot of wood. It will also make it somewhat harder to maintain high temperatures. A thick coat of the perlite alumina render is good, having some loose perlite or even better a layer of ceramic insulation underneath is even better. Ideally, you'd also have some kind of insulation under the floor to keep the foundation from sucking too much heat out of the hearth.

I'm sure that your oven will get hotter & hotter as the mortar cures (dries out), but try to get as much insulation in as possible & everything will work better.

Cheers.

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[RTflorida]

Sounds like you have zero insulation under your hearth, you may be losing most of your heat to the 4" concrete base. It takes alot of fire to heat that entire slab, sucking the heat away from those red clay bricks.
As for the dome - how thick is that alumina/perlite/sand render? It is recomended to use a 5:1 of perlite & portland, no sand - at a thickness of at least 3" (4 or more is optimal). You have replaced the portland with alumina (no big deal, makes it more heat stable, but it adds nothing to the insulating value) and added 2 1/2 parts sand (again, no insulating value). This mix actually cuts your overall insulation value by about 40%. You would need at least 4" to be effective...then your colored top coat render.

Sorry if it sounds like I am raining on your parade....just calling things the way I see them.

RT