Feedback from Perlite/Cement oven owners?

[UtahBeehiver]

I see you are still considering a perlite mix for your core dome rather than Dave's suggestion of a dense refractory whether it be a commercial product or a home brew version, neither one has perlite or vermiculate in it for the core dome.. I just want to point out that Dave does cast oven for a number of customers so he knows his stuff.

[david s]

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!

This is great. There’s nothing like doing your own tests. Keep them damp for at least one week then another week drying in the sun. It is instructive to weigh the samples daily to measure the water loss. Here is a table of vermicrete strength and insulative values with a range of different mixes. Perlite and vermiculite are pretty much interchangeable in their qualities.Not sure how volcanic Crete is, but many Greek islands abound in pumice which is free on beaches and a pretty good source of insulative aggregate, being quite a bit stronger than perlite it would be a preferable alternative if you are still sold on making an insulative inner dome.

[mkasap]

Hi UtahBeehiver. After reading David's comments, navigating through different posts in here and taking notes, i can clearly understand that a perlite oven is not even close to a proper pizza oven. As i mentioned in a previous comment, i will go ahead with the perlite project just to get some experience with mixing and develop some building skills. Hopefully, after the perlite oven i will go for a proper wfo with firebricks.

[mkasap]

David, thank you very much for the table. Could you kindly explain what the wet density, dry oven density and K factor is?

[UtahBeehiver]

K values are Thermal Conductivity, in other words, how heat transmit in a given material, K values are either in W/m C (metric) or Btu in (hr ft2 F) standard. In the case of the table that Dave posted the K units are in standard. So the higher the number the more quickly the heat transfer thru the material. A dense material will store heat but not insulate well and a insulated material will keep heat from transferring but won't store heat as well, hence a dense material for dome followed by a insulating material over the dome is the best method for an oven.

[Gulf]

i will go ahead with the perlite project just to get some experience with mixing and develop some building skills. Hopefully, after the perlite oven i will go for a proper wfo with firebricks.

Mkasap,

I understand the feeling of wanting to rush ahead and and get what seems to be quick results. The infamous gym ball/perlite video makes it all seem so possible. The truth is, David's reccommendation of the sand mold is much quicker and easier. It is cast in place with no need to move the shell after the pour. The formulas that you are experimenting with are only a couple of ingredients away from a very great performing and long lasting oven. Just figure in a little sand, and some clay. I would save the perlite for the insulating layer.

I dont mind losing some thermal mass as i will be cooking pizzas only, for family and friends.

That's all good, if that is all that you are interested in. I never refer to my oven as a "pizza oven". I use the term "wood fired oven". My first interests in wood fired cooking resulted from repeated losses of electrical power due to tropical storms and freezing rain. Pizza is just something I do when firing the oven up to cook real food lol. My wfo has really taken a load off of the conventional oven. It has greatly expanded the number of dishes that we can put together for a family get-to-gether or a soiree. In the summer months, it has take a load off of the airconditioning needed to cool the house. The breads, the roasts, too many dishes to note in one sentence all have been made much better in the wfo. Although it does great pizza, I'm glad that I did not limit myself to a "pizza oven".

Your willingness to ask questions leads me to believe that you want to do this right. There are two ways to get there. Neither one of them require that you build an oven to learn from your mistakes. A dense castable is one and firebrick is the other. The proper methods to both are available here on this site. It just takes some reading and asking some questions. In my opinion, the best documented ovens on this site were built by folks that had no building skills. But, they were willing to do their resarch on the site and ask questions.

That said, I'm looking forward to the feedback on how well the gym ball/perlite oven performs over time.

[mkasap]

Dear UtahBeehiver and Gulf, thank you very much for taking the time to answer my questions and also give me vital advices/recommendations. I greatly appreciate it. Something i forgot to mention, another reason i want to start with the perlite gym ball oven, is because next year we will move to a new house so im planning to build the “good version” of my oven there, following your comments and posts from other members. So, for now i just want to experiment with the perlite version and cook some pizzas at the end to reward my self lol.

Gulf, your oven sounds really great! A beast! I wish someday i will be able to build an oven simiral to yours!

