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WoodchuckDad 05-21-2010 11:49 AM

Who gets the most cracks? Square vs Angled cuts
I see some folks with exacting cuts and nice tight mortar joints.....and at the ed of their build they talk about cracks. And I see some who just cut the bricks in half and use them and blob on tons of mortar and they don't say anything about cracks. So who gets the most cracks and the most stable build?

fxpose 05-21-2010 12:02 PM

Re: Who gets the most cracks? Square vs Angled cuts
My joints are very, very wide on the outside of the dome using homebrew, but the joints in the dome are fairly tight where it really counts.
Regardless, I'm looking forward to getting some cracks once the curing process begins... :p

Tscarborough 05-21-2010 12:45 PM

Re: Who gets the most cracks? Square vs Angled cuts
There are several causes of cracking, and they are not mutually exclusive or additive.

1. The number one cause is shrinkage from too wet of a mortar/too wet brick at placement.

2. The number 2 cause is thermal shock. As a rule, this one will be less with wide joints, more with thin joints that can not absorb as much movement.

3. Structural movement. Also more common with thin joints.

As you can tell, thin joints are not a good thing with the exception of refractory applications, where the advantage of protecting the joint is greater than the possibility of movement and cracking.

Dino_Pizza 05-21-2010 01:40 PM

Re: Who gets the most cracks? Square vs Angled cuts

Originally Posted by WoodchuckDad (Post 90180)
just cut the bricks in half and use them and blob on tons of mortar and they don't say anything about cracks. So who gets the most cracks and the most stable build?

This sounds good to me! I've come to really appreciate the simplicity of doing this. From the inside of the dome, it looks exactly the same. From the outside...quite different until you insulate. But you use a lot less bricks, less diamond blades but more refractory mortar. But I say this just for comparing building methods....(I went with mostly tight joints, lots of tapering)

As for does it affect cracking? I don't know. I got a few hairline cracks in the dome (as viewed from the outside before insulating) and 1 big thermal-shock vertical crack that split my bricks due a curing "accident" on my 3rd curing day that was easily avoidable. Other than than that, no cracks that you can SEE at the arches or arch walls. Those may or may not happen as Tscarborough said: due to too wet mortar or shifting problems.

IMO I wouldn't pick the building method based on hairline cracking. I would be more likely to pick on other just as hard to pin-down criteria: Do you believe the tighter joints make an oven with slightly better heat retention having 90% firebrick instead of 85% firebrick/15% refractory mortar? Does the extra work make a difference?

I think the most important decision is spending time on good vent transition design that is stable and sized large enough to draw excellently. Hairline cracks disappear for all kinds of reasons (insulation covers it, soot masks it, oven weight closes them) but smoke out the front is always...smoke out the front. If obtaining your bricks is easy to do, you could install your floor, soldier course, the 1st row of half bricks, then your inner arch, by then you can decide if you think the 2nd row and the rest will be larger joints or not.

Lastly, make both inner/outer arches tight. Cut them slightly pie-shaped and fitted well. The cracks there tend to come from rectangular pieces slipping slight which cannot happen if they are all slightly wedge shaped (especially near the top of the arch).

You'll be really happy with whichever method you choose, Can't wait to see it!, Cheers, Dino

Tscarborough 05-21-2010 01:44 PM

Re: Who gets the most cracks? Square vs Angled cuts
Good point, Dino. Unless the crack is at a structural joint (like the center of your arch) and is getting bigger, then it is no big deal. Some cracking is to be expected.

splatgirl 05-21-2010 05:00 PM

Re: Who gets the most cracks? Square vs Angled cuts
The super meticulous people who cut their bricks so that there are the tiniest of joints are also the people who obsess about cracks. Those of us that that were less meticulous have no idea whether we have cracks or not :)

eprante 05-21-2010 09:29 PM

Re: Who gets the most cracks? Square vs Angled cuts
I did quite a bit of reviewing builds with this question in mind. My decision to use the homebrew mortar made me look at people who changed the formula. I came to the conclusion that the recipe listed in the plans was best, and slow controlled curing seemed to be an important factor. I tapered my bricks with compound cuts on the sides but not on top or bottom. I had very thin joints on the inside of the oven, and fairly large mortar joints on the outside( I also did not cover the outside of the oven with mortar so I could see if cracks developed during curing). I cured with a natural gas burner, low and slow. I had only some hairline cracks on the exterior. I think the curing process with a burner is a great advantage due to the heat control you have. Controlling a wood fire is difficult ( as I found out tonight as I was cooking a pork shoulder and trying to keep it at 350-400).
My 2 cents

