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Les 09-12-2007 07:50 AM

What’s the secret to minimizing mortar cracks? I’ve started adding a little mass to the outside of the bricks and the mortar is cracking. It was only applied ½ to 1 inch thick. I kept it wet and covered. Would a thicker or thinner mortar mix help? Are many thin applications better than one thick one? Any suggestions are appreciated.


El Puaco 09-12-2007 03:45 PM

Re: Cracks

What mortar are you using? I had to punt after I ran out of the Insulfrax mortar and had some bad luck with a brand that I bought from Mutual Materials but then had great luck with Heat Stop. Heat Stop gave me very minor cracking. The Insulfax worked like a dream but I ran out before I could really start the main cladding phase. Good luck.

Acoma 09-12-2007 03:59 PM

Re: Cracks
Les, I have been speaking with CanuckJim about insulatin and he states "Matrilite is a far better insulator than vermic/Portland, and you simply mix it with water, no cement needed. I think if you added three inches to the sides and about five at the apex you would be fine for a framed enclosure". Is your oven going to be framed? Jim, does this sound about right?


Les 09-12-2007 08:14 PM

Re: Cracks
Mike / Robert,

I'm using the Forno mix for the cladding. I bought a 50 pound bag of heat stop to use on the dome construction - but at $85 a bag, I'm trying to use it in the critical area.

Robert, this isn't insulation - the purpose is to add a bit of mass and lock in the rings.


Les 09-12-2007 08:18 PM

Re: Cracks
Robert - missed your question about the enclosure. I'm going with the igloo to minimize the profile.


Bacterium 09-12-2007 09:32 PM

Re: Cracks
wouldnt the key be that all layers (thick or thin) are applied before the previous layer that it cures as one whole mass, rather than one layer is much drier than the adjoining layer - and move about at different amounts as they dry and "shrink".

.........probably not the best technical explanation but maybe someone like Uno or CJ could shed some light :D

Les 09-12-2007 10:38 PM

Re: Cracks
Damon - there in lies the problem. It cracks... When I leveled my island it required around 1.5 to 2 inches of mortar. I kept it wet and covered - it cracked. That wasn't a problem because it would be covered with tile. I'm concern with the dome because I don't want the heat / smoke to escape. I have noticed that if I "skim" a surface with mortar it's fine. There must be a magic bullet to lay it on one inch thick and get the desired results.

Thanks for your input.


maver 09-13-2007 01:56 PM

Re: Cracks
Do fine mortar cracks in that layer even matter? Aren't you covering it with insulation? I might be missing something...

Bacterium 09-13-2007 04:01 PM

Re: Cracks
yeah I tend to agree that cracking (large) would be a problem on the "mortar" layers above blanket/vermiculite........are you going to use any of that Les?

ok, I suppose one big factor is the mix content.....:

For a "general bricklaying mix" the bricklayers sand has some clay in it and it does crack somewhat......I'm talking distances generally larger than a standard mortar joint. Whereas the sand used in a stucco (render) mix is much sharper and the mix (if done properly) only seems to crack if the base it is applied to moves.

A bricklayer once explained to me how it all bonds together in the drying process....the clay has a degree of shrinking going on as it dries (even though one might minimise water content), whereas the sharp sand doesn't tend to shrink any further than once its wet.

Les 09-13-2007 04:10 PM

Re: Cracks
I'm going to cover the dome with 2 inches of inswool and then some vermiculite (around 4 inches of loose), the dome will eventually be stucco.

Damon, so you think if I mix it on the dry side it will help. The stuff I applied was fairly wet because I was also using a grout bag.


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