[mkasap]

Hello everybody. I have updates regarding the experiment i did one week ago with the three different types of mixing ratios with perlite, portland, lime and cement enhancer. We had some warm days so the 3cm thick bars were fully dried so i decided to test them out today. The perlite/portland bar and the perlite/portland/lime bar were a total disaster. As david mentioned in previous post, i was able to scratch them off with my fingernails. The third bar with the cement enhancer was looking sturdy and i couldnt scratch it even with a wire brush. But, then i remembered David's comment (THANK YOU) that most probably the enhancer will be useless in high temperatures so i've put scrap wood in my bbq, placed the bar at the top and light the fire. After 2 hours on high temperatures i removed the bar, let it cool down and disappointedly i noticed that it became the same weak as the other two. So, because my family's health is above every fun project, i decided to quit perlite for the dome. When i initially started this post i wanted to get some feedback from people who used perlite for the dome but couldnt find any, so now i'm giving a feedback/friendly advice to all folks out there that are considering doing the same. JUST DONT USE PERLITE FOR THE INNER DOME unless you want pizzas seasoned with cement/perlite. Dear david s , since you are the casting guru, i would appreciate if you could give me a recipe for a cast dome. Unfortunately, bricks are out of budget and the materials i have so far are: portland, lime, perlite (for insulation layer), refractory cement (bought a 25kg sack today). Will get some sand as well on monday. Thank you in advance and thanks again for all the precious advices so far. Regards from Cyprus.

ps: when i say cast dome i mean the one with the sand mould (unless you suggest otherwise)

[david s]

Results as predicted, but there’s no better learning than proving it for yourself. You have a couple of options.
1. Use the refractory cement to make a castable refractory. Be careful though as i’m Not sure exactly what you have. It could be pure calcium aluminate cement, a refractory mortar (ref cement and fine aggregate), or castable refractory (ref cement, powdered clay, both fine and coarse high temperature aggregate and burnout fibres)
2. Make your own castable refractory using the homebrew ie. 3:1:1:1 sand, Portland cement, hydrated lime, powdered clay, Also highly recommend adding polypropylene fibres to this mix (they burn out >160c creating a network of mini pipes to help steam escape and because they’re so fine do not reduce the density of the mix).
Read the thread I linked to on an earlier post which will address many questions.

[mkasap]

Good morning David. I’ve taken a couple of photos of the refractory cement pack. Im not sure what is exactly lol. When you say powered clay you mean the refractory cement? Thanks.

Results as predicted, but there’s no better learning than proving it for yourself. You have a couple of options.
1. Use the refractory cement to make a castable refractory. Be careful though as i’m Not sure exactly what you have. It could be pure calcium aluminate cement, a refractory mortar (ref cement and fine aggregate), or castable refractory (ref cement, powdered clay, both fine and coarse high temperature aggregate and burnout fibres)
2. Make your own castable refractory using the homebrew ie. 3:1:1:1 sand, Portland cement, hydrated lime, powdered clay, Also highly recommend adding polypropylene fibres to this mix (they burn out >160c creating a network of mini pipes to help steam escape and because they’re so fine do not reduce the density of the mix).
Read the thread I linked to on an earlier post which will address many questions.

[mkasap]

[david s]

What you have is a high temperature refractory mortar, probably around 20% calcium aluminate cement and 80% high temperature fine aggregate and some other goodies. It’s designed for thin joints between bricks, not really suitable to use as a castable mix.You would need something like 5 bags for an oven and as you’ve already found, the stuff is quite expensive. IMO the homebrew mix would suit your needs far better as well as being far cheaper.

When I say powdered clay I don’t mean cement. I mean dry powdered clay. Most any clay should suffice, but avoid Bentonite as it’s extremely small particle size leads to excessive shrinkage. You may be able to dig, pulverise and sieve your own. Alternatively try Bricklayers clay from building suppliers, or powdered clay from a pottery supplies outlet. If there are local potters they will be able to direct you.

[mkasap]

Okay got it. How many days should i let the homebrew mix to dry before perlite layer and then for the first small fire? UPDATE: This question is answered by reading other posts and James' guidelines for firing.

One more question regarding powdered clay; A few kilometers away from here i live there are fields with red natural soil. If i sieve this red soil and get the powder, is that okay to use? Thanks.

[david s]

The homebrew should be kept moist for at least a week. Then allowed to dry for at least a week after sand mould removal. If adding a perlite layer over it use 10 parts perlite, one part Portland cement, three parts water and a little powdered clay to help bind it. Cover the casting in this mix in a layer of around 35 mm thick and allow to dry for another week. Do a second and maybe a third layer again with a week to dry in between so the total thickness is around 100 mm. Doing it in layers like this and allowing it to dry in between is important so water doesn’t get locked in, as it’s hard to remove. Apply with the hands from the bottom to the top, but wear rubber gloves.
Your red soil may contain some clay, but the clay layer is usually beneath the topsoil. Consult a local potter for advice on access to local clay deposits.

The curing thread is under “Oven Management” read it all to get an idea of what it’s about. Start here
https://community.fornobravo.com/for...curing#post833

[mkasap]

Hi David. For once more, i cant thank you enough. Will update the post after having ready all the materials needed. Have a great day.