Percrete falling into pizza

[jkaiser1]

I just completed a percrete WFO and have cooked in it twice and the second timeI found small bits of perlite fell into the pizza.

Does anyone have a suggestion on how to seal the interior dome so that this doesn’t happen.

Thanks. jkaiser1

[david s]

Perlcrete or vermicrete lack strength and because they are insulators also lack thermal mass, mixtures made containing them also lack thermal mass and strength (see table).The problem you describe is a common one for inner domes built using these materials. I've not seen one report of an oven of this type having any longevity. For these reasons we recommend that builders stay away from building the inner dome from this material and use a dense, conductive and strong material instead. keep your perlite for insulation only.
If you coat the inside with something, being thin it is likely to crack and fall off. Bonding to the perlite surface also introduces problems as it will suck moisture from any mix applied to it. Perhaps a non water based product like ceramic fibre blanket rigidiser could be an alternative, But I don't know about its safety with food application. Someone else may have a suggested solution.

[fox]

You can try water glass or a water glass product, most commercially sold water glass is expensive but you can YouTube how to make your own.
I am not sure how well it will work with perlite but it is used to make vermiculite boards so it definitely hardens and seals vermiculite .

[Gulf]

Hi jkaiser1,

Welcome to the forum. I'm very sorry to hear of your problem. Many have been influenced by the notorious youtube videos depicting the quick and easy gym ball and other methods of building a WFO with pelcrete or vermicrete for the cooking chamber. That method is very popular on social media and has led many to waste time and money. If you do some searches of this site for "gym ball", "exercise ball", or "medicine ball, you will find others who have come here with the same problems. Most of those will be located in Other Oven Types (for lack of a better category). There you will find many suggestions to rescue this type of build. Sadly, we don't get much feedback on how well the repairs work for the long term.

As fox advises, "water glass" is a substance some knife makers use with perlite or vermiculite for small gas fired forges. Normally it is mixed in with perlite or vermiculite as it is formed. I'm not sure that this will have the same effect as a painted on surface over a portland mix.. Note: forging metal does not require heat retention. On this forum, we recommend perlite and vermiculite only as insulation. We do not recommend it for the cooking chamber which should be a dense material. Please, let us know how what repair you decide on works in the long term.

[Petter]

I could try Sodium Silicate today. Got some perlite concrete and will bake in the evening.

[Petter]

Diluted the SS 1:1 with water. About 5 mm deep in the jar. Let a 1:25 cementerlite cast absorb for one minute.




Got a capillary rise of about 20 mm.



Lifted it out do dry on the jar edge, wet side down. Will cure it later today and put it in the fire for a few hour to see what happens.

[Petter]

After 1 h drying under the chimeny and 2 h in the fire/coals (700-800 C):

the right is the SS-coated perlcrete. The left (untreated) side show signs of wear at the edges from log drops. It is also brittle and falls apart when poked on. The treated side is way more robust and though, a 1:25 mix!!. It does not crumble when poked and is still intact.

I would with no doubt recommend you to coat it. Can you remove the dome and turn it up-side-down?

Use a garden sprayer to apply. Wear mask and eye-protection. Coat it twice with a 1:1 blend and then a single coat of only Sodium Silicate only.

Let it dry one day between coatings.

Cure very very slowly. Keep a dome temperature of 50-70 C for a few h using coals. Then increase to 120-150 C keep for 2 h. Then you are set to go. Too fast temperature increase will make the bonded water go to steam too fast and form blisters.

Good luck!

[Petter]

I would guess you need 2 liters of SS for a 92 cm oven.

[wolfmoonex]

This is awesome Petter, thank you for taking the time to do this. I am going to make a chimney out of this mix for my oven experiments.

So again the mix is 1 part of 1:1 SS:water, 1 part cement, 25 parts perlite ? can't believe that your mix was just one part cement but if it was, that is such a small part, i bet i could swap out portland with something more refractory like hydrated lime or washed wood ash

[Petter]

Welcome, glad you found it helpful.

Yes, I had 1:25 in volume ratio. However, it is nothing I recommend since the mix is very sensitive to homogenity and water addition. It was as lean as I could go and still get a firm mix to render against. A standard 1:10 mix is a lot more workable but has less insulating capacity.

My Sodium Silicate was about 40 % solid. I diluted it with equal amount of water, so the final mix had about 20 % solid content.

I don't recommend to do a oven of simple a cement perlite mix with sprayed Sodium Silicate. This is a way to rescue an already existing oven with a bad design and poor choice of material. If you're building a new, do it with 2" homebrew cast and then apply the cement perlite mix. It will distribute the heat better and be a lot easier to maintain constant temperature.

[Ronstarch]

I too built one of these perlcrete ovens in 2017. I didn't have the issue of perlcrete falling on pizza, but I wanted to see if I could improve the performance of the oven. So, I turned it upside down and coated the inside with a ~1 cm layer of castable refractory (I sieved out the larger aggregate). This is basically building a pizza oven from the outside in, starting with the insulation layer, and then adding the dense refractory layer. It held up for as long as I had it (until last year), when the house was sold.

I think Petter has a great idea as well, and I imagine the sodium silicate will adsorb/adhere to the perlcrete and not flake off. There's always the risk that if you add castable or homebrew, it cracks and crumbles, being a thin layer (although that didn't happen in my case).

Just my experience, YMMV

[Gulf]

Petter, Ronstarch,
Thank y'all both for the follow ups. A lot of folks have built these type ovens. Any tips on retro-type repairs that work will help someone else.

[wolfmoonex]

Appreciate the input @Petter

Will not be doing a full oven with your mix, just the chimney pipe

Hi @Ronstarch - mighty fine idea. It can rescue all those endless perlcrete ovens on youtube

I too built one of these perlcrete ovens in 2017. I didn't have the issue of perlcrete falling on pizza, but I wanted to see if I could improve the performance of the oven. So, I turned it upside down and coated the inside with a ~1 cm layer of castable refractory (I sieved out the larger aggregate). This is basically building a pizza oven from the outside in, starting with the insulation layer, and then adding the dense refractory layer. It held up for as long as I had it (until last year), when the house was sold.

THIS ^ @jkaiser1, post some pictures of before and after. would love to see

[chrisrizzo]

Thank you so much for taking the time to do this. For my oven experiments, I'm planning to create a chimney out of this mixture.

[david s]

Because the high proportion of perlite in the mix reduces the strength enormously, some fibres in the mix would help to improve strength. I’d be using 19mm AR fibreglass fibres as their melting point is well outside the service range of the chimney and their easy to work with.