Firing 31, the third weekend of April, the damper sat at four inches open for the whole climb from 600 to cone 08 and the stack thermocouple never dropped below 340F. That is the number I keep coming back to. On firing 24 the same damper position gave me 280F at the same point in the schedule, and firing 24 was the one where the back stack stalled for two hours at 1750F and I burned an extra half cord getting out of it.
The kiln is a two-chamber cross-draft I built in the back field in spring 2023 with a friend who does masonry. Chimney is eleven feet six from the grate, nine-inch flue, cast damper on a steel rail with the inches scratched into the rail so I can read the position from the stoking side. Pyrometer probe goes into the stack twenty inches above the exit flue. I write everything on graph paper in the barn and copy the useful parts here afterward.
Weather turns out to matter more than I assumed when I started. Cold and dry pulls hard. On the January firing, 19F at dawn with a north wind, I could not close the damper far enough to slow the draft and the front of chamber one ran 130 degrees ahead of the back all day. Warm and humid does the opposite. Late August, 78F and heavy air off the river, and the fire wanted to lie down in the firebox instead of climbing.
Stack temperature above 400F during the body reduction means I am wasting wood up the chimney and should close the damper an inch. Stack below 250F with visible smoke at the top means the damper is too far in and the chamber is choking. Between those two the fire behaves and I stop fiddling. Took me about fourteen firings to trust that band instead of chasing every reading.
Barometric pressure has shown up in the ledger four times now. Every firing where the pressure was falling through the afternoon, the draft went slack around 1900F and I had to open the damper further than my notes said I should. Rising pressure firings never needed that. Four is not many but the pattern is clean enough that I now check the glass on the porch the night before and write the reading at the top of the page.
Wood matters in a way that confuses the damper record. The pine from the lot clearing in 2024 burned fast and hot and made me close the damper more than oak ever has. I have started noting species and rough moisture in the margin so the damper column means something later.
Next firing is scheduled for the second Saturday in October, weather allowing. I want to try holding the damper at three and a half inches through the whole reduction rather than adjusting by feel, just to see what a flat setting produces in the cone packs at the back.