Feedback from Perlite/Cement oven owners?

[mkasap]

Good evening david s , It seems that is not that easy to find powdered clay in my area. I asked quite a few pottery shops and they don't sell powdered clay. The only one i was able to find is soft ready-made clay sealed in nylon bag. I guess i could take that and let it dry and then somehow smash it until it becomes powder. It costs around $10 per 8kg. I guess the actual powdered clay is half the weight when dried? Here comes my question: Could you please let me know approximately how many kg of powdered clay i will need for a 65cm dome and 5cm thick, using the 3:1:1:1 ratio? I want to make the calculations just to see if it worth it or i will go with the fire bricks solution. Thanks.

[david s]

Potters require powdered clay to make their glazes and make slip. You should ask where they get their supplies. They should also be able to tell you where you can source clay from a local deposit. Usually there is a layer of clay beneath the subsoil. Building sites when foundations are being dug is a good source.
The volume of your casting is around 20 litres, so as 1/6th of this is clay (3:1:1:1) you’‘ll only need 3 litres. However there is a reduction in volume by around 20%, when you mix all the materials together with water, plus you will need some more clay to help bind the vermicrete or perlcrete layer so let’s say 6 litres. Powdered clay is also used by Bricklayers and is available from builders suppliers labelled either as “fire clay” or “Bricklayers clay”. I find this is the cheapest source. You could buy a block or two from your potter, cut it into blocks no bigger than your fist with some fishing line, allow it to dry for around a week in the sun,then pulverise and sieve it. Just takes a fair bit of effort.

[mkasap]

Got the ready-made clay tonight and cut
it in pieces and let it dry. When it’s fully dried and processed, i will know how many litres i can make out of the 8kg. Btw, what type of sand shall i use? I only have access to the common blueish all-purposes sand. Hope it does the job.

Potters require powdered clay to make their glazes and make slip. You should ask where they get their supplies. They should also be able to tell you where you can source clay from a local deposit. Usually there is a layer of clay beneath the subsoil. Building sites when foundations are being dug is a good source.
The volume of your casting is around 20 litres, so as 1/6th of this is clay (3:1:1:1) you’‘ll only need 3 litres. However there is a reduction in volume by around 20%, when you mix all the materials together with water, plus you will need some more clay to help bind the vermicrete or perlcrete layer so let’s say 6 litres. Powdered clay is also used by Bricklayers and is available from builders suppliers labelled either as “fire clay” or “Bricklayers clay”. I find this is the cheapest source. You could buy a block or two from your potter, cut it into blocks no bigger than your fist with some fishing line, allow it to dry for around a week in the sun,then pulverise and sieve it. Just takes a fair bit of effort.

[david s]

Any sand should be ok. A mixture of grain sizes is preferable but go with what you can obtain. Try to obtain some polypropelyne fibres to add to the mix. Proprietary castable refractory mixes usually have similar fibres already added to the mix. They melt at 160 C leaving a network of tiny pipes that excess steam can escape through. This gives some protection from steam spalling without reducing the density of the casting. Whilst not essential they are certainly helpful. These fibres are around 15 mm long and finer than human hair. They are commonly used in the concrete industry as a replacement of steel reinforcing. They usually come in a pack of two different fibre types, the really fine ones and some longer (30 x.05 mm) nylon ones, but don’t add these ones. There are many different brands, the ones I usually use are Novomesh 950. For every 10 litres of mix you only need a small handful, but they need quite a bit of mixing to properly disperse. For brick builders using home brew I’d recommend adding them to the home brew mortar, especially helpful where thick joints are filled in the outer gaps between the bricks.

[mkasap]

Alright, cool. Once I have news i will update the post accordingly. I will make a small testing bar like i did with the perlcrete just to make sure that my mix works as it should. Thank you David and have a lovely day.

[mkasap]

Good evening and Happy New Year everyone! Today i pulverized the dried pieces of clay and sieved with the sand, lime and portland. I made a small bar and covered it with a wet towel. I checked it after 5 hours and you can already feel that it's becoming solid. That put a smile on my face!! I will let it dry for 1 week and hopefully i will get the output i want.
QUESTION: My sand has quite a few small round rocks (2-4mm). Shall i filter them out of my mix or it's not a problem?

