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Wet mix mortars are considered a problem in oven building. For one thing, they are heat setting, and they aren't waterproof in our application. Also, they are designed for thin joints: they are not so good at filling the wide gaps on the outside of the dome. If you're getting the stuff cheap, it may be out of date, and wet mortars don't store well. It might also have been damaged from too hot or too cold storage.
I was ignorant to the act that the dry-mix Sairset is not waterproof and used it to build a firepit with firebricks this past summer. The firepit went together without a hitch and performed well on a series of successive fires. Unfortunately, the firepit was not covered and was drenched after the first fall rain. Curiously, I tested a brick after the weather eased and the mortar crumbled like dry oatmeal.
On the other hand, I let the first two courses of my oven "cure" in a light drizzle for almost a day and the homebrew mortar I used dried diamond hard.
Glad I asked you guys about this. I have been trying to get a bro who is building a square brick oven to read some posts. He is using house bricks, no fire clay and no vermiculite. He help me today and we got the floor and first course down. He wanted to leave the lime out of the fireclay,portland and sand mix. He is great help but I guess we have different ideas. Thanks for all the great help so far!!