SCChris 05-22-2010 07:11 AM

Re: Who gets the most cracks? Square vs Angled cuts
I cut on the sides and top with the exception of the soldiers. My reasoning really related to minimizing the joint sizes to maintain something about 1/8”. I used 3 HF diamond blades and used Sairset wet premix mortar, premix wet mortar is not widely recommended around here. I used home brew mortar on the top 2 courses of the stack area and at the chimney flange. I had run out of the Sairset , didn’t need a full bucket and wanted to give the homebrew a try.

Throughout the oven I was able to maintain the joints under the self imposed ” limit although some outside joints approached this size.

For curing I ran a quartz worklamp in the oven for several days and then a propane burner for a total of something like 60 hours in 4 or so burns to my little propane burner’s limit of 540f. After several hours at this temp I started running a wood fire that slowly pushed the dome temps to 850f with no significant cracking. I think I remember a hairline crack or two.

My feelings about full tailored, 3 side cut, bricks is that even without any grout these bricks can’t move very far. If, heaven forbid, the grout fell out or evaporated, the tapering and gravity will have them nest tight again. For me I’d cut 3 sides again. This said, it doesn’t seem to matter for oven stability, or function, as long as you somewhat diligent and not sloppy about brick placement and not assuming that replacing a brick with mortar is ok.

Your question about cracking is valid but I think the long slow curing is at least as important. It's easy to get caried away and push the curing too fast and pop you have a crack. Slow and low followed by long controlled increasing temp burns worked for me. Get the water moving out, keep it moving out, and keep it out by design.


kebwi 05-22-2010 10:45 AM

Re: Who gets the most cracks? Square vs Angled cuts
Yep, my OCD-built oven with very tight joints has cracked all to hell. I want to post a bunch of photos of it at some point.

However, I take solace in the fact that since the whole oven is virtually free-standing due to the way in which the bricks fit together that it almost doesn't matter if it is all cracked up.

I am mildly concerned about one particular brick, a keystone in one of my arches (there are three arches in my build) that is literally loose but it only rocks about 1/16" of an inch...and again, due to the wedge-shape of that stone the others in the same arch, it could absolutely never fall through, so unless the arch buckles outward (which would have to compress the InsWool and vermicrete, much less break the exterior hardibacker and stucco), I really don't see it ever becoming a problem.

Aside from the chemical reasons (too much, too little water), I have noticed that the tightest joints are the ones that cracked "apart". There are hairlines through my thicker joints but the ones that developed a gap are very very tight, and I think that in combination with a relatively large sand grain (#30 if I recall) in my mortar, these joints just didn't work. I needed much finer sand (or wider joints). Think of it as the same reason you can spread creamy peanut butter thinner than crunchy peanut butter. Same effect.

So what does all of that boil down to? Is it an argument against precisely cut bricks and thin mortar gaps? In my opinion no (but it ain't an argument in its favor!). I take great satisfaction in the concept of a mechanically free-standing self-supporting arch. I just find the notion "beautiful" from an engineering point of view, and to that end, mortar is completely totally unnecessary (you can build an arch with no mortar at all). That's my taste. Others have different tastes. If someone is going to be tremendously hurt by the presence of cracks they better thick hard about their priorities.

All of these ovens survive from what I've read on this forum, so from the point of view of properly cooking food and standing up for many years without major degradation, none of this matters.

dmun 05-22-2010 12:40 PM

Re: Who gets the most cracks? Square vs Angled cuts
This is a most interesting question.

From the perspective of someone who built a cut-every-brick thin wall oven, with heatstop mortar, it's just not worth it. I got a bunch of cracks, and I think the tiny mortar joints are a contributor to that. I know it's not a water problem on start up, because my oven cured for a better part or a year while a built a two story chimney on top of it.

Were I to do it again, I'd use the standard thickness of a cut brick, trim the inner edges so it looks good on the inside, and leave the big gaps on the outside, where nothing is ever seen. I'd also carefully consider the homebrew mortar, mostly because it's lime based, and lime based mortars are said to be more flexible than other types. I don't think rigidity is a plus on something as subject to thermal stress as a brick oven.

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