So my oven cracked. Advice needed.

[ebbbe]

Hey everyone

So a 2 years ago I decided to build a cast homebrew oven. I made it on a wooden structure because I was living at a rented place and, though I expected to live there at least five years.

Link to the original build is attached.

I got got it insulated and fired it one time before fall came, but I did have steam coming off it once during the firing process though I tried to actually go slower than every guide I could find. I packed it up for the winter. Last year some things changed, and we bought a house. My brother in law works as a fork lift technician, so he was able to borrow a massive trailer and a fork lift, so I was actually able to have it moved to my new house.

Now I have a cack in it. In the inner dome from the front opening and maybe 2/3 back. There is also a Crack ind the outer dome, so I actually have some smoke coming out when starting a fire.

What would you suggest I could do from here? I figured I would remove the insulation around the Crack and replace that and then when everything is dry I would render it. I would just let the Crack in the inner dome be and then be more careful when I eventually upgrade it.

Any tips are greatly appreciated.

[david s]

Visible steam is an indication that you are pushing the water out too fast and it is highly likely to create cracks. I prefer to push the water out after insulating but before rendering the outer shell as it allows the moisture to escape more easily. There’s not much you can do about the inner dome crack. You can try to repair it but it’s rarely successful. Fill the cracks in the outer shell after all the moisture has been eliminated. Throw some sheet plastic over the dome during firing to see if it condenses on the underside. This will tell you if It’s dry or not.

[ebbbe]

Alright thanks David. I figured repairing the inner dome was a lost cause.

I did try to be diligent when heating up. Apparently I failed.

Oh well. The pizza is good.

[david s]

As they say, there are two kinds of oven owners, those that have cracks and those who lie about their ovens having cracks.
I’d be interested though, to know if you added burnout fibres to your homebrew mix to assist in safer water removal.

[UtahBeehiver]

I am uncertain from the narrative if the cracks are from heating or moving the dome?

[ebbbe]

Last year the weather changed shortly after me posting this, and I never got around to fixing the oven. This weekend I removed the top half of the prelcrete. It seemed that it was much more porous than the perlcrete on the bottom half and the opening. I think I may have added to much water and washed the cement off the perlite? I don't exactly remember but it didn't really stick well together.

I am uncertain from the narrative if the cracks are from heating or moving the dome?

With regards to this I guess it's both. From what I could see a crack formed the first time I fired it in my old place, but it definitely got worse after it was moved. My take away definitely is to not move an oven once it's built.

Anyways now I'm not really sure how to proceed. Last year it seemed that the crack down the middle of the oven expanded slightly when it was fired and over the first few times I used it, it caused the perlcrete to crack as well. My instict tells me that if I just replace the perlcrete, this will happen again. Another, more expensive, option would be to remove all the perlcrete on the dome, buy some ceramic insulation blanket and use that instead? My idea is, that this could take the small expansion from the inner dome thus not result in cracks in the ourter render?

Am I completely off here? If I have a high risk of cracking the outer render either way, I would just use the perlite I already have.

[ebbbe]

As they say, there are two kinds of oven owners, those that have cracks and those who lie about their ovens having cracks.
I’d be interested though, to know if you added burnout fibres to your homebrew mix to assist in safer water removal.

I did add burnout fibers as per your instructions. I also tried to follow the drying cycles I could find, but apparently, for my oven, that wasn't good enough.

[david s]

It sounds to me like you might need to build your second oven. Because the homebrew is so cheap and you've done it before, you'd be able to build a second one both better and quicker. The addition of ceramic fibre blanket especially for the first layer is a well worthwhile advance.

[ebbbe]

It sounds to me like you might need to build your second oven. Because the homebrew is so cheap and you've done it before, you'd be able to build a second one both better and quicker. The addition of ceramic fibre blanket especially for the first layer is a well worthwhile advance.

I guess you're right. I could probably reuse the underfloor insulation, the oven floor and the flue. that would probably cut down the cost quite a bit. then it's basically just concrete and homebrew.

Do you have any idea, if there is a specific mistake that causes it to slightly expand when fired? Is that just the nature of a cracked oven? I would really like to avoid making the same mistake again.

[david s]

I guess you're right. I could probably reuse the underfloor insulation, the oven floor and the flue. that would probably cut down the cost quite a bit. then it's basically just concrete and homebrew.

Do you have any idea, if there is a specific mistake that causes it to slightly expand when fired? Is that just the nature of a cracked oven? I would really like to avoid making the same mistake again.

