To bed or not to bed firebricks

[Darrell]

Hello,
I tried to find some definitive answers about this topic but ended up with more questions than answers. My oven will sit on a concrete slab. On that slab will sit 4" of cal sil board. I have watched videos by Melbourne brick company and a few other Aussies who place aluminum foil over the cal sil then use a hybrid refractory mortar on top of that to then bed the firebricks. The claim is that they don't want the bricks to move as they do on sand/fireclay. Is it beneficial to bed the firebricks or is using sand/fireclay adequate for my application (precast dome)? I was wondering about cracking from expansion/contraction between the firebricks if I bed the in refractory hybrid mortar. Thanks in advance for your assistance.

[david s]

The reason for having a layer between the floor bricks and the insulation is simply to have them all level as there is often a slight difference in brick thickness. If you do it with a wet mix they will be stuck hard and that will make removal if required down the track, extremely difficult. Far better IMO to use a dry mix (50/50 powdered clay and fine sand works well) which also allows for free expansion of individual bricks.
regarding the foil, if placed against a conductive layer it’s use to reflect radiant heat is completely negated because aluminium is highly conductive. It is the equivalent of placing foil hard up against metal roofing sheeting, useless. It needs to be surrounded by either an air space or a highly insulating material.In addition the foil layer acts as a moisture barrier which may prevent some water entry, but also prevents its removal.
I’m open to any argument to the contrary.

[MarkJerling]

Hello,
I tried to find some definitive answers about this topic but ended up with more questions than answers. My oven will sit on a concrete slab. On that slab will sit 4" of cal sil board. I have watched videos by Melbourne brick company and a few other Aussies who place aluminum foil over the cal sil then use a hybrid refractory mortar on top of that to then bed the firebricks. The claim is that they don't want the bricks to move as they do on sand/fireclay. Is it beneficial to bed the firebricks or is using sand/fireclay adequate for my application (precast dome)? I was wondering about cracking from expansion/contraction between the firebricks if I bed the in refractory hybrid mortar. Thanks in advance for your assistance.

I can't see how the bricks can move on a well compacted sand bed. I deliberately laid my floor on a much thicker than most sand bed as I wanted the option to take some of the sand out in future and replace the floor and, when replac ing the floor, I may go with thicker fire bricks for the floor, so I wanted the ability to do that without altering the overall (top of) floor height. As it is, my thinner floor bricks are working fine, so I don't think I'll need to go thicker in future, but, be that as it may, the floor bricks have not moved on the compacted sand layer.

[Darrell]

Thanks for the input. The aluminum foil over the cal sil was just to create a water barrier between the refractory mortar and the water-hungry cal sil. It seems like bedding in clay/sand vsmortar is the ability to change out the bricks if ever needed. There doesn't seem to be a definitive reason to bed them in mortar is what I'm gathering.

[Giovanni Rossi]

Hello Darrell,

Thanks for asking the question. I’m seriously considering buying the kit from Melbourne because I want to build my oven but avoid most of the brick cutting. As you stated, the foil is to keep the water from the refractory mortar heat bank layer from absorbing into the CaSi board. That kit uses large floor brick (look to be about 250mm square) that are mortared to the refractory layer. The info states: “The fire bricks used in the oven floor are 38% Alumina, rated to 1350°C with an exceptionally hard-wearing surface, and are 50mm thick.” Not sure if these are fired differently from standard firebrick. The experts on the Forum generally say to not bed the floor allowing the option to replace a damaged brick but will admit that the need to do so is exceedingly rare if proper firebrick is used.

The thing about the Melbourne kit that has my attention is that everything is mortared, floor to refractory layer and dome bricks on top of floor. From what I’ve learned on the Forum the dome is not typically mortared to the floor. In fact, when the dome is laid outside the floor, a gap is recommended to allow for expansion. I would think mortaring everything together would be asking for expansion cracks. However, Melbourne states the refractory heat bank is added to balance the thermal mass of the dome.

Maybe they figured out that creating a single connected mass works for their design and that’s why they bed their floor bricks?? I don’t have the engineering credentials to assess that.

