This is a follow up to my article on raw glazing single firing experiments.
Single firing results
Whilst loading the kiln I had a problem with glaze flaking off, especially around the rims. I had to rebrush glaze onto most of the pots. The clay content of both glazes is quite high which I believe is the problem.
I’ve included my notes on the firing. I think the firing schedule itself was ok, I had no breakages, and can’t see any pinholing or bloating which are associated with firing too fast. I did have a major problem with glaze crawling. I think every pot had at least a little bit, and some a lot of crawling. With crawling, the glaze is not adhering properly to the pottery, islands of open clay appear in random locations.
In my case, from what I’ve read it seems the high clay content of the glaze is probably the culprit. As I was working with bone dry pots, the shrinkage of the glaze doesn’t match that of the body, and a weak bond is created. These glazes have been ok when used on bisque pieces, but it seems they are not going to work for raw glazing.
I need to do some more reading, but I think I will reformulate my glazes with around 10% clay next time. I may also look at additives which will give a more flexible glaze.
I was pretty disappointed when I opened this kiln load, but at least I have learnt some things. It gave me an idea of how my slip decoration looks once fired, and which slip formulas are working. I’m still keen to make single firing work for me, so i think it’s worth doing some more experimenting.
Single firing manual kiln schedule
10am start, bottom 2 switches at number 3, lid propped open. Kiln is 75% full with bone dry small pieces.
10.30am water condensing on mirror above spyholes.
1pm no condensation visible on mirror, close lid, turn top switch to lo. May have dried sooner, but I was out shopping so couldn’t check.
2pm top of kiln started to glow dull red, water vapour present at peephole, no change to controls.
3pm a little vapour, bottoms two knobs turned up to 4.
4pm whole kiln red/orange bottom switches turned to 5, top switch to 2. No vapour
5pm top switch to 3, bottom switches to 6
6pm bottom switches to full, top switch to 5
7pm all switches full, top side spyhole plugged.
9pm cone 7 down, I decided to end firing, all switches off