Martz Memories, by Stanley Lee

Reduction Effects in Neutral Tale #13

Magazine publishers for the ceramics industry chose to print their magazine covers in color. Sometimes those color pictures included pottery that had been fired in an out-of-doors reduction kiln. It was only natural for readers to want to know how the colors were obtained. In the 1950’s, Indiana University Bloomington had no outdoor kiln, but students were interested to know how the colors were obtained. Their master-teacher, Karl Martz, knew of the student interest, and he wanted to devise a compromise that he thought might satisfy.

There was no easy way for their new indoor gas kiln to be further ventilated, and there would be no easy way to prevent their kiln from damage caused by reduction. The students needed to be taught about the reduction process. What could be done? Karl examined the steps that needed to be followed to obtain the desired effects.

Contrary to the usual oxidation atmosphere of a customary kiln firing, a reduction atmosphere stole oxygen from the kilns contents, and that included the oxygen in the glazes. However, the reduction did not happen all-at-once. The theft began slowly, although one could easily tell it was going on, for flames and fumes came out of every crack and hole they could find.

Reduction glazes were formulated to react to the theft of their oxygen, although all glazes showed results of some stealing. Karl, and his students, wanted to know what would happen to the glazes they were currently using. Karl also wanted to know what would be evidenced at different times as reduction took place. There was one critical period during the kiln firing that caught his attention. It was when the atmosphere changed from oxidation to reduction. This change-over time could be slowed down, speeded up, or even held steady at what was called a “neutral” atmosphere. When at neutral, the air was neither being sucked into the kiln, nor was it being forced out.

Karl experimented. With his pots and test-tiles glazed, he loaded and fired the kiln. Then at the desired firing temperature for those glazes, he held the pressure at neutral for half-an-hour. Finally he turned off the gas, closed all ports and spy holes, and let the atmosphere slowly re-oxidize. The slow cooling was said to enhance reduction.

As students we were well pleased with the results, but we were even more pleased to know we could get glaze reduction effects without the need to go through the dangers of the full reduction cycle. We enjoyed the lesson and the visual examples, and we used the techniques with success. For me, there was extra instruction. Karl Martz had shown me yet another way to approach problem solving.

Thank you, Karl.