neilestrick, I tested out one of the chimney pipes and after about 1 hour, it cracked and I replaced it with ceramic fiber tube.
I also did not use my home made "flue thingy". Instead, I used 2 thin fire bricks as a support for the bottom shelf and made a half pipe out 25 mm fiber to direct the exhaust out to the brick chimney.
Anyways, the results for our first firing where awesome. Porcelain looks so much better if fired in gas kiln.
We started out really slowly with a smallest flame possible for about 1 h. Then started to increase the flame slowly.
After about 9 hours and maybe 9-10 kg of gas the prssure got too low and we decided to stop.
We had additional 11 kg as backup but we decided not to use it. It was probably a mistake because we did not reach cone 7.
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Few changes I have to make.
1) Because the fire bricks, inside the kiln, sank to the floor, the flue opening shrank too. I need to use 2 more thin fire bricks as a "floor" for the flue inside the kiln.
2) Lift the kiln higher off the ground so I we have more room to play with the burner distance.
3) Probably spend some more money and buy lower temperature
50 mm 6.5 mm (and put 3 layers) blanket to make the walls thicker. All I need to do is open up the wire cage and wrap it around the existing kiln body. Front part of the kiln has a overlap so it is not a problem.
4) Borrow a kiln pyrometer. Sight and smell will not do it yet