Solar fired electric kiln, reduction glaze firing

We have fired the solar-powered electric kiln to stoneware in reduction for the first time.

This all happened a few weeks ago, but I got distracted by the arrival of the new electric car, so I’m a bit behind in writing all this up.

The kiln worked perfectly, just as I had hoped. No problems at all. I had it packed with domestic sized items, cups and bowls, plus a few clay and glaze tests to see how things worked. Plus a spread of cones to see the temperature variations throughout the setting. This is only the first attempt at a reduced stoneware firing in this kiln, so it is new territory for me. I will need to test out the many options available to me to get the most efficient firing time with the best results, using the least amount of gas for the reduction, while achieving the best reduced colour.

I decided to try a fast firing, just to check out how fast the kiln can fire. It took 3 1/2 hours to get the 1,000oC and then I set up the pilot burners to start the reduction atmosphere. The burners clipped into their mountings easily. I was careful because the kiln was quite hot already at this stage in the firing. I made the mounting so that  one simple bolt can be slid into a hole like a pin, and the mounting is secure.

The burners lit easily off the kilns heat, I didn’t have time to click on the lighter to get them started. I had built a few different sized flue holes in 3 separate damper tiles. I had made a few tests at room temperature with the different sized flue holes.

A 12mm hole with a 5 kpa pressure. A 25mm hole with a 10 to 20 kpa gas pressure and a 32mm hole with a 20 to 35  kpa gas pressure. These settings were made at room temperature in a cold kiln, so I expected to have to make a decision based on the new volume of expanded gasses at the higher temperatures.

I chose the damper tile with the flue hole outlet in the top of the kiln of 25mm sq , as a starting point. Then adjusted the gas pressure to 10 kpa. This established a slight back pressure at the burner hole and a small flame at the flue exit. I also tried 12 and 15 kpa. This achieved what seemed to be a good back pressure and reduction atmosphere with a small flame at the spy hole as well, but the temperature was still climbing slowly. I decided after an hour of this to increase the gas flow and use the larger flue hole of 32mm Sq. This kept the same reduction atmosphere and back pressure, but increased the rate of temperature rise.

The kiln reached temperature in just over 2 hours and consumed 700 grams of gas at a cost of about $2.00. The electricity cost was nil, as we are totally solar here and I fired in the middle of the day, at the greatest solar productivity. But the cost foregone of lost sales of kilowatts to the grid was about $4.75, so a total theoretical firing cost of $6.75

Total firing time of 5.25 hours. A very fast firing. Not the best for good quality glaze quality, but I proved to my self that it could be done. I can now choose to fire any length of time slower than this.

The glaze result was OK. The reduced glaze colour of the rock glaze celadon could be richer and deeper, but it is excellent for 2 hours of reduction. The translucency of the porcelain was very good – considering the very short maturation time at high temperature of approximately just one hour. It was a very light pack, just to try out the kiln and all three sets of cones went over evenly at the top, middle and bottom shelves. Cone 10 right over = more or less 1300oC

The next firing will be a slower one.

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