[MarkJerling]

Giovanni, your comments made me do some maths!
It would appear that thermal expansion of fire bricks depends very much on the make-up of a firebrick so different bricks will have different expansion rates. Other factors to consider would be that the top of the dome gets hotter than the sides and hotter than the floor, so the expansion will vary throughout the dome and floor.

Be that as it may, it seems that many different fire bricks have an expansion curve of about 1% at 800deg F and about 0.5% at 400degF. Using those figures I can see that, on a 42" dome, the whole dome may become about 0.3" bigger at max temperature to the top of the dome and the floor about 0.15" bigger. That would seem to indicate that the dome will "grow" more with heat than the floor. So, it seems, the better advice is to have the dome and floor disconnected from each other.

My maths may not be up to scratch though, so I stress this is what my first attempt at it shows.

[Giovanni Rossi]

Good stuff Mark. Knowing all that, I assume the mortar would expand at yet a different rate. I recall reading what someone wrote on the Forum that there are two kinds of oven owners, those who’s ovens have cracks and those who lie about their ovens having cracks. Seems like cracks are inevitable and are fine as long as they don’t continue to expand. And as many of you say over and over, the cracks you need to avoid and manage are in the external dome treatments. I really want to keep my dome exposed, but that will be a challenge in NE USA. I’ve been collecting a lot of information and knowledge from all of you on the Forum. Thanks!

[MarkJerling]

When you say you want to keep your dome exposed, do you mean you don't intend to cover it with insulation or do you mean you don't intend to build a "house" over it?

Mine has three small cracks which do not appear to have gotten worse with time. I suspect you're right - most ovens will have some cracks.

[Giovanni Rossi]

Yeah, trying to avoid a house-like structure. I like the look of the dome shape. Probably going to use a roof over a larger area to keep the big stuff off and really work on preventing and managing water infiltration. Yours was one of the first builds I reviewed when I started this quest. Thanks for being a resource!

[MarkJerling]

You're most welcome Giovanni!

I did a 3-coat cement / sand / lime plaster render, with a plasticizer added. That seems to work well and is pretty waterproof. Under it, I have building foil, which most others here frown about as it may delay the drying out. For me, it's worked well as it's a second defence to moisture ingress.

I'm still debating whether I should paint the plaster to match our house or do a final colour plaster render over the top. I'm leaning toward the colour plaster render.

[Pizzarotic]

s it beneficial to bed the firebricks or is using sand/fireclay adequate for my application (precast dome)? I was wondering about cracking from expansion/contraction between the firebricks if I bed the in refractory hybrid mortar. Thanks in advance for your assistance.

Fact is it doesn't matter, if you bed it with mortar it will crack but you will never see the cracks, if you bed with a dry mix there is nothing to crack but the effect is the same. Mortar or dry mix is just to provide a level adjustable base to achieve a smooth floor. Tiling is done using a wet mix so that is the easiest way to achieve a level floor but either method is fine.

[Pizzarotic]

s it beneficial to bed the firebricks or is using sand/fireclay adequate for my application (precast dome)? I was wondering about cracking from expansion/contraction between the firebricks if I bed the in refractory hybrid mortar. Thanks in advance for your assistance.

It doesn't really matter, if you bed it with mortar it will crack as the floor expands and contracts but you will never see the cracks, if you bed with a dry mix there is nothing to crack but the effect is the same. Mortar or dry mix is just to provide a level adjustable base to achieve a smooth floor. Tiling is done using a wet mix so that is the easiest way to achieve a level floor but either method is fine.

[mrotter]

Thanks for the input. The aluminum foil over the cal sil was just to create a water barrier between the refractory mortar and the water-hungry cal sil. It seems like bedding in clay/sand vsmortar is the ability to change out the bricks if ever needed. There doesn't seem to be a definitive reason to bed them in mortar is what I'm gathering.

Btw. I just laid first course of my WFO - half soldiers. What I did was that when I was putting mortar to contact edges of those bricks, I put little less mortar to bottom part, so that no that much of mortar pressures agains CaSi board. I see no real problems with CaSi sucking out moisture from mortar, maybe it is also because my mortar is really not that "wet" consistency but rather "butter" consistency - it is calcium-aluminate mortar.