blanket / lath--- vermicrete or stucco?

[joeykramer]

hi all, i am second guessing my curing process. i was originally going to cure it when it is not covered with the insulation blanket. now i am thinking to insulate it, add a vent and then cure it. i am only one fire into the process and then it rained. i wanted to cure it all week and then make pizza this weekend, but it looks like its going to be a washout this weekend...

i have about 3 inches of insulation blanket to put on top. i will then cover it with a steel lath.... now the question, do i then coat it in 3 inches of vermicrete/cement at a ratio of 10:1 or go straight to the stucco. i think i have seen people just going right to the stucco after the lath. i bought a 3/4" galvanized nipple with a screw on cap that i am going to put at the very top so to act as a vent and i'll keep it open when curing. good idea? better ideas?

from what i found, scratch and brown coats are 1 part portland, 1 part lime and 5 parts sand. for the finish coat add another lime to it. sound right?

i plan on finishing the dome with some brick flats like Gulf did. love the way that looks.

this is still on track to be an igloo.

thanks
Joe

[david s]

You can stucco directly on to the blanket, but a 10:1 vermicrete will provide a firmer substrate to stucco against and even out all the bumps and lumps in the blanket allowing you to keep the outer stucco layer thinner. For the stucco layer each coat should be thinner and either the same strength or weaker than the last.If you are planning on covering over the dome with brick splits you could skip the outer stucco and lay them directly against the vermicrete.
I agree with your plan of drying the oven out after insulation, but make sure you keep the rain off it in the meantime.

[joeykramer]

Thanks David. Sounds like you can do either of the two, or both. Will the vermicrete be better in terms of having a harder more durable structure ? I.e. Will just a stucco later crack if let's say a basketball hits it ?

How soon after the vermicrete can you add the stucco on ?

[david s]

vermicrete requires a lot of water in the mix (3 parts water to 10 parts vermiculite) and only around half of that will be used up in the hydration process so you need to eliminate it. Best to leave it a week in sun and wind before doing any drying fires, then apply the stucco layer. Because it's a dome form it will be way stronger than you'd think. If you make ithe stucco layer only 10 mm thick (that's pretty thin) you'd be able to stand on it ok.10:1 vermicrete is very weak however, but it's solid enough to work against, unlike the blanket which is more elastic.

[Larry P]

I did perlcrete over my blankets, but that's because I had a low dome, so the igloo wasn't near hemispherical. I wanted to build up the shape to be a hemisphere before stuccoing. If your blanket surface is smooth and round enough, then you can stucco right over it. The layer of perlcrete might make it a little more structurally sound, but 10:1 is very crumbly even when dry. I doubt it adds much.

For curing, it's endlessly debated. I think the tradeoff is, the oven will hold heat better during curing if insulated, but if not then you can see (and stress out over) cracks. You certainly want to cure after your masonry is done, so if you haven't yet built your vent, then get that done first. After that, my personal opinion is, cure before insulating. Two reasons: One, if something goes really wrong structurally, you'll have less to undo to get it fixed. Two, you're trying to drive moisture out of the masonry. I think that's easier to do with less stuff covering it. Then again, it doesn't rain here so YMMV.

[david s]

If the outside of the refractory (bricks or casting) is uninsulated it is exposed to ambient temperature. The resulting heat loss from the outside surface sets up an enormous difference in temperature between the inner and outer surfaces which leads to enormous stress. In addition there will be a greater difference in temperature between the top and bottom of the dome if left uninsulated during firing. An example of problems created from uninsulated firings is an uninsulated chimney which develops cracking. Hence the use of flue tiles to reduce the temperature difference. Because a dome is self supporting any cracks in the inner parts are really cosmetic, but if you want to reduce the likelihood of creating them in the first place it is better not to fire any refractory that is uninsulated.

[UtahBeehiver]

A good solution to both Larry's and Dave's preferences is one I used from Karangi Dude experiences. This only works if your first insulation layer is a ceramic blanket. He suggested to lay the blanket over the dome, unfitted, during the curing process. This tempers the dome from temperature extremes yet allows you to remove and inspect the dome after the curing is complete. Then you can fit the blanket or install the v or p crete if you wish.