Using vermicrete against the inner dome can leave moisture there, which will suddenly turn to steam if it's not dried out first. I used not to use blanket and only used vermicrete because the earlier generation of blanket was classified as a class 2 carcinogen. The new generation of blanket is now exonerated as a carcinogen (check the MSDS on your product). I learnt that it was vital to dry the vermicrete out properly or it would swell and crack and in one case the steam pressure cracked the outer shell of an oven I'd built. My solution was t apply the vermicrete in layers of an inch and a half with a week drying after each layer. I now use blanket against the inner dome and it's dry so problem solved. It also acts as an expansion joint. I only do a layer of 10:1 vermicrete as the last insulation layer to act as a firm substrate for the render, restoring the hperfect hemisphere and as an additional insulation layer.

Not sure why your inner dome cracked, but it was probably due to firing too aggressively too early (if you see steam you're going at it too hard) or possibly the burnout fibres were not dispersed properly. When mixing them in you need to mix about double the time that you would if you didn't have the fibres. You can see that they clump together and take quite a while to disperse throughout the mix.

[ebbbe]

earlier generation of blanket was classified as a class 2 carcinogen. The new generation of blanket is now exonerated as a carcinogen (check the MSDS on your product).

First of all, thanks for the info. I will definitely check this.

My solution was t apply the vermicrete in layers of an inch and a half with a week drying after each layer.

Not sure why your inner dome cracked, but it was probably due to firing too aggressively too early (if you see steam you're going at it too hard) or possibly the burnout fibres were not dispersed properly. When mixing them in you need to mix about double the time that you would if you didn't have the fibres. You can see that they clump together and take quite a while to disperse throughout the mix.

With regards to the perlcrete layers and the thorough mixing of the fibers, I did do that (as per your advice). The culprit is probably my lack of patience with the firing schedule, though I did try to go slow. Do you have any recommended procedures, or do you just go by experience at this point? I believe I saw two different approaches with one recommending firing up to a certain temperature (increasing day by day) and then letting it die out. The other method required multiple very long temperature rests. This seems hard to do, without accidentally firing it too hard.

I now use blanket against the inner dome and it's dry so problem solved. It also acts as an expansion joint. I only do a layer of 10:1 vermicrete as the last insulation layer to act as a firm substrate for the render, restoring the hperfect hemisphere and as an additional insulation layer.

Is is necessary to add perlcrete or can I render directly onto the blanket? I believe I have seen people do this, but I may be wrong. At this point I really don't like the stuff..

50mm of blanket insulation is not way off, right? I have the option of buying 50mm in one layer or two layers of 25mm at the same price. Any preference?

[david s]

Some folk have rendered directly over the blanket, but as it’s lumpy youend up with using way more render to get a smooth hemisphere. It is also not a firm substrate to render against. I’ve only used the 25mm thick stuff so not sure how you’d go with 50. The perlcrete layer allows you to restore the perfect form again as well as providing additional insulation, saving on an additional blanket layer.

[fox]

As David points out you can do it either way, I build mine with 4” of blanket and plaster directly on top.
I prefer this method as it is much quicker and I find it easier but if you are not in a hurry or your budget wont stretch then perlcrete is fine too.

[Kris S]

50mm of blanket insulation is not way off, right? I have the option of buying 50mm in one layer or two layers of 25mm at the same price. Any preference?

I've used the 25mm blankets, found them easy to apply, I would go for 25mm layers...

[ebbbe]

Some folk have rendered directly over the blanket, but as it’s lumpy youend up with using way more render to get a smooth hemisphere. It is also not a firm substrate to render against. I’ve only used the 25mm thick stuff so not sure how you’d go with 50. The perlcrete layer allows you to restore the perfect form again as well as providing additional insulation, saving on an additional blanket layer.

Alright. Thanks again for all your advice. Really appreciated.

As David points out you can do it either way, I build mine with 4” of blanket and plaster directly on top.
I prefer this method as it is much quicker and I find it easier but if you are not in a hurry or your budget wont stretch then perlcrete is fine too.

I'm willing to stretch my budget a bit to avoid perlcrete. I really hated working with it.

I'll probably add a couple of layers of 25mm and se how close I can get to a hemisphere.

Thanks.

I've used the 25mm blankets, found them easy to apply, I would go for 25mm layers...

Alright thanks.

Did you add 2 layers or more?