Thursday, May 18, 2023

Carbon trapping with help from a little friend


Shino Minnow Mug made by Bruce Gholson

Bruce writes, 
      "I’ve been talking recently with a friend and fellow potter Jared Zehmer, about the vagaries and elusiveness of consistent carbon trapping in shino glazes.

      During my last cone 9/10 reduction firing, I learned a serious lesson from a mouse about kiln control and carbon trapping in shinos. The firing got off to a good early start looking to be “on schedule“, going great through body reduction, and it was early enough to ensure carbon at right time, albeit feeling maybe a little richer than my normal firing. The problem seriously reared its head when I tried to get the kiln into a cruising mode -- post body reduction -- to fire on up to the final temperature.  I still desired a continuous reduction “lite”, and I judge this from the visual feedback of flame and pressure out of the bottom spy which is the opening in the kiln's door. The back pressure wouldn’t go away no matter how far out I pulled the damper!

     Normally slight damper adjustments meant strong pressure changes, but I was stuck with a jet of back pressure flame shooting out of the bottom spy hole, with the inevitable slowing of temperature rise. 

     After a bit of time, and soon well behind my preferred schedule of climbing temperature, I realized that pleading with the kiln wasn’t going to cut it. The remedy that ended up working was to partially open two upper spy holes, front and back, until the desired back pressure was achieved. The damper was open and out of play for the rest of the firing until closing time. Essentially the kiln had now become a hybrid, an updrafted downdraft kiln; lets call it sidewinder drafting.

     The firing results were beautiful, and there was very rich carbon trapping on many pieces. 
The problem wasn’t a choked bottom shelf stacked too tightly in front of the flue like I had feared. The culprit turned out to be a strategically inclined little rodent that had been collecting pieces of kaowool from the gasket of the chimney, and stashing them out of sight in the center back half of the flue. It was enough of a pile to drastically influence the draft as the firing progressed, but was great for strong carbon trapping effects captured during early body reduction in shinos.

Nest made by mouse out of kaolwool looking through bottom of kiln

Bulldog Pottery’s kiln firing log

Looking down to the bottom of the kiln from the top of the kiln's chimney

Grouping of carbon trapped shino mugs made by Bruce Gholson

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