[mkasap]

[david s]

The sand should be a mix of grain sizes so don’t discard the larger grains. The clay should be ground to a powder.

[mkasap]

Great, thanks. Clay was transformed to powder. What you see in the photo it’s the “leftovers” from pulverizing.

[mkasap]

Im thinking to create a vertical mould for the first 20-30cm of the dome and after 2-3 days continue building the rest of the dome based on the sand mould. Would it be a good idea to put a chicken mesh so i have a good bonding between the two parts or the heat will expand the metal mesh and crack my dome?

[david s]

Im thinking to create a vertical mould for the first 20-30cm of the dome and after 2-3 days continue building the rest of the dome based on the sand mould. Would it be a good idea to put a chicken mesh so i have a good bonding between the two parts or the heat will expand the metal mesh and crack my dome?

Chicken wire, being thin should not present problems from expansion because the greater surface area allows more heat dissipation than thicker steel. However, being coated with zinc which has a lower melting point than the oven might see, could be a problem. Once the protective coating has gone corrosion might start. Not sure though as the only way to tell would be to try it and break the oven apart after years of use. I tried using chicken wire reinforcing for the fourth oven I built and it’s still operating ten years later, no idea what the condition of the chicken wire would be. Another consideration is that zinc vapours are highly toxic although being embedded in the middle of the casting should prevent release. The recommended reinforcing for castable refractory is melt extract fibres which are made of stainless steel. They are around 0.5 mm thick and being stainless are resistant to corrosion. I use them in the manufacture of my product, but they are not absolutely necessary. Most manufacturers don’t use them as I’m sure they’d advertise if they did; they are a pain to work with as the make handling the mix difficult as well as being expensive. Sorry about the long winded reply, but you did ask.
Dave

[mkasap]

Thank you very much for your long informative reply, that’s exactly the reason why i’ve asked you specifically. Im taking your suggestions very seriously because after the perlcrete catastrophe i want to build something long-lasting but also eliminate any possible sources of contamination. My chicken wire is indeed very thin but i’m not gonna risk it because i can’t be sure that i will install it under the inner surface all over so, it’s rejected. I will only use it in the insulation layer. Another concern i have is regarding making my oven portable. I want to create a metal base approx. 140x90x90cm (LxWxH) to build my oven on, with four heavy duty wheels. Do you think my oven will suffer from vibrations from moving it around the house, increasing the risk of cracking? Thanks again and im sorry for bombarding you with all these questions.

[CoastalPizza]

...create a vertical mould for the first 20-30cm of the dome and after 2-3 days continue building the rest of the dome based on the sand mould....

This is exactly how I built mine - see the build thread link.
Post #4 shows the finished ring mold
Post #9 shows the cast ring after the mold is removed
Post #10 shows the sand filling the ring and the casting of the domed top
The casting was done using the 3:1:1:1 formula home-brew that David S recommends with about 2% by weight stainless steel needles added.

[mkasap]

Thank you very much CoastalPizza. Your oven looks rock solid - well done!! Did you have any issues with those small holes on the dome after several fires? Photos are saved for future reference

[mkasap]

david s May i have your feedback please, regarding making my oven portable? Thank you.

Thank you very much for your long informative reply, that’s exactly the reason why i’ve asked you specifically. Im taking your suggestions very seriously because after the perlcrete catastrophe i want to build something long-lasting but also eliminate any possible sources of contamination. My chicken wire is indeed very thin but i’m not gonna risk it because i can’t be sure that i will install it under the inner surface all over so, it’s rejected. I will only use it in the insulation layer. Another concern i have is regarding making my oven portable. I want to create a metal base approx. 140x90x90cm (LxWxH) to build my oven on, with four heavy duty wheels. Do you think my oven will suffer from vibrations from moving it around the house, increasing the risk of cracking? Thanks again and im sorry for bombarding you with all these questions.