Thinking a lot on how to start an Earth oven

[siruami]

Hello all...

Finally decided to start building my own earth oven, I already build it's base, a 4x4' base made of concrete,I have bought perlite, fire clay (Hawthorne) and 1" ceramic fiber blanket, and I also have a 1/4" cement board (hardiebacker), my original plan, according to what I have read, was to put ceramic fiber, then the cement board, then a layer of slip with perlite, then placing the firebrick on top of this layer (perlite) with some building mud mix on the bottom.

Once this floor is done, build a 28" diameter dome with 3" thick mud, then a small layer of refractory cement, then a layer of more ceramic fiber blanket and then maybe a mix of perlite and portlant cement for "waterproofing"

So how does this sound as a plan? I would love some guidance at least on the floor so I can get started.

Advice is greatly appreciated.

[SableSprings]

Welcome to the Forum Siruami! In order to have your cooking floor insulated from the base there are several options. DO NOT put your ceramic fiber blanket underneath any part of the oven. The blanket will compress and lose a major part of its insulative quality (the air pockets are important). The recommended way to do the bottom insulation is to put down a water barrier above the base, then a rigid ceramic board (or 5:1 perlite:cement), then your oven floor/dome.

One of the original, low cost floor insulation methods was to embed beer/wine bottles upside down, vertically in a bed of sand. This produced an air gap insulation which was reasonably effective. If you use the 5:1 perlcrete mix, then 4-5" is considered adequate. Ceramic board is twice as efficient and only needs to be 2" thick underneath your cooking floor.

Ceramic fiber (board or blanket form) loves to absorb water, hence the need for waterproofing the oven...or at least making your best attempt to thwart water coming in from the bottom or seeping through cracks in the final outside material coating (without maintenance...water always will win in the long run ). Many methods and material are covered here in the forum for placing these water barriers. Perlite and Portland cement DO NOT make a waterproof material, this perlcrete is also water absorbent and will hold onto that water fairly tenaciously.

It is important to think about your dome material..."mud" is not the correct way to go. You need to use something with a high percentage of clay and although the term mud is often applied to wet clay...you need to be aware of the distinction. Earth ovens are not really long lived and prone to lots of maintenance issues. If you don't want to do a brick oven, then consider casting one with a mixture of 3:1:1:1 (sand:lime:fire clay:cement). It will be much stronger, with less maintenance, and a much longer service life. Lots of good casting examples on the site and our casting guru is David S.

Hope that helps a bit, but I'd suggest you start reading some of the cast oven threads before you go much further...it's time well spent.

Good luck and we'll be looking forward to answering more of your questions and watching your build progress.

[siruami]

Mike, can't thank you enough for that advice, I was about to do something very wrong.

I used the general "mud" term to refer to my building mix, which will be sand and clay, didn't think about adding cement or lime, thanks for the advice... now on that... will it be refractory cement? or can I use portland cement?, and as far as the lime, I think you are not referring to the soil amendment correct?

Thanks again, I hope to send pics soon.

[SableSprings]

The 3:1:1:1 mix is called home brew on this site. To be specific use builders sand, hydrated lime (also known as builders lime...NOT agricultural lime), fire clay (available at most pottery supply shops), and Portland cement. Home brew can be used as the mortar for a brick oven or used as the casting material for one. When casting, polypropylene fibers and/or stainless steel needles have been recommended as additions to make the home brew stronger (among other reasons). However there have been many cast ovens that are just done with the home brew mix. Do NOT use the soil amendment lime...you need to find builder's lime, it is a totally different product. Amend your profile to show where you are in the world/US and then someone on the forum can recommend places to get various materials (as well as help adjust comments to your climate area). The builder's lime that you need is quite corrosive and you will need to where gloves when working with it. Not that it's going to eat your pinkies off if you touch it...but it will cause skin damage if you don't follow the standard precautions. Just FYI, in the home brew, the Portland cement will eventually break down with high heat cycles...but the builder's lime actually increases in strength and is slightly flexible. The home brew ends up being very sturdy and will endure pretty damn well with constant use over many, many years.

You can also generally source refractory cement which is designed/mixed for you to "just add water". It's a bit more expensive, and is a significantly different material than the home brew. (Fiber and stainless steel needle additions are still recommended.) The primary issue with the refractory cement (besides the higher cost) is that it sets quite a bit more quickly. Again, looking at some of the cast oven threads here and particularly noting comments by David S will be very helpful to you.

