mortar gap

[Mike D]

So I have been reading through everything over and over again. I have a mortar question. When putting the dome together, is there supposed to be mortar showing on the inside of the dome? As in do the bricks touch on the inside of the dome, or is there a 1/8 of an inch mortar showing (depending on what type of high heat mortar you are using). I have read the plans, as well as many threads, it looks like some have them and some don't. The plans seem to say to say not to have a seem? Looking at Les' pics, I can't see a seem, he is no ordinary builder.
Mike-

[GianniFocaccia]

Re: mortar gap

The FB plans (pg 38) do call for a flush mortarless joint:

"Do not allow for an inside mortar joint, as you will be setting
the edges of the bricks facing inside the oven flush with
each other."

but in reality, not too many of us (except Les, of course) have the time, patience nor skill to cut every brick to facilitate this design. Nevertheless, the great majority of builds seen here have minimal joints and given the functionality of the refractory mortars being used, no harm, no foul. Subsequent analysis of ovens built by Italian masters reveals joints not unlike the builds here.

[Tscarborough]

Re: mortar gap

A lot depends upon the type of mortar you are using. With wet refractory cement, the joint size should be 1/8" which is almost impossible to achieve in this design. With good home brew and products like Heatstop you can get away with normal sized mortar joints, 3/8" to 1/2". The less joint at the face the better, but the reality is that you will have wider joints than 1/4" when building a 36-60" diameter circle with 4-1/2Wx2-1/4Hx4-1/2L firebrick.

The important things are that your joints are staggered course to course, and the inside joints are minimized. The gaps on the backside need to be completely filled either as the brick are laid or after the fact.

[Mike D]

Re: mortar gap

How did Les do it. Did he cut every brick on all sides? As in, cut a whole brick in half, then cut each half on both side at angles and then cut an angle on the top to give it a wedge shape?
Mike

[SCChris]

Re: mortar gap

Mike,
Les's build is spectacular, yep he cut to shape every brick. I came after Les's build and got frustrated with the custom cutting without some sort of tool. I had the JCG31's "Bevel and Angle Calculations" and this helped but I was still wasting a bunch of time. Here is a link to the tool that I built to facilitate custom cut dome bricks.

http://www.fornobravo.com/forum/f28/...ool-12478.html

I don't think I can find JCG31's original post with his info so I'm including a copy of the graphic. Thanks Again JCG31!!


Although when you think about it, it becomes obvious that if you custom cut you're going to use more bricks, but it bears mentioning.


Chris

[dmun]

Re: mortar gap

A perfect fit dome is an exercise in pure craftsmanship. I stand in awe of people who do it without access to tapered firebrick. It is by no means a requirement for building an oven. The original plans called for brick-set cut bricks and angle iron over the doorway. Many ovens have been built by this simple method, which gives BIG mortar joints as you go up, and they cook just fine.

[SCChris]

Re: mortar gap

Dmun is absolutely correct. Brick fit is mostly about aesthetics not function. Custom cutting dome bricks won’t fix or avoid lack of insulation an inadequate foundation or chimney. For me it was about the challenge of maintaining precise grout lines and about solving the "brick cutting" puzzle and implementing the solution. My feeling is that, above the soldiers, the dome is really self supporting and the grout is more about sealing the gaps and less about brick support.

Chris

[Tscarborough]

Re: mortar gap

In normal masonry construction, the joints are sized according to several factors, but it is not normally desirable to use less than 3/8". In refractory applications, on the other hand, it IS desirable to reduce joint size as much as feasible, and fireplace standards call for 1/4" Maximum.

Several things are in play here.

First, one of the main criteria determining joint size in any application is the uniformity of the units. Brick specifications allow for size variation that would probably surprise you (FBS spec is 1/2" in length between units for example). This is the primary reason that firebrick are manufactured by the dry press method, while 90% of all other brick are extruded: it allows for precise sizing and minimal warping, which allows for minimal joint size.

Secondly, the smaller the joint, the more cracking will be expected, simply because a thin joint can not support the modulus of elasticity of the mortar (As an aside, this is also one of the reasons you should always use the weakest acceptable mortar possible for normal masonry; they will normally have a higher modulus of elasticity).

Lastly, refractory cements are more friable than refractory units, and thus are more subject to abrasion and temperature shock failure so minimizing the area covered by them is the goal (in a normal wall up to 20% of the wall face will be mortar, for example).

Sorry for the book, can you tell I am stuck in the office today?

[Mike D]

Re: mortar gap

So, does one place the inside edge brick to brick and then use a shim (wooden or a small brick chip?) to get the desired angle and mortar it in? The smaller the inside joint the larger the outside joint. Aren't these large outside joints a problem? Is this why people add a finishing layer of mortar around the outside of the dome?
Mike

[brickie in oz]

Re: mortar gap

You could always cut your bricks smaller to alleviate any mortar joints whats so ever..

[dmun]

Re: mortar gap

So, does one place the inside edge brick to brick and then use a shim (wooden or a small brick chip?) to get the desired angle and mortar it in?

You can do this, but the mortar forms a pretty self supporting base for the brick, hence the multitude of brick positioning jigs (indispensable tool)

The smaller the inside joint the larger the outside joint. Aren't these large outside joints a problem?

No.

Is this why people add a finishing layer of mortar around the outside of the dome?

Sometimes people parge the outside of the dome with a layer of refractory mortar to get a little more thermal mass. It's by no means necessary.