stucco cracked

[plug2000]

I built a barrel oven all fire brick and then 3'' of perlite 7-1 insulating cement with 2 layers of stucco on top. after firing the oven slowly the last firing the stucco cracked along the top. i thought i had all the water dried out but guess i didn't as there was steam coming out of the cracks. The oven is in perfect condition, just the stucco render is cracked. I was planning on firing it a few more time and let the cracks act as vents and then add 2'' of insulation on top and re-stucco it. What do you think of this ??? or I was gonna fill the cracks in with heat stop mortar while the oven is hot and the crack is expanded. any advice is appreciated. thank you.

[Petter]

(I would) Dry it first and evaluate if you think the heat retention is to your likeing or not. If so, cut a 90 degree V in the crack and fill it with stucco again. Otherwise, try get the stucco off with a hammer and chinsel carefully. Add a second layer of perlcrete. Make it as lean as possible to make it crack rather than the stucco. Add the stucco in one go. I experience it gets stronger this way. Cover it for a week to cure.

[Mongo]

...guess i didn't as there was steam coming out of the cracks. ...

Not sure if it's too late but consider adding a vent to alleviate this very problem. Search the forum for 'breather' for a lot of discussions about venting.

[david s]

I built a barrel oven all fire brick and then 3'' of perlite 7-1 insulating cement with 2 layers of stucco on top. after firing the oven slowly the last firing the stucco cracked along the top. i thought i had all the water dried out but guess i didn't as there was steam coming out of the cracks. The oven is in perfect condition, just the stucco render is cracked. I was planning on firing it a few more time and let the cracks act as vents and then add 2'' of insulation on top and re-stucco it. What do you think of this ??? or I was gonna fill the cracks in with heat stop mortar while the oven is hot and the crack is expanded. any advice is appreciated. thank you.

It sounds very much to me like the moisture in the stucco has turned to steam and the pressure has cracked the outer shell. Vermicrete swells if the water is driven out too fast. As you've reported steavisible steam this is the most likely cause. This happened to me on the third oven I built and since then I always drive the moisture out of the insulation layer before applying the stucco. The problem is always much greater if you use very or perlcrete directly against the inner oven rather than a layer of blanket first.Just keep using the oven until all the moisture has gone. You can test this by throwing some sheet plastic over it and inspecting it for moisture. Your oven will improve in efficiency as it become drier. To fill the cracks dampen them and wait 5 mins before filling to give the water a chance to penetrate, this will improve the bond, or in addition stucco over the whole oven in another layer then cover for a week.