Hello,
I am building the base layer on top of my reinforced slab. Does the perlite need o be mixed with refectory cement or can i use regular portland cement?
Also when I get to that stage can I make an extra thick perlite portland layer over the brick dome instead of using a ceramic thermal blanket?
Thanks,
Philippe
Hello, Philippe, and welcome to the BrickWood forum!
You have two different insulation needs going on here. The insulated base keeps heat from conducting into the cold (and thermal energy-hungry) concrete below it. The firebrick that form the hearth are refractory and are sufficient on their own to provide a sizzling hot surface for food. Using refractory mortar below that would be a waste, so stick with the Portland cement and save yourself about a hundred bucks.
Before I get to the second part of the question, there is a related fact: the Perlite/Portland cement mix is not particularly strong and would eventually break down under prolonged direct exposure to high heat and flame. Where we are putting it in the base, that’s not an issue.
The arch presents a different set of conditions. You have direct flame, conductive heat, convective heat, and the tendency of convection for heat to rise as cold air is drawn in. Perlite would provide some insulation but the level of thermal energy is enough to heat it up and make its way on through. The mineral fiber blanket is not prone to that. Both Perlite and mineral fiber work by trapping air, but the fiber is orders more efficient at doing so.
On balance, you are investing resources where they are needed by pouring the simple (and cheap!) Portland/Perlite mix into the void, and blanketing the arch with mineral fiber where its superior (and more expensive) properties are needed.
Do that, and your oven will perform like a champ.
Hope your build goes well, and that you’ll come back to let us know about your progress and show us a few photos.
Listen to Matt he has the right take on this
One thing I am condsidering is adding 13/16" Cordierte floor to the oven
it will add insulation and bring the floor closer to the dome.
Cordierte is the material that they use in the more expensive dome type brick oven
Just and idea
I 'll keep everybody posted
Thanks guys,
My concern is that the oven is outside here in Bermuda and it is all really damp and rains a fair bit. It is not easy for me to get the thermal blanket imported but I am more worried about a leak happening eventually in the outer plaster shell and then the blanket being a void that soaks up moisture across teh inner surface of the whole outer dome. We make our roofs here traditionally out of Bermuda limestone slabs or tiles with a coat of plaster and then cement paint. If a crack occurs it can saturate the limestone at the crack but it doesn’t spread and is easy to repair. Newer roofs use cem ply and and foam layer with a fiber reinforced plaster and paint, its strong but when that leaks it spreads across a larger area of roof gets under the foam and never dries out !
That is my concern with putting ceramic blanket over the brick dome - can I just go thicker with multiple layers of the perlite portland mix ? or is there something else? Or am I concerned for nothing?
Thanks for your help,
Philippe
Thanks for the additional information, Philippe.
In a wet climate, it’s recommended to do annual maintenance: clean the outer shell of soot and grime, repair any cracks that may have developed using stucco patch, and paint with good quality exterior deck paint. Waterproofing the chimney and exposed brick with a non toxic sealant is also recommended. I think that would address your concern, which is not at all misplaced.
The BWO designer lives near Seattle, Washington, with a wet climate most of the year, and I live in Connecticut, which similarly is no stranger to deluging rain and tropical storms. We also have the added attraction of -20 C winter temperatures and occasional ice and hail storms. My oven has held up nicely over 4 winter seasons with no leaks.
One other thought: if your design would allow for it, is it possible to build a roofed structure over your oven to keep the rain off? There are a number of those in the photo galleries here. The idea is not to build an enclosure, but at its simplest a sloped roof supported by posts that offers clearance and keeps the rain off.
The thought of a pizza oven in Bermuda makes me happy, and I hope you can put your worries to rest on this score.
Thanks for the encouragement - actually I found some 50 year old galvanised roofing that could look pretty cool as a lean too over the oven area - like a bauer shed style thing - I want it to look like it was always there - her is a picture of the base from salvaged old Bermuda stone - each big one weighed 60 pounds. kind of cool you can cut it with a hand saw with wide teeth but still solid.
I am building the form work for the slab.
So to be clear - can I use just a perlite portland mix in a much thicker set of layers over the inner dome or should I import a blanket? I would rather not as i am scrounging and salvaging most materials - including the dome parts.
Ooo, I like that! And galvanized would be better over heat. I imagine the rain will beat a bit of a tattoo on it, but that adds to the atmosphere.
And in general I like your commitment to scavenging what materials you can.
I’ll say you could, but it’s going to affect the thermal performance to some degree. I’m at the edge of my expertise as to how much. Ovens in warmer climates do fine with a single layer of mineral fiber blanket, because icy weather is not your ongoing challenge in Bermuda. So I’d be okay with “much thicker,” and caution that you need to make sure the thermal layer has dried and cured thoroughly before you cover it up. Even though Perlite sheds water much more readily than Vermiculite, the Portland cement will still retain some moisture over a period of days. We want that gone before you seal things up, or else the oven will try to bake it out (= steam).
In my view this ends up being the same seven day curing period that you would use for the hearth thermal layer built with Vermiculite.
The photo you shared is tantalizing, and I hope you’ll come here often and share your experiences as you pull together this epic oven, Phillippe!
