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kebwi 10-21-2009 11:56 AM

Maximum mortar joint on inner oven surface?
I apologize in advance that my questions are often long and verbose. Such is my way...

My understanding is that the most basic FB Pompeii design is very forgiving of large mortar joints, prescribing an extremely simple half-rectangular block design for the purpose of enabling as many people as possible to build ovens. I admire the motive, but nevertheless have some questions about the assumptions it rests on.

Now, mind you, that for the purpose of this thread, I am not inquiring about the dome's structural strength or problems that may or may not arise from using rectangular blocks with fat mortar wedges as opposed to trapezoidal blocks (on one or two axes) with thin mortar wedges. That is not my concern in this particular thread. Rather, I am asking about the problems that might arise from exposing large (what is large, 1/4", 1/2"?) areas of mortar to the super-heated interior of the oven?

While this may be less of an issue with true refractory mortars like Heatstop and Refmix, and I am going homebrew, and I believe the general wisdom on the subject is (correct me if I'm wrong) that the Portland in homebrew will effectively dissolve away under high heat.

So, does such a problem mean that we should really strive to minimize exposed homebrew mortar on the interior surface of the oven? The most effective way to do this is to angle our bricks not only on the azimuth axis but secondarily on the altitude axis as well, which of course gets into some pretty fancy cutting.

To people who only angled their bricks on the azimuth axis, thus leaving large triangular mortar gaps exposed to the interior: what is your opinion of such an approach after the fact? Does the mortar spall and fall away from the sides and roof of the oven or behave in any other undesirable way?


Tscarborough 10-21-2009 12:28 PM

Re: Maximum mortar joint on inner oven surface?
The best mortar joints for fire exposure are the smallest ones you can make. For fireplaces 1/4" or less. For an inside curve joint, why would it need to be any wider?

kebwi 10-21-2009 12:50 PM

Re: Maximum mortar joint on inner oven surface?
It's just a mathematical property of the geometry; it isn't something I "need" per se. Given bricks of a certain dimension, cut only on the azimuth axis to some form of trapezoid (which at one end of the spectrum is a rectangular solid and at the other end is a triangle), but not cut on the altitude axis at all, wrapped around an azimuth of some radius and tilted by an altitude of a second radius, will implicity create triangular gaps of some width (at the bottom of the triangle). Whether that gap is larger or smaller than 1/4" is just numerical.

This problem is "easily" alleviated by created secondary cuts on the altitude axis such that the bricks are trapezoidal on two axes, and perhaps that simple solution is your reason for assuming that 1/4" tolerance should be easy to achieve, but the basis of my question is that "easy" is a relative term, especially on secondary cuts across first cuts. The edges are no longer square; such cuts represent a new level of complexity over the first cuts.

It seems a little odd that you would brush off gaps of 1/4" so casually when the photos on the galleries and in the forum posts clearly illustrate many ovens created with very large gaps, upwards of 1/2" or more.

...which is, of course, the origin of and motivation for, my original question. That is exactly what I am trying to determine: whether such loose tolerances are acceptable.

If you are suggesting that 1/4" is a maximum tolerance for thermal reasons, as per my original concerns, then that is very helpful information and I can shoot for a design that is within that tolerance...but it begs the question as to whether or not the numerous ovens shown on FB that exceed that tolerance should be expected to exhibit any problems.

Tscarborough 10-21-2009 01:36 PM

Re: Maximum mortar joint on inner oven surface?
I am a fireplace and masonry guy, not an oven guy, but the principle is the same, only more-so.

kebwi 10-21-2009 01:40 PM

Re: Maximum mortar joint on inner oven surface?
Hmmm, okay. I guess the only thing about small ovens relative to fireplaces is that they have a tight radius of curvature which introduces these risky mortar gaps. I'll see if I can make a design that minimizes them.

Tscarborough 10-21-2009 02:22 PM

Re: Maximum mortar joint on inner oven surface?
From the Residential code:

R1001.5 Firebox walls. Masonry fireboxes shall be constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete. When a lining of firebrick at least 2 inches (51 mm) thick or other approved lining is provided, the minimum thickness of back and side walls shall each be 8 inches (203 mm) of solid masonry, including the lining. The width of joints between firebricks shall not be greater than 1/4 inch (6 mm).

Of course, this is for fireplaces inside dwellings, but the reason for requiring the joint thickness is still relevant.

Mitchamus 10-21-2009 02:35 PM

Re: Maximum mortar joint on inner oven surface?
The best sized joint - is no joint at all.

Since the home-brew mortar is non-refractory, it will let heat escape at a greater rate than a true refractory mortar.

If I was running a kiln or blast furnace 24/7 I would care about the excess running costs.

Since I am running my oven on wood... once or twice a week at the most.
I can handle the increased cost of 1 or 2 extra pieces of wood per firing :)

I don't think that spalling is an issue.

dmun 10-21-2009 04:42 PM

Re: Maximum mortar joint on inner oven surface?
Successful ovens have been built (Frances' springs to mind) with just fireclay and sand. Lime adhered mortars for thousands of years before the invention of portland cement. I think that the portland serves a secondary function in the homebrew mortar, that even if the bit close to the oven deteriorates, there is still plenty to hold the dome together.

I would deny no one the fun of building a cut-every-brick oven, but it really isn't necessary to have a fine working oven. It's been five years plus that this has been going on, and a LOT of ovens have been built, many of them with fairly crude joints, especially at the top. If any of them have collapsed, I haven't heard about it.

Tscarborough 10-21-2009 06:35 PM

Re: Maximum mortar joint on inner oven surface?
Always design perfectly, build practically, and accept the reality.

As someone in the industry, I can't and won't make recommendations that do not represent best practice. As noted above, that does not mean that perfection is required to achieve desired results.

The oven I plan on building breaks every rule on design for example (although not on construction details). The reality of cutting every brick to create equal 1/4" mortar joints in a dome is not practical for example, but the goal of 1/4" joints is a design factor that should be considered.

Les 10-21-2009 06:45 PM

Re: Maximum mortar joint on inner oven surface?

Originally Posted by Tscarborough (Post 68428)
The reality of cutting every brick to create equal 1/4" mortar joints in a dome is not practical

I have a little bit of an issue with this - I cut every brick to avoid any mortar joint (on the inside). :eek: Then again, reality alludes me at times.


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