first firing!!!!

[paulages]

so i returned from japan yesterday, and though my brain was still 16 hours ahead in tokyo and i hadn't slept a wink, i had to fire the oven up for the first time.
i started with a small fire, though it has been sitting in the sun for three weeks. it doesn't seem like there was any moisture to drive out---no steam from the mortar at all. however, cracks did open up in several places, like other people have described, seemingly from wherever the mortar had time to sit and dry up for a bit before the next course went up. the cracks are small, but when firing it this morning, they were smoking a bit until the fire was large enough to draw properly out of the opening. i checked, and from the smell it was definitely smoke and not steam. the plan is to wait until they are open fully again, then squirt some refractive putty insulation in the cracks, then float another layer of mortar over the whole thing.

does this seem like a sensible plan of action anyone?

it didn't seem to heat up anymore at the point of the cracks than the rest of the dome did (i used the infrared gun). the entire dome got about 150-200deg.F, hottest at the top. the inside of the dome reached about 500 deg. max, but really was probably stable at about 350deg. without the flame licking the surface. today i will go for a slightly hotter fire.

[paulages]

also---if you look at the picture i posted of the top of the dome from the inside, you'll see that there are some fairly large gaps between the last bricks. in particular, if you look closely, there are three in a row very tightly fitted (these are the #1 arch brick i had planned in my dome cross section layout). then there are two that snug in perpendicular to these. the four ensuing gaps, i filled with triangular keystones, but the weren't as deep as the others, leaving the large gaps.

does anyone think this could cause any issues with thermal mass/ radiation? my thinking is that as long as there are no airgaps, less thermal mortar would be beneficial (as long as there is good insulation on the other side of it), meaning less heat-up time, and as long as there are bricks there to radiate heat back downward, there should be no issues with heat loss in these gaps.

any thoughts???

[ColonelCorn76]

Originally Posted by paulages

however, cracks did open up in several places, like other people have described, seemingly from wherever the mortar had time to sit and dry up for a bit before the next course went up. the cracks are small, but when firing it this morning, they were smoking a bit until the fire was large enough to draw properly out of the opening. i checked, and from the smell it was definitely smoke and not steam. the plan is to wait until they are open fully again, then squirt some refractive putty insulation in the cracks, then float another layer of mortar over the whole thing.

does this seem like a sensible plan of action anyone?

Be fine. Everyone I know uses refractory furnace cement for repairing these (and every builder I know - Scott oven or Pompeii - has experienced minor cracking). You can get it in a caulking tube and it works fine because it cures under heat. Don't really need another layer of mortar over it again as you're introducing a new discontinuous interface (one of the reasons for the cracks is that the mortar dries out at different cure rates which is one reason to soak the bricks as you build so the bricks don't draw the moisture out of the mortar).

Jim

[paulages]

i actually think the worst of my cracking was from that slight shift forward of the arch i mentioned, because you can follow the crack in a step-like pattern up and around the dome, where a big chunk just shifted at once and found its permanent home. the unsawool was indeed in a caulk style tube like you mentioned.