tiling over a clay oven?

[downthevalley]

Hello! I have finished making a pizza oven out of clay, sand and cobb. I'd like to waterproof it and put a mosaic over the oven, and I'm wondering whether I need to do something like a concrete render over the clay before gluing the mosaic down? Or would it be ok to glue straight onto the clay? Since the clay is quite sandy I imagine this might not be a great way to go

[david s]

Unfortunately tiles and ground are not flexible so any thermal expansion might produce some cracking and provide entry for water. If you have a roof over the oven then this is not a problem. But as a roof involves a fair amount of labour and expense, it's a rather extravagant solution. Tiles will not adhere to the cob mixture at all well so a cement render would provide a suitable substrate. Although until all moisture has been eliminated, which may take many many firings, do not attempt to render or tile the oven now or you will simply lock in any moisture that's there.

[downthevalley]

Thanks David! I'll definitely be drying it out as much as possible before tiling - have fired it up multiple times and it's amazing how much moisture is still in there! I was looking a little more into it and I wonder if I could just seal the clay with a waterproof tile primer, something like this , and tiling over that with grout and glue that has a little flexibility which might help if there's any cracking

[david s]

If you’ve fired the oven multiple times with no problems then you are presumably not in danger of steam spalling damaging it.
The product you posted about sounds suitable. As an under tile sealant it probably has no UV protection, but if you are tiling over it that wouldn’t matter. Try it out and let us know how it goes. It could be a good idea to fire the oven up after applying it and before tiling to see how it withstands some heat and how it copes with some thermal expansion.

[Pizzarotic]

Thanks David! I'll definitely be drying it out as much as possible before tiling - have fired it up multiple times and it's amazing how much moisture is still in there! I was looking a little more into it and I wonder if I could just seal the clay with a waterproof tile primer, something like this https://www.crommelin.com.au/product...fing-membrane/ , and tiling over that with grout and glue that has a little flexibility which might help if there's any cracking

Yes, use a waterproofing application first then tile over it. There are plenty of flexible tile adhesives available, in fact almost all have some flexibility so some movement shouldn't be an issue.