March 2020 Soda Firing

After my last soda firing at Northern Clay Center, I was excited to get back and fire the kiln again and NOT oxidize the kiln too much during the down-firing and also try to skip body reduction all together. Leading up to this firing, others had been firing the kiln and not having good luck with the thermal couple, as it was not picking up the heat produced from the pilot burner and shut off the burners. I was a little nervous about this but went on anyways later to find out that this created a bit of a problem for me during the firing. On another note, I was lucky enough to fire this kiln less than a week before we closed our doors at Northern Clay Center due to Covid-19, the Minnesota Stay at home orders, and the general interest in the safety of the public and our NCC community.

Materials:

• My Porcelaneous Stoneware and Continental Clay’s Woodfire Porcelain with 12 mesh brick grog wedged in.

• RF Studio East Soda Flashing Slip from UWRF.

• RF Studio Blue Celadon from UWRF.

• Basic salt/soda wadding and Newman Fireclay wadding.

Loading:

Notes from the firing up in temperature:

• The kiln had about 13 hours to preheat before the active firing time.

• The kiln was at about ^011 (1786°F) in the morning.

• At hour 2.5 and 5.5 the burners went out due to the thermal couple.

  • We replaced the thermal couple the second time.

• At hour 8.75 (2214°F) and hour 9, I sprayed 800 grams of

• soda ash mixed with one gallon of water into the kiln.

• The kiln soaked in reduction for an hour after spraying.

• Overall took about 10.5 hours

Results / Post Unloading Notes:

• The firing went well, but the burners going out certainly set back the firing a little bit.

• A great range of surfaces:

  • The top of the kiln had lighter areas of soda and nice reds / oranges with the silica sand shining like stars.

  • The bottom of the kiln was very dark with lots of reduced carbon trapping

  • The heavily carbon trapped areas went to a nice stony, smoky, grey color.

  • In between the heavy carbon trapped stony areas and flashing was a nice matte brown. I think that this may have been influenced by the water vapor in the kiln during the cooling.

  • The surfaces were very rough and needed a lot of sanding and some oil.

  • The flame marks had tighter banding to them. I seem to think that this may be due to the proximity in time that I sprayed the soda ash into the kiln.

  • The extra soda sprayed into this firing was not needed. Some things got a little to juicy. Beads formed on the bottoms of some shelves and dripped onto pots and there were some wads that stuck.

  • There was some reoxidation in the brown and grey areas creating deep reds.

Next Time:

• Use less soda.

• Don’t reduce as much during the down firing. A super smoky atmosphere may not be needed.

• Reoxidize the kiln at 1900°F on the way down to hopefully promote more color variation and colors that appeared in my first firing at NCC.

• Pack the kiln a little looser, specially near the top to promote heavier build ups of soda.

• Turn the kiln up a bit more overnight so it is hotter in the morning and requires fewer active hours to fire up in temperature.

• Think about reformulating the clay bodies or the slip to prevent the roughness that occurred in the high soda and reduced areas.

Summary:

This firing was a really exciting one as halfway through I had to change the thermal couple. The kiln was buttoned up for a couple minutes and only lost one hundred degrees or so, setting me back a little bit. Other than that, I think that the firing was very successful. There was a lot of successful down-fired soda surfaces. Looking back, I am really stunned with the variety of surfaces; deep reds and darker tones coming out of this one. This is probably attributed to the heavy reduction during the down-firing. Between the heavy reduction and firing to 1615°F, the soda seemed to blister a bit with my high alumina slip over my not so high alumina clay body. Because of this and wanting to continue firing this way, I am looking into reformulating my clay body, as well as formulating some new clay bodies that are higher in alumina to possibly not use my slip anymore or fit it better. I am hoping in my next firing to reoxidize the kiln on the way down at 1900°F and maybe prolong the down-firing beyond 6 hours down to 1650°F or so.