Why slather mortar on the outside of the dome

[ThermoJax]

The guy at the firebrick yard asks me today why put any stuff on the outside of the dome. He thought that maybe it had to do with filling any holes from the initial build. He says that I should build the dome with SAIRSET mortar (premixed) and then cover the outside of the dome with insulating cement.

So, what do you guys think. mortar all the way, or a combination of mortar in between bricks and insulating cement over the dome?

Thanks

[ThermoJax]

surely, someone must have an opinion. First, I assume that the slathering of mortar, or cladding, it used to stop smoke perhaps from going thru the brick. It could also be to add extra mass, although, if that were true, then, why are we cutting our bricks in half for the courses. it could be that the cladding is extra assurance that the dome won't move or shift. I believe that the guy at the firebrick store was implying that a well built oven hardly needs mortar. As in "the mortarless oven build"

So, does anyone have advice as to the type?

thanks

[Tscarborough]

Primarily to fill the gaps and make it a solid unit.

[ThermoJax]

So, should I clad the outside in Sairset mortar, or the insulating cement?

[Tscarborough]

You should fill the gaps flush with refractory mortar, then use insulating material to coat it (unless you want to add more mass, mine is 3" of brick and 1" of stucco).

[dmun]

I think the exterior cladding is an attempt to solidify the unit, but I haven't heard of any less cracking in clad compared to unclad ovens. Cover it or not, as you choose.

The insulating concrete is different: if you use this instead of the blanket you want to layer this on at least four inches thick.

[Neil2]

It is a good way to use up excess mortar and does add a bit of thermal mass. Otherwise it is not necessary.

[ThermoJax]

Dino had a nice explanation:
Just to be sure everyone is on the same page, if by "skim coat" you mean the high-heat mortar you smear on the outside of the dome bricks while you build it, then that "cladding" is part of the ovens "mass". You have to decide how much extra mass you want just like you decide on the oven width and height. I wanted a 1/4" of extra mass added around the dome to 1) add mass that heats up and releases the btu's just like the fire brick, 2) gives you the feeling of somehow adding some structural integrity to the dome although it really doesn't but I suspect it might make the joints stronger if you add it to the outside of that joint while it's still moist. And 3) unfortunately it shows all the hairline cracks you'll get when you 1st fire up the oven but that's OK.

Some have added 1/2" of extra cladding, others barely a skim coat. Your oven will heat up faster without it, but with it, you retain more heat for longer cooking times and works better for bread baking.

You can add the cladding while you work on the rings to use up the extra mortar or go back after you're done make sure you have an even amount.

Happy WFO building, -Dino

[DrakeRemoray]

I added an extra inch of refractory mortar to mine so that I would have more mass. This allows me to bake a couple of extra batches of bread.

Also, I think that sari set premixed mortar is an "air set" mortar. Air set mortars are not appropriate for outside building as they remain water soluble even after is has dried...

Drake

[luca]

This raises the issue of cladding vs. vermiculite for insulation. I understand from Allen Scott's book that instead of putting a cement cladding layer over the dome bricks, he recommends 6 inches of vermiculite piled around the oven. Will this really cut down on the thermal mass and length of time the oven stays hot? Is only a one inch layer of refractory mortar really enough "thermal mass" to make a noticeable difference?!