Help! Cracks in newly cast homebrew oven

[taz]

Hi everyone. Was hoping I could get some advice for a problem I've encountered today.



I cast an 85cm diameter homebrew mix oven yesterday. All was well until about 2 hours later when I noticed cracks that appeared from the top of the dome an extended to the back in a 'Y' shape. I'm in the UK and it was a rarely hot and sunny day yesterday. Luckily the mix was still malleable so I gently compressed through the cracks and immediately covered with wet towels.



I've been trying to see if anyone else has had similar issues and trying to figure out the cause.



​​​​​​I mixed in batches and the final two were for the top of the dome and were slightly wetter than the previous mixes. At the point I was doing this bit the sun was directly over. On looking further into the forums I've seen some people using SS needed and Polypropylene fibres. I haven't used either of these and was wondering how important they are?



Are these cracks going to be an issue? What's the best way of preventing or minimising the chances of them happening?



Thanks all



Taz

[downunderdave]

They are shrinkage cracks created by the casting getting too warm too soon. This was no doubt caused by the sun exposure. I get the same problem if I do my casting in the morning and the concrete gets early afternoon sun exposure. I usually cast in the late afternoon so it cures slowly overnight. The other problem is making the mix a bit too wet because as it dries it will have greater shrinkage. You did the right thing by troweling over the cracks and it may be sufficient to have fixed it. The fine polypropylene fibres although for our purpose are primarily added to act as burnout fibres to allow better elimination of excess free water are actually designed to reduce early shrinkage in standard concrete so their omission doesn’t help. The casting can’t collapse because of its form. If it were mine I’d continue, although you could start again. Use the experience for the second oven you build.

[taz]

Thanks for the advice Dave!

The wet towels seem to have done the job for now. only one small hairline crack.
I'm annoyed with myself for not using the fibres. I actually bought a pack a while back but completely forgot to mix them in!

Knowing it shouldn't collapse is reassuring and therefore I'll dust off and carry on! My plan is to remove the sand tomorrow and let dry for a week before putting on a ceramic blanket and vermicrete layer. Then dry for a week and start some small curing fires for a few days before using it.

[downunderdave]

When you remove the sand you will almost certainly have some voids that will require filling. This needs to be done when the cast is somewhere between wet and dry, so still a bit moist. If you sieve some homebrew mix to remove the coarser sand, making it more workable and a richer mix, mix it to a peanut butter consistency, then force it into the voids, you’ll get a good result.

[taz]

Jun 9, 2021 21:08:55 GMT downunderdave said:

When you remove the sand you will almost certainly have some voids that will require filling. This needs to be done when the cast is somewhere between wet and dry, so still a bit moist. If you sieve some homebrew mix to remove the coarser sand, making it more workable and a richer mix, mix it to a peanut butter consistency, then force it into the voids, you’ll get a good result.

Brilliant thanks Dave!

[taz]

Hi Dave,

It's been about 3 weeks since I cast my dome and I've left it to cure. No new cracks. I filled the voids as you suggested. To be fair there weren't many of them.

However I've done my second small curing fire today and have noticed some new small voids in the apex of the dome that weren't there before. They are approx 1cmx 1cm and probably about 5ml deep.

Are these anything to worry about? Do I try and fill them with some homebrew mix?

Would really appreciate your advice

Thanks

Taz

[downunderdave]

Jul 2, 2021 15:50:21 GMT taz said:

Hi Dave,

It's been about 3 weeks since I cast my dome and I've left it to cure. No new cracks. I filled the voids as you suggested. To be fair there weren't many of them.

However I've done my second small curing fire today and have noticed some new small voids in the apex of the dome that weren't there before. They are approx 1cmx 1cm and probably about 5ml deep.

Are these anything to worry about? Do I try and fill them with some homebrew mix?

