A few questions

[csnewton8]

Hi All,

I have built my dome using a mix of 5:1 perlite/cement fondu, with a cement fondu/sand render over the top. In the next few weeks I intend to cover with insulation blanket (secured with chicken wire) and finish with a waterproof render and paint.

If I knew what I know now, perhaps I wouldn’t have chosen a perlite / cement fondu mix for the dome as a lot of people advise that a castable refractory should be used as this has a higher thermal mass.

My question is...is cement fondu classed as a castable refractory (mixed with cement) or is this something completely different? Will a mix of perlite and cement fondu cause issues down the line?

If issues are foreseen, is there anything I can do to salvage my oven? I have thought about skimming the inside of the dome but I am not sure what materials I would need.

A few other questions I have if anyone can help:

- When placing my oven on top of firebricks, what are my options if I do not have the tools to cut the bricks? Could I Just build a square platform using the bricks and mortar the dome on top? Would this cause issues with heat transfer to bricks that are exposed outside of the dome? And to resolve this, would you advice that I cover the expose bricks using a refractory cement?

- Is there anything people recommend to place on top of fire bricks to provide a smoother surface or are fire bricks sufficient to cook on directly as long as they are level with no raised areas.

Any advise provider would be really appreciated.

Thanks,
Csnewton8

[david s]

The problem with a 5:1 perlite cement mix is that it is not very strong. You will be able to scratch it away with your fingernails. This means it will be subject to knocks and abrasions in use. In addition, as it is an insulating mix it will be difficult and slow for heat to penetrate the whole dome because the mix is not very conductive and because it has little thermal mass it won’t be able to store heat.
The Ciment Fondu is a trade name for calcium aluminate cement. Castable refractory is a mix of high temperature aggregates combined with calcium aluminate cement plus other goodies in the mix. Skimming the inside of the dome with a render mix is unlikely to be successful as a thin layer is likely to crack or flake off. Also the moisture in the mix will be sucked out of it so fast it may not get a chance to hydrate properly.
Regarding the floor, firstly make sure you have insulated under the floor bricks, or you’ll not be able to maintain decent floor temperatures. You can place the dome over the floor and cut off any bricks that protrude beyond the outside of the dome. An angle grinder with diamond blade and a brick bolster will do the trick. It can be pretty rough because they’ll be covered by the insulation.The whole oven under the floor and over the top needs to be encased in insulation.Floor bricks are best laid loose to allow for thermal expansion. If they are a bit uneven a dry levelling mix of 50/50 sand powdered clay say 1/2” thick can do the trick.

[csnewton8]

Thanks for the advice David!

It does make me wonder why there are so many videos on YouTube which instruct people to use perlcrete or vermicrete domes if they can’t hold heat. It’s just a shame I came across the Fornobravo forum after watching and following these videos.

I’ll make sure I buy an angle grinder to cut the fire bricks when I reach that stage of my build.

If anyone can let me know how to protect the perlite/cement fondu mix on the inside of my dome that would be great. I would prefer to continue rather than scrapping everything I have done so far, as I’ve spent a fair bit!

[UtahBeehiver]

David pretty much sums up your options. Yes, it is too bad builders get sucked into building a pcrete or vcrete domes off of YouTube. We try and caution builders that there are a lot of videos out there that are not necessarily good builds. For the floor under the bricks, you can use the same mix and ratio of pcrete to insulate the floor, at least 4" thick.

[david s]

What you have will work to cook pizzas, but there won’t be sufficient thermal mass in the oven to do retained heat cooking. However, by maintaining an active fire you still may be able to roast in it. The strength of the dome or lack of is still an issue so be really careful not to damage the surface. I have no idea how long it’s likely to last, but no one has reported long term survival, despite invitations to do so.
The attachment shows relative strengths of perlite/vermiculite mixes. Note the 5:1 mix is around 200 psi. By contrast a proprietary castable refractory is around 3-4 psi. As I mentioned in my previous post the rendering of a stronger mix on the inside is not likely to be successful, but if you want to give it a go you will have to thoroughly wet the dome or it will suck the moisture out of the applied mix and not leave enough for the hydration process. Also if you make it too wet there will be issues with the mix bonding. It requires somewhere between wet and dry, difficult to judge.I'd be most interested to see how it goes if you want to give it a shot, so report back if you do.

[Gulf]

It does make me wonder why there are so many videos on YouTube which instruct people to use perlcrete or vermicrete domes if they can’t hold heat. It’s just a shame I came across the Fornobravo forum after watching and following these videos.

When I first started researching, a search for a DIY pizza oven, would bring up about 8 out of 10 hits for the Forno Bravo Forum on the top lines. Social media has changed that.

To directly respond to your statement, It was kind of like an epedimic. One person put together a well crafted video of their build on youtube. Structurally wrong, but it was a nice put to gether video for those that did not know any better than to follow along. That begat copycats on social media that wanted to show off their easy builds. Many folks were and are looking for the easy way out. There are now (too many to count) on youtube and other video hosting sites. Social media has many wood fired oven groups which have helped spred the epidemic.

Truth is, I would have probably fallen for the BS, if I had been looking for a solution this day and time .