Cracks in Cob

Hi all,

Ive succumbed to using high temperature silicon to seal cracks in the cob surrounding the burn chamber. Today, i thought i would take it out and see if i could seal the cracks with something not so ugly. The cracks are big and they run all the way down to underneath the barrel, which is where the smoke is coming from. I apologize for not taking a photo..

Can i seal the cracks with refractory cement? Will it bond well with the cob or should i use something else?

I would suggest trying refractory cement with some fiberglass in it. But before you do that, acid wash your cracked cob with some HCl (1 part pool acid diluted with 3 or 4 parts water) to rough up the surface and make it better for the cement to adhere. I suggest adding some fiberglass to give the cement some more tensile strength because cracking is what happens when the mud encounters tensile forces during drying. Here is an example of one of the commercially available products I am referring to.

Hi John,

Thanks for your help on this. When you say "pool acid," is that hydrochloric acid? Then mix it with 3-4 parts water?

Karl Meisenbach wrote:Hi John,

Thanks for your help on this. When you say "pool acid," is that hydrochloric acid? Then mix it with 3-4 parts water?

Yes. Straight out of the bottle, pool acid is quite nasty and gives off strong fumes that you don't want to inhale. Mixed with 3-4 parts of water, it will no longer fume as bad and you can handle it. However, it is still strong stuff and you will want to use rubber gloves and not get any on your skin (it stings like a son-of-a-bitch if you do).