Feedback from Perlite/Cement oven owners?

[mkasap]

Hello everybody. Im ready to start building my very first wood fired pizza oven using gym ball, perlite, portland cement, cement enhancer, chicky mesh and firebricks for the base only. Ratio 5:2. I would appreciate any advices/recommendations and i would also like to hear feedback from owners of similar ovens about the performance and if perlite started falling on to the pizza after several fires? Thanks in advance.

[Gulf]

If you haven't already, read through the threads that others on here have started pertianing to this design. We haven't got a lot of feedback on just how well this type oven performs. From what I see, there are very few pros and some very serious cons to this method.

Good Luck

EDIT: I just read your post in its entirety. I should have done that to begin with. Like you, I am looking forward to any and all feedback from builders who have used this method.

[mkasap]

Good morning Gulf, thank you for your quick reply. Could you please share with us the cons you’ve read regarding this type of pizza oven?
Thanks again and have a great day.

If you haven't already, read through the threads that others on here have started pertianing to this design. We haven't got a lot of feedback on just how well this type oven performs. From what I see, there are very few pros and some very serious cons to this method.

Good Luck

EDIT: I just read your post in its entirety. I should have done that to begin with. Like you, I am looking forward to any and all feedback from builders who have used this method.

[UtahBeehiver]

We see the use of inflatable gym balls as cast molds here and there. If you read some the build post, especially from David S of Australia (our resident casting expert) you will see the he has commented about gym balls often and he recommends a sand form over a gym ball. He also recommend a "dense" cast refractory for the dome then either ceramic fiber blanket or pcrete 8 to 10 to 1 for insulating the dome, We see a number of build using a p or v-crete low ration cement for their main dome but never get any long term feedback from them on durability. If it were my build, I would have more faith in David S recommendations that a one off builder, Dave builds dozens of these cast ovens.

[mkasap]

Thank you very much for the very informative and
detailed response. I will go ahead and read the posts from David S and take my notes! Thanks again, have a great weekend!!

We see the use of inflatable gym balls as cast molds here and there. If you read some the build post, especially from David S of Australia (our resident casting expert) you will see the he has commented about gym balls often and he recommends a sand form over a gym ball. He also recommend a "dense" cast refractory for the dome then either ceramic fiber blanket or pcrete 8 to 10 to 1 for insulating the dome, We see a number of build using a p or v-crete low ration cement for their main dome but never get any long term feedback from them on durability. If it were my build, I would have more faith in David S recommendations that a one off builder, Dave builds dozens of these cast ovens.

[mkasap]

Dear david s i've read quite a few of your comments and it seems that you are not fan of the perlcrete and gym ball ovens In my case, i have no experience in building ovens so im planning (havent started yet) to give it a try as it seems the most doable for persons like me without any experience. The mix would be with perlite, portland and cement enhancer for the first layer of about 70-80mm thick and then for the second layer i plan to add chicky wire and another layer of 50-60mm of perlcrete. The ratio would be 5:2:2. My question and biggest fear is if that mix will start flaking off after several fires? I dont mind losing some thermal mass as i will be cooking pizzas only, for family and friends. Many thanks.

[david s]

Hi Mkasap,
No I’m not a fan of the perlite/gym ball method for a couple of reasons. Firstly, as it’s only the top half of the ball that you want to cast then you will have to create some extensive form work around the ball or dig a hole in the ground, bury the bottom half and cut a plate to fit half way. After completing the casting it will have to be demoulded and moved into position and bedded onto the insulating slab. This is where the casting will be subject to damage. By contrast a sand hemispherical mould built where the oven will be requires no moving of the casting, the sand is simply dug out after the casting is completed. Secondly a perlite and cement mix has drawbacks, primarily in strength as it produces a casting that is not very strong, you could scratch it away with your fingernails. While this will work, it won’t stand up to knocks and abrasions, neither will it hold much thermal mass so it is imperative to keep a reasonable fire going constantly on the side, so forget retained heat cooking. As Portland cement degrades under anything north of around 300 C it’s anyone’s guess as to how long this mix will stand the test of time. It is usual to add lime to the mix so it can take over where the Portland cement fails. Not sure what your recipe calls for where you said “cement enhancer”. Whilst the perlite itself is good for up to 1100 C, plenty for ovens, one of its qualities is that it takes up lots of water on mixing and most of this needs to be eliminated or you will have problems if it suddenly turns to steam.
Have a read of this well documented build. https://community.fornobravo.com/for...and#post396954 if you are still set on doing the gym ball/perlite thing, will you report on its performance as no one else has? I suspect they work ok but not long term.

[mkasap]

Thank you for your reply David. By "enhancer" i mean the liquid you put into the mix (you replace part of the water) to make your build more strong, "elastic" and durable over vibrations, cracks, etc. so the risk of cracking while moving and setting on the slap is somehow reduced.I totally understand your concern regarding portland cement; Do you suggest any other kind of cement? My local store has a cement that is heat resistance (up to 1110c i think), but it costs $40 per 25kg. Do you believe if i use this type of cement will eliminate the risk of flake off in the future? Thanks again for taking the time to reply to my comments.

