cracks

[emsoven]

hello em here from australia i have just finnished building a 50 inch pompeii oven and am very happy with the finnished product. however i did not use a fire blanked but used 4 inches of vermiculite concrete then chicken wire and and more than 1 1/2 inches of concrete.i left oven to cure for about 2 weeks and then followed the 5 step curing fires as recomended in forno bravo plans . however the out side of my dome has cracked quite badly but the inside brick are perfect.i would like to rerender my dome and am thinking of useing a slip layer any advice for me and how to render a slip layer. thanks em

[Neil2]

"the inside brick are perfect"

This is what counts. Cracks on the outside of the dome are common.

At one time it was advised to have several layers of heavy duty aluminum foil as a slip layer, although this practice seem to have fallen out of favor.

[emsoven]

thanks neil i am thinking just to rerender over dome with plenty more chicken wire once it has fully dried out there is still plenty of water in the vemiculite

[david s]

Hi Em,
Because you used vermicrete for your insulation you will have added hundreds of litres of water in the application of this layer (around a third of the volume of vermiculite that you used) To evaporate this much water you need way more than a couple of weeks. Ask yourself how long it would take to boil dry a 44 gallon drum of water. The cracks have occurred because water present in the insulation layer has created steam pressure. Don't be in a hurry to render over the cracks until you are 100% sure the water has been removed.This usually takes about 10 decent firings after the initial curing. If you apply some kind of slip layer, you are also sealing in some of that moisture. I foiled over my vermiculite layer prior to rendering the outside shell. In two places I punctured the foil with quite a few holes and it is in these spots that the oven gets hot on the outside after rainy periods, which is an indication that the vermicrete layer has moisture and steam is finding its way out there.This is a handy indication of what is going on inside but I wouldn't do it again. It slows the drying process too much.
Dave

[emsoven]

thanks david. yes i agree totaly with you the other thing i did was to render with concrete two days after i rendered the vemiculite, in the middle of winter.traped all the water in .i think i will render over it in a few mouths time by then should have used it over 10 times regards em

[david s]

Yeah, don't stress over it.The outer shell is only to keep out the weather. But should it rain it would be best to cover it because the rain will enter through the cracks.

[Neil2]

I also put a vent in the top of the dome thru the stucco and into the vermicrete layer. This is a 1 in dia vent with a rain cap. I also have four 1 inch vents around the perimeter at the bottom of the insulation layer. During firing after a wet spring, I often see steam coming from the top vent.

[david s]

Originally Posted by Neil2

I also put a vent in the top of the dome thru the stucco and into the vermicrete layer. This is a 1 in dia vent with a rain cap. I also have four 1 inch vents around the perimeter at the bottom of the insulation layer. During firing after a wet spring, I often see steam coming from the top vent.

This is a good idea Neil, I've also incorporated a vent into the insulation space via a system that also supports the flue pipe

[emsoven]

venting the insulation intresting, but here where i live we only get 13 inches of rain per year here

[david s]

In our wet season we might get that much in one week.