Would really appreciate your advice

Thanks

Taz

Hi Taz,
Not sure what mix you used, but a high clay content creates shrinkage which can lead to cracking as well as a mix that is overly wet. Pushing the water out too fast is also a cause. The addition of polypropylene fibres that are well mixed and dispersed into the mix gives a measure of protection from shrinkage cracks during early drying as well as melting and burning away north of 160C, leaving a network of tiny pipes to assist safer steam removal. As the structure is a dome it can’t collapse and as they say there are two kinds of ovens, those with cracks and those who have owners who lie about their ovens having cracks. Hairline cracks are normal and characteristic of large one piece castings. Cracks larger than you could fit a playing card into are not, but will not affect the oven’s performance and will probably not get any bigger.

[downunderdave]

Jul 2, 2021 15:50:21 GMT taz said:

Hi Dave,

It's been about 3 weeks since I cast my dome and I've left it to cure. No new cracks. I filled the voids as you suggested. To be fair there weren't many of them.

However I've done my second small curing fire today and have noticed some new small voids in the apex of the dome that weren't there before. They are approx 1cmx 1cm and probably about 5ml deep.

Are these anything to worry about? Do I try and fill them with some homebrew mix?

Would really appreciate your advice

Thanks

Taz

I also should have added that it is also advisable to do the drying fires after insulating the dome because this evens out the huge difference in temperature (and resultant thermal expansion) between the inner and outer surfaces of the casting. Reducing this difference is less damaging to thermal shock. For this reason my preference is to always do the drying fires after insulation layers have been applied.

[taz]

Hi Dave,

Thanks for all the advice. It's incredibly helpful!

I used the homebrew 3:1:1:1 mix that's been talkied about on the forums. Only thing I'd forgotten to put in was the fibres.

I've lit a third fire now and it seems to be going ok.

I saw your previous advice on the forum and had insulated the dome with a ceramic fibre blanket and vermicrete (10:1) prior to firing it up. Unfortunately the Welsh weather has returned with buckets of rain today so have had to cover with a plastic sheet so the vermicrete doesn't soak up the rain. Once the weather dries up and the insulation is dry I'll coat with a waterproof render (leaving a hole for ventilation)

Thanks again Dave! I've read a lot of your advice on the forums and you've helped loads of people. Amazing stuff.

Taz

[downunderdave]

Jul 3, 2021 7:57:59 GMT taz said:

Hi Dave,

Thanks for all the advice. It's incredibly helpful!

I used the homebrew 3:1:1:1 mix that's been talkied about on the forums. Only thing I'd forgotten to put in was the fibres.

I've lit a third fire now and it seems to be going ok.

I saw your previous advice on the forum and had insulated the dome with a ceramic fibre blanket and vermicrete (10:1) prior to firing it up. Unfortunately the Welsh weather has returned with buckets of rain today so have had to cover with a plastic sheet so the vermicrete doesn't soak up the rain. Once the weather dries up and the insulation is dry I'll coat with a waterproof render (leaving a hole for ventilation)

Thanks again Dave! I've read a lot of your advice on the forums and you've helped loads of people. Amazing stuff.

Taz

Hi Taz,
Just remember to take it slow with the drying fires. It is exciting to get to this point and you'll be really keen to cook stuff, but to avoid damage it's precisely the time to go easy. Long gentle fires are required until all the black soot has been burned from the entire oven interior. You can use this heat to cook stuff like roasted meats, just don't shoot for pizza temperatures straight away.Thanks for the accompanying accolades. You'll find yourself keeping an eye out for free fallen timber now, remember "the best wood is free wood" and there's tons of it everywhere. On principle I don't buy wood and have only broken that rule once in 13 years.
Dave

[taz]

Yes I've kept the fires fairly low and let them slowly burn. Funny you mentioned the black soot. There is loads of it! Assuming this will eventually disappear from what you've said?
I've managed to accumulate a fair amount of wood over the last year or so. Will see how long that lasts. I've already had plenty of our friends "making booking" to come and enjoy some pizza! 😂