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Re: Render cracks

From the WFO board

Posted by Rado (

In Reply to: Re: Render cracks posted by Ed

As you say it's not changing size on heating and cooling you can try to continue BUT below I write more on layers that go under the render and what can be done. You can also do further tests just by watching thing during next firing. Sometimes it can be only not using metal trowel smoothing the render when it dries and then heat continues with what drying shrinkage didn't do. Expansion cracks would show some closing. I never used glass fibers in mortar but if it can help why not.

In unboxed ovens it is extremely common that the render gets thin long cracks if insulation under is not properly applied. Luckily it can be fixed always everything is possible. The insulation under will still move pushing outwards onto render, and also the heat can shrink the render further if it reached it - linear shrinkage on heating.

To cover oven this way you need to fire it 3 times and applying thinner insulation layer after each of these firings getting slowly into desired thickness. This way the insulation gets used to movements and absorbs it before the final render. The last insulation layer will most likely not create cracks and expansion cracks under get overlapped making no spots for higher heat come through up right to the render (after next heat up try if you can feel heat around these cracks, if not then it could be only the render, gaps can be filled, the hollow sound on tapping is okay often under insulation.) This was for mixed insulation. You can also use ceramic fiber blanket layers, these also shrink (linear shrinkage again as it's called) so should be cut rather longer and placed not stretched laving wrinkles in surface and small folds on ends.

I will see but I could build one rectangular brick oven covered this way into the barrel shape on outside and do 1 page on this too. r

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Re: Render cracks

by Rado Hand on Google+

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