Olympic updraft gas fired kiln with alteration.
Original pipe burners replaced with MR-750 Venturi burners.
The pyrometer & thermocouples are Tru-View Digital from L&L Kilns with protection sheaths http://www.hotkilns.com/tru-view.html
Notice the cone packs on top of the kiln demonstrating a very even firing top to bottom.
Here is some helpful information to achieve even firings:
This'll be long, but some variation of what I write I'm sure will be helpful, though I think the only thing for more even reduction would be a slower firing.
Much of what I've learned came from fellow Clayarters Charles Moore, who passed along information from Paul Vernier, and then other messages from Bob Mesnard.
- A dual thermocouple pyrometer. With thermocouple at top and bottom of kiln, I can watch temperature difference and adjust damper accordingly. A digital pyrometer provides instant readings.
- Damper. Some use a single piece of kiln shelf. I use two insulating firebricks, one on either side of the flue opening. I've marked the lid, on either side of the opening, from center out, lines (I just used pencil) spaced about 1/8" apart. This lets me set the damper bricks at specific settings that I can easily repeat for other firings.
- Use gas pressure to induce reduction. Increasing gas pressure will inject more gas, creating more back-pressure, which will induce reduction. But you'll have to be careful, because too much gas pressure will cause too much back-pressure in the kiln and temperature will stall or decrease.
- Set the primary air (disks on back of burners) to 1/4" open and leave them there forever.
- Baffle shelf, a shelf smaller than your regular shelves is good (broken shelf will work). I have some 6" x 6" square shelves rescued from an old kiln that were used to cover burner openings. This small baffle shelf is set 1 1/2" below the flue opening. I use three posts set on my half shelves (I only use halve shelves in my kiln) to hold the baffle.
- Bottom shelf 4" off bottom. I'm going to experiment in future firings of laying a small shelf across the gap, over the middle burner to direct the flame away from center of kiln (remember, I'm only using half shelves).
- Burners - the original Olympic burners simply never worked properly at low gas pressure for me. After a couple of failed firings and fighting back burning (flame coming out the back of the burner!) I removed the installed burners and ignition tube that was just causing all sorts of soot and other problems. I replaced these pipe burners with cast iron venturi burners and replaced the ignition tube with a heavy-duty thermocouple/mini burner on the first burner in the gas line. I have to manually light two burners, but it's not that big a deal.
Through most of the firing the bottom is 200° cooler than the top. When I see the first cone bending on top (you need cones top & bottom!), I start adjusting the damper (closing) until I see temperature on top stalling and temperature on the bottom rising, with the dual thermocouples I can flip back & forth, reading top & bottom in an instant. In the end cones are almost exactly the same top & bottom.