Spacing between the firebricks

[Nick in Laval]

Hi everyone

I've been getting mixed oppinions about the spacing between the firebricks for the actual wall and dome of the oven.

As per the fornobravo design, the bricks are tight and come into contact with each other (face side of oven) and the back side of the bricks often have a small half inch empty space wedge of air that is supposed to get filled with high heat resistant mortar. I've been told that the mortar properties are different from the properties of the actual fire bricks in that the mortar is only able to resist heat where as the bricks are made to retain the heat. For this reason, because the two properties are quite different, the mortar is supposed to be used sparingly (no thick layer) as a high heat resistant glue to hold the bricks together. I was also told that because the mortar is supposed to be used thin layer as opposed to thick layer, the latter would imply that I am supposed to cut the bricks so as to reduce the amount of spaces between each brick on all sides. Basically the tigher the fit between bricks the better it would be because it would require less mortar, which is a good thing because if too much mortar is used, it becomes counter productive.

In the Fornobravo plans, the pictures of the brick laying appear to have healthy spaces of air between the backside of the bricks, which then get filled with mortar. Is this a concern for the effectiveness and longevity of the oven? I realize it's a lot easier to go this route but is it better construction to take the time to cut each brick with the necessary angles from all sides to minimize the space between bricks and use less mortar? Someone even told me that because the mortar resists heat and the bricks absorb the heat, because the two properties are so diiferent that if too much mortar is stuffed between the bricks that over time they could fall off.

Can someone please shed some light on the above because cutting every brick to measure, to fit as close as possible in every way to the neighboring brick, will take a looooong time!!

Thx in advance

Nick

[Les]

Re: Spacing between the firebricks

Nick,

In my opinion, the characteristics between mortar and brick are more likely to be similar than different. Keeping a tight joint would obviously be a good thing but as you have seen, many, many people have used a lot of mortar to fill the space. I think at the end of the day, either route you take, your oven will last a long time for a homeowner. You are correct on the fact of cutting every brick - it does take a lot longer to build.

[Laku]

Re: Spacing between the firebricks

Maximum gap between the bricks also depends from the mortar you are using. For example for some refractory mortars the maximum gap between the bricks is just 3mm (0.118110236 inches). I believe this depends from the coarseness of aggregate used, finer it is the less thickness a seam can have?

Cue brickie.

[brickie in oz]

Re: Spacing between the firebricks

Can someone please shed some light on the above because cutting every brick to measure, to fit as close as possible in every way to the neighboring brick, will take a looooong time!!

Welcome to the world of the perfectionist, the gaps if you have any will fill with ash and wont be detrimental to the ovens performance.

My own oven started with no gaps on the hearth but with thermal expansion I now have gaps that are filled with ash.

[Tscarborough]

Re: Spacing between the firebricks

Wide joints are inevitable in a round oven using square bricks. Minimize them on the interior and fill them on the back. Aggregate is normally sized to expected joint thickness, most normal applications of refractory mortar have joints less than 1/8" so the aggregate is sized accordingly.

You can use premixed dry refractory to lay the brick, then make a home brew with a coarser aggregate to fill the backsides, or even add up to 25 percent coarse sand to the pre-mix.

[ATK406]

Re: Spacing between the firebricks

Nick in Laval wrote - "Someone even told me that because the mortar resists heat and the bricks absorb the heat, because the two properties are so diiferent that if too much mortar is stuffed between the bricks that over time they could fall off."

Nick, it is important to make a distinction between heat resistence and resisting heat. It may sound like symantics but the former denotes a material that can withstand heat (will not break down due to heat exposure) and the later implies the material has significant insulating properties. The brick and mortar are both designed to withstand heat. It is true, the mortar does not have the same heat capacitance as the brick itself (i.e. it does not absorb as much heat - think thermal mass) but it will not break down due to the heat you are likely to generate in your WFO. That said it is better to minimize the amount of exposed mortar on the inside of the dome as the mortar is not as heat resistant as the brick itself.

Like Wotavidone I cut the vertical edges of my dome bricks (starting at the 5th course) to form trapezoids that would minimize the gaps between the bricks on the inside of the dome. I used the left over wedges created by these cuts to parially fill in the gap on the outside of my dome. In this way my mortar joint was reduced to ~ 1/8" instead of 1/2" to 5/8". I have posted a pictures to illustrate what I mean.

Good Luck, Regards,
AT

[Prairie Valley Bread]

Re: Spacing between the firebricks

So it looks like it's safe to have a 1/2 inch gap at the back, right?
Do you pre-soak the bricks?
Instead of filling them in at the back, can you just add more insulation?

[stonecutter]

Re: Spacing between the firebricks

So it looks like it's safe to have a 1/2 inch gap at the back, right?
Do you pre-soak the bricks?
Instead of filling them in at the back, can you just add more insulation?

1) yes. A mortar with larger aggregate is better for backfilling though.

2)some do, some only dampen them( I'm in this group) the trick is to balance the content of water in the brick vs the hydration of your mortar. Too much water either way will make a poor mechanical bond...which lends itself to sliding bricks( if you don't have a form) and a longer dry out schecdule.

3) what do you mean? Fill the gaps with perlcrete or vermicrete? I wouldn't recommend that. The dome is under compression from it's own weight for one, and you want material that has a compression rating close as possible to the building units. Second, and I'm just speculating....filling gaps with insulation could screw with even heat saturation, because your gap thickness will vary a lot throughout the dome.

Recommendation: backfill the brick with mortar, preferably with larger aggregate than preblended refractory mortar. Limit the amount of water absorbed by the brick. Do not backfill with insulation.

[Prairie Valley Bread]

Re: Spacing between the firebricks

Thanks so much for clearing this up.
I did have some mortar that did not stick to the brick; it looked like it shrunk away, and I did get a few cracks.
I'll get back to the oven later and let you know how it goes.
Thanks again,
Tom

[Ciabatta Lover]

Re: Spacing between the firebricks

So would I be right to assume that the ultimate oven would have no mortar and prefectly cut bricks locked in place by key stones (with the outside slatered with mortar)?