Hi Mkasap,
No I’m not a fan of the perlite/gym ball method for a couple of reasons. Firstly, as it’s only the top half of the ball that you want to cast then you will have to create some extensive form work around the ball or dig a hole in the ground, bury the bottom half and cut a plate to fit half way. After completing the casting it will have to be demoulded and moved into position and bedded onto the insulating slab. This is where the casting will be subject to damage. By contrast a sand hemispherical mould built where the oven will be requires no moving of the casting, the sand is simply dug out after the casting is completed. Secondly a perlite and cement mix has drawbacks, primarily in strength as it produces a casting that is not very strong, you could scratch it away with your fingernails. While this will work, it won’t stand up to knocks and abrasions, neither will it hold much thermal mass so it is imperative to keep a reasonable fire going constantly on the side, so forget retained heat cooking. As Portland cement degrades under anything north of around 300 C it’s anyone’s guess as to how long this mix will stand the test of time. It is usual to add lime to the mix so it can take over where the Portland cement fails. Not sure what your recipe calls for where you said “cement enhancer”. Whilst the perlite itself is good for up to 1100 C, plenty for ovens, one of its qualities is that it takes up lots of water on mixing and most of this needs to be eliminated or you will have problems if it suddenly turns to steam.
Have a read of this well documented build. https://community.fornobravo.com/for...and#post396954 if you are still set on doing the gym ball/perlite thing will you report on its performance as no one else has. I suspect they work ok but not long term.

[david s]

Thank you for your reply David. By "enhancer" i mean the liquid you put into the mix (you replace part of the water) to make your build more strong, "elastic" and durable over vibrations, cracks, etc. so the risk of cracking while moving and setting on the slap is somehow reduced.I totally understand your concern regarding portland cement; Do you suggest any other kind of cement? My local store has a cement that is heat resistance (up to 1110c i think), but it costs $40 per 25kg. Do you believe if i use this type of cement will eliminate the risk of flake off in the future? Thanks again for taking the time to reply to my comments.

Your “enhancer” sounds like an acrylic PVA additive designed for concrete. While it works quite well for standard concrete I would expect it not to be able to withstand the temperatures experienced in an oven. You might like to research this aspect. Also I think that it will interfere with the removal of water once your casting is set and you want to dry it out, because it acts as a waterproofing agent as well as enhancing strength. I use it in areas that are not subject to anything north of 200 C and where I want to retain porosity.

The cement you have access to sounds like calcium aluminate cement and yes it is expensive as well as being more difficult to work with in that it has a very short pot life (hardens fast). A cheaper alternative is to use 50/50 Portland cement and hydrated lime, as previously posted the lime takes over where the Portland starts failing.

[mkasap]

As this is my first trial, i think i will go with the 50/50 solution of portland and hydrated lime and once i got some experience i will go for the sand hemispherical mould design and proper firebricks for the dome's inside. I will post some photos and provide feedback in a few days. Thank you David.

Your “enhancer” sounds like an acrylic PVA additive designed for concrete. While it works quite well for standard concrete I would expect it not to be able to withstand the temperatures experienced in an oven. You might like to research this aspect. Also I think that it will interfere with the removal of water once your casting is set and you want to dry it out, because it acts as a waterproofing agent as well as enhancing strength. I use it in areas that are not subject to anything north of 200 C and where I want to retain porosity.

The cement you have access to sounds like calcium aluminate cement and yes it is expensive as well as being more difficult to work with in that it has a very short pot life (hardens fast). A cheaper alternative is to use 50/50 Portland cement and hydrated lime, as previously posted the lime takes over where the Portland starts failing.

[mkasap]

david s I just noticed that the cement i bought says “Portland limestone cement”. Does that make any difference or i still need to buy a hydrated lime? I apologize if thats a stupid question but as i mentioned above i have no experience. Cheers.

[david s]

You will still need to buy the hydrated lime. Portland cement is made from limestone. Make sure that the lime you get is hydrated lime used for building, plastering walls etc. dont use agricultural lime used for gardens. The lime is very harsh on the hands so wear rubber gloves. No problems, it was a good question.

[mkasap]

Got it, thank you.

[mkasap]

I decided to do a stress test on my materials before starting my project. I made three cement bars approx. 25x5x3cm using three different ratios and materials. On the attached photo below, the bar on the left is made using a 5:2:2 ratio (Perlite, portland, water). The one on the right is 5 perlite, 2 portland, 1 water and 1 acrylic cement enhancer. The top one is 5 perlite, 1 portland, 1 hydrated lime and 2 water. Underneath i’ve put nylon to replicate the gym ball and I covered them with wet towel. In 7-10 days i will scratch each one with a screwdriver to test their strength and i will burn them using a heat gun. I hope this test will also help other folks that are planning to build the perlcrete oven. Stay tuned hahah!

You will still need to buy the hydrated lime. Portland cement is made from limestone. Make sure that the lime you get is hydrated lime used for building, plastering walls etc. dont use agricultural lime used for gardens. The lime is very harsh on the hands so wear rubber gloves. No problems, it was a good question.

[mkasap]