Many cracks in cast dome - Please help!

[cheyne]

Hello,

I am casting a 33" oven with home-brew refractory cement, and am getting an unusual amount of large cracks. I was planning to put the final thickness layer of home-brew on the top of the dome today, but am worrying about structural integrity. There are smaller cracks in the sides, and bigger cracks on top where is thinner. It was 60 degrees here in New Mexico yesterday, and kept it covered with a blanket and tarp, and didn't let it freeze last night. I've tried searching as much as I can on this forum already :/ What am I doing wrong? Any Advice? Thank you!

I Used Paper mache form around an exercise ball to get the shape

Home brew 3, 1, 1, 1
Play sand
Portland cement type 1/2
Type S lime
30 mesh fire clay

[david s]

Hello,

I am casting a 33" oven with home-brew refractory cement, and am getting an unusual amount of large cracks. I was planning to put the final thickness layer of home-brew on the top of the dome today, but am worrying about structural integrity. There are smaller cracks in the sides, and bigger cracks on top where is thinner. It was 60 degrees here in New Mexico yesterday, and kept it covered with a blanket and tarp, and didn't let it freeze last night. I've tried searching as much as I can on this forum already :/ What am I doing wrong? Any Advice? Thank you!

I Used Paper mache form around an exercise ball to get the shape

Home brew 3, 1, 1, 1
Play sand
Portland cement type 1/2
Type S lime
30 mesh fire clay

From the appearance of the surface it seems to me that your mix was too wet. The wetter the mix the greater the shrinkage. Use the "ball up" consistency test. ie if you throw a handful of the mix two feet in the air, you should be able to catch that ball again. Too wet or too dry won't work.Also the clay content imparts shrinkage, and there is quite a lot of it in the homebrew mix. I used to sometimes get shrinkage cracks in the mix when used as a mortar and now modify the brew to 3:1:1:0.6 sand,OPC,lime,clay, so you could try reducing it by around 30%. The fine polypropylene fibres, if well dispersed, apart from providing assistance for safer moisture removal on firing, also hold the mix together to reduce early shrinkage cracking. This is their primary use when used with a standard concrete mix.

[gregoru2]

Hi cheyne I'm wondering if you found a solution? I also have a dome with cracks in it...

[Macrinehart]

Use the "ball up" consistency test. ie if you throw a handful of the mix two feet in the air, you should be able to catch that ball again. Too wet or too dry won't work.

david s tried this tip with my furst batch of home brew and the ball was catchable but flattened when caught, is that still too wet? Thanks!

[david s]

Sounds about right. You need to wriggle each handful against the side of your mould so you don’t get too many voids. Start at the bottom and go all the way around, making a flat ledge on the top to take the next row. Wear rubber gloves.

[Steve48!]

Hi newbie on here, preparing to cast my first oven over a cardboard mold - was wondering if first layer should be clay rich to provide more heat resistant and use std home brew mix for second layer to provide structure. Have seen some ovens on line that are made out of pliable clay that seem to work. molding clay balls onto a mold should work if a long curing process is used shouldnt it?

[david s]

Although clay imparts refractory properties its extremely small particle size leads to a lot of shrinkage. If building over a form that shrinkage is likely to result in a lot of cracking.. Potters expect up to around 10% shrinkage from their clay bodies. This can be reduced by the addition of grog or in the case of oven building, sand because we don’t reach temperatures high enough to turn the sand into glass. Cob ovens usually open the clay body by a mix of two parts sand to one part clay and a generous dose of straw. This is usually enough to reduce, but not eliminate all shrinkage cracks. The sand form is usually removed once the mix has set a bit so further shrinkage can take place without damage.
Bricklayers sometimes use a little clay in their mortar, but too much leads to the shrinkage crack problem. 7% is the industry recommendation limit. Mortar strength can be increased by clay addition up to this amount, but decreased if it exceeds it.
My experience with using the 3:1:1:1 formulae has led me to halve the clay content to 3:1;1:0.5, which is still a bit over the 7%, but eliminates shrinkage cracks.
By all means give your idea a go, I wouldn’t, but I’d love to hear how it goes.