Here's four links to good posts/threads on casting ovens.

https://community.fornobravo.com/for...er-sand/page11

https://community.fornobravo.com/for...ven-by-the-sea

https://community.fornobravo.com/for...becomes-a-kiln

https://community.fornobravo.com/for...-chicago/page7

[david s]

Hello all...

Finally decided to start building my own earth oven, I already build it's base, a 4x4' base made of concrete,I have bought perlite, fire clay (Hawthorne) and 1" ceramic fiber blanket, and I also have a 1/4" cement board (hardiebacker), my original plan, according to what I have read, was to put ceramic fiber, then the cement board, then a layer of slip with perlite, then placing the firebrick on top of this layer (perlite) with some building mud mix on the bottom.

Once this floor is done, build a 28" diameter dome with 3" thick mud, then a small layer of refractory cement, then a layer of more ceramic fiber blanket and then maybe a mix of perlite and portlant cement for "waterproofing"

So how does this sound as a plan? I would love some guidance at least on the floor so I can get started.

Advice is greatly appreciated.

Search "cob ovens" for info on building a clay oven. You may also want to consider using a home-brew castable which I believe is a superior, more durable and longer lasting mix, but more expensive than using free clay and straw. Better again, but way more expensive is dense castable refractory which is used as an industrial mix for high temperature furnaces and kilns.
Do not use the blanket under the floor as it will compress and substantially reduce its insulating capacity. Firebricks for the floor are better laid with a 50/50 dry mixture, (making removal and replacement far easier should you need to do that years down the track), of powdered clay and sand, which allows the floor bricks to move independently with the uneven thermal expansion they will receive. This is only required to level the fire bricks should they be of a slightly uneven height, if not the bricks can simply be placed on the calcium silicate board. Alternatively you can cast an insulating slab with either perlite or vermiculite using 5:1 ratio with portland cement.
If you have blanket keep it for insulating the dome, over which you can lay a weak mix (10:1) of vermicrete or perlcrete which will be light enough not to compress the blanket, but insulating enough to provide more insulation and a firm layer to stucco against.
I think you need to read some more builds on the forum before proceeding, post some pics and ask some questions. We all have the same objective which is to have a good functioning oven that will cook great food. But, there are many different ways to get there.

[Brwilliams88]

Is there a difference between "mortar clay" and "fire clay". I went to a building supply store and asked for fire clay and the stuff they gave me was this mortar clay. Is it the same ?

[SableSprings]

BrWilliams88: The correct fire clay used to make the refractory mortar and casting material has Ca, Al, & Si compounds. I do not believe the mortar clay you pictured is the correct type (check the back ingredient list). Hawthorne Fire Clay which is mentioned in this thread's opening post is the material type needed. Check around at a pottery/ceramics supply business for fire clay. It is intended for creating "high firing temperature" ceramic items and is what you want to get. It should be pretty inexpensive and also available in 50# sacks. I've attached a link to Hawthorne Fire Clay so you can see the high Al & Si content that is in a fire clay.

David S is our resident expert on casting and various material types...hopefully he will confirm or "adjust" my response.

[UtahBeehiver]

I used the HC Muddox mortar clay for my homebrew mortar.

[david s]

For the temperatures we fire to virtually any powdered clay should be ok except Bentonite which has extremely small particles and therefore has more shrinkage which we don’t need. The other factor is price and I’ve found that the cheapest apart from digging and processing your own, is bricklayers clay, which is selected and designed to be added to mortar to improve (albeit in small quantities) mortar workability.
The term “fireclay” used by builders is for this purpose Ie as an additive to mortar. When used by potters or refractory engineers “fireclay” is a different animal. It is a high alumina clay that can withstand extremely high temperature ie low in fluxes and silica which want to turn to glass in the upper range.

[SableSprings]

David & Russell, thank you for correcting me. I thought we'd always talked about using fire clay as the component to home brew, so I assumed regular clay was not up to snuff...I'm glad to get that straightened out in my head! (Probably was just a reference my beady brain picked up out of an old post that "all the fire clay residue from cutting the oven bricks could be used in home brew"...not that it was required.)

[Chach]

Im always learning in this forum..We have some smart people on here!

[Brwilliams88]

So I should be good to go with the mortar clay I got for the home brew mortar?

[david s]

Yes you should be