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Re: Render cracks

From the WFO board

Posted by Ed

In Reply to: Re: Render cracks posted by Rado

Thanks for replying so quickly Rado.

The oven is a firebrick barrel with solid reds for the dome and I wanted to have a shaped render layer rather than a boxed in oven. This has reinforced 2 inch concrete cladding followed by cement/lime/vermiculite layer of about 3 to 4 inches built up over 3 days. I had fired the oven a few times before making the render which is 6 sand/1 portland cement/1 lime. First render is about 10 to 15 mm thick on chicken wire. This was cured for 3 days under plastic. Cracks were not evident at this point.

Fine long cracks now run diagonally across the area above the dome and there are none in the sides. Longest crack is almost 50 cm. When I tap on the area where the cracks are, oven sounds hollow underneath, but not so where there are no cracks. They do not appear to change size on oven heating or cooling (although I haven't had any severe fires since rendering).

If you think this might be from a wet mixture, I might try making the second render coat on chicken wire also for maybe a little extra stability. What would you think about adding some glass fibre into the final render coat? Would this give the render some directional strength?

If all else fails, I guess I will have to box the oven in but woud really like to retain the shape I have now.

Thank you again.

by Rado Hand on Google+

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