I have to repair some cob

[Rocko Bonaparte]

Right at the top of the mouth of the oven, a good fist-sized chunk of the cob washed out in the recent rain here. I'm a little alarmed that it had not really cured well enough in the time I've had the oven, so I hope I could do a repair and cure it in properly. I have pottery sand and fireclay with which to make a repair. I was wondering what a good ratio is for doing repairs--same as I used to make the oven? I'm also looking at good techniques for making the repair permanent.

What I am intending to do is mix some cob, clean out the whole, fill it up, cover it with some wet paper, and then wedge up some bricks and stuff up against it for a few days to adhere. After that, I'll start doing some small fires there over a few days before moving on to cooking fires. I guess it's out of commission for at least a week while I do all this.

[Rocko Bonaparte]

Re: I have to repair some cob

Mate of mine had the stone work in his century old farmhouse cellar repointed by a mason who specialises in the old stone houses.
He built it up in layers, as thick layers either fall off altogether, or at least don't adhere very well.
I guess there are parallels in clay v old style lime mortars? Have you considered gradual build. Each successive layer would go some way towards filling the cracks in previous layers?

I had thought about it after the fact. When I did the cob originally, I took what I had left, got it real thin, and spread it all over the dome. For the repair, I plugged most of it up with a regular piece of cob and just kept wedging the stuff in there with the help of a trowel; it otherwise wants to stick to my hands.

With the leftover, I spread it on one side of the wall where I had chipped it a bit while manhandling the oven before it really cured. Honestly, I probably could have done that smear thing a little every day and done the repair that way too. It's probably what I'll do if this repair fails.

I had to extend the mouth of the dome from what I originally made, and unfortunately this crack is along that line.

[Rocko Bonaparte]

Re: I have to repair some cob

I am going to try this gradual approach now. I first tried slapping a bunch on and that failed. I slapped some wet cob on top of some wet paper, on top of a board, on top of a car jack, pressed up against the roof of the mouth of the oven. That was looking good for a day, and then that completely failed. So I sprayed the expose bit so that it was workable again and brushed on a thinner layer of cob. I'll try using that for awhile before spreading some more on.