Mortar on ceramic fibre board

[Thaitim]

Hi all, I'm new to the forum so please forgive me if I'm asking something that has already been covered. I'm tackling my first build ( 40" internal dia, Tuscan style). I've watched loads of videos online but still a little unsure about bedding dome bricks and floor bricks onto the fibre board because it supposedly almost instantly sucks the water out of the mortar, causing cracking. I watched a YouTube vid about a professional oven builder in Australia who covers his fibre board with tin foil first to avoid the water being sucked out of he bedding mortar. Any advice/links would be much appreciated. Also what is meant by opening "reveal"
Many thanks
Tim

[SableSprings]

Welcome Tim! We do not recommend mortaring the cooking floor bricks to the ceramic insulation board. At most, use a thin layer of fine sand with a little added fireclay on top of the ceramic board. Lightly dampen the mix and use a notched trowel to put a grooved pattern in the sand mix (like laying tile). That way you can lay your cooking floor bricks on top and tap/set them level. When the firebricks are heated and cool down during each cycle of use, they expand and contract. Not a lot, but enough to shear some of the stress point bonds. This is particularly true if you lay your cooking floor bricks and then mortar the base dome bricks down on top of them. If you choose to start your dome on top of the insulation board but on the perimeter of the cooking floor (bricks are cut so the floor is roughly circular), you leave a gap between the cooking floor and the outer ring dome bricks...for expansion. The recommendation is to leave a piece of cardboard in that perimeter gap...it burns out and leaves about 1/4" of a gap...

Both the ceramic board and upper insulation batting do readily absorb moisture. There are many methods to protect your insulation from moisture. Here's a link to a build that incorporated all the current best practices (slab drain holes, mosaic tile bed, foamglas, then the ceramic insulation board).

https://community.fornobravo.com/for...andpoint-idaho

The reveal is the little brick "lip" at the entrance of the oven. Simply there to provide a nice seal for a door. Many ovens do not have a reveal but simply build a door that fits into the oven opening.

Take the time to go through the following link that has a great collection of well documented builds.

https://community.fornobravo.com/for...n-the-archives

Looking forward to your build and questions

[Thaitim]

Cheers Mike, most helpful. If I can not get hold of any fireclay where I am, is there an alternative?
Many Thanks
Tim

[SableSprings]

If you have access to any powdered clay (local potters) that will work. You don't need much...it just allows the sand to be a bit more workable and to be stable (hold) when dry. If that's not available, just use builders sand which includes a variety of grain sizes. The varied grain size allows the dampened sand to "soft set" when dried. I used just builders sand when I did mine and it worked fine. If you have been cutting firebricks (with a wet saw) for your build, you can collect that brick dust and use it instead of the fireclay...because that's basically what it is.

The other thing I didn't mention about not using mortar to attach the cooking floor to the ceramic board, if necessary, you can more easily remove and replace a damaged brick in the future. Doesn't happen very often, but it's nice to know you can. The Australian builder you noted in the first post used the aluminum foil primarily to create a slip layer for the expansion/contraction movement of the cooking floor even though the "sponge effect" does exist.

p.s. For those in the forum, it's very helpful to know the country/region in which you live. It makes a difference in the answers you will receive if we know (can guess) the climate in which you live. Go into your profile and add that info to help us help you...

[JRPizza]

One other thing to consider for the floor bricks. My hearth was very flat, and the CF board laid on top was uniform in thickness so it came out flat also. I sorted my bricks to get enough for the floor that were uniform thickness and just laid them directly on the board and did not need any leveling compound.

[Thaitim]

Thank you for the information, it's vey helpful. I've now got a much better idea on how to proceed.
Cheers
Tim