[Clayart] Clayart Digest, Vol 83, Issue 32

[Village Lady]

> Re achieving reduction of glazes throughout a kiln:  The most satisfying for me were achieved consistently in a 30 cu ft sprung arch downdraft kiln I built in about 1982.  The earlier kilns I built were all cross draft and worked well but I needed a larger kiln.  The most helpful parts of the design of the later kiln - as far as achieving both even reduction and even temperature - were the bagwalls and the flue channel (which is in the floor).  

> There were four naturally aspirated venturi burners fueled with LPG at about 11”wc. The air intake on the burners was, as has been said already, set to produce a clean oxidizing flame, then never changed. The gas pressure was set with a regulator and left where it was.  The gas volume was adjusted through the firing with a valve on each burner.  

> Two burners were aimed into each side of the kiln, with the flames hitting a bagwall about 4 or 5 bricks high but perforated, with the arrangement finetuned over many firings.  The flames were thus mostly forced up to the arch and then drawn down to the channel in the floor, which is a brick wide and a brick tall and runs the full distance from the front wall of the kiln to the back, where it exits to the flue.   Across the channel are laid bricks (some half-bricks), their placement also finetuned over multiple firings to produce evenness in the results.  

The horizontal damper in the vertical flue was adjusted during the firing and was the only control for reduction. The amount of reduction was monitored using a carbon monoxide analyzer, which gives a reading in inverse relationship to the oxygen present.  Using the analyzer was cumbersome but extremely helpful, especially during the first firings to know what was happening at various places in the kiln so that the arrangement of the bagwalls and flue channel cover bricks could be finetuned.