Monday, 26 March 2012

A very mixed bag!

It was a really warm day yesterday, so I had to wait until the evening to let the kiln cool down enough to open it. Some nice pieces came out, but on the whole it was a rather disappointing firing. I thought I'd fixed the problem of my leaking sagger by sealing the top with kiln fibre, but air is still getting in somewhere. Several of my shino pots were completely oxidised, and this neph sye. shino glaze really doesn't like to breath, especially when it's laid on bubbles like crazy.

A few pieces were fully reduced, and they came out pretty well. This carved sake cup was probably the best of the lot:

It has a transparent ash glaze on the outside and the shino on the inside..the ash glaze has brought out the iron specks in the clay nicely.

Anyway, I'll say more about this firing later, but the sun is blazing outside and it's way too lovely to be sat by the computer!

Some time later..

Well, I just pulled the saggar out of the kiln, and I now know why most of the pots were oxidised. There's an enormous crack running the whole length of the container and about 3 mm wide! Oh's lasted just one firing, whereas the last one did around 15 before cracking. The rim was on a slight tilt, so I think the weight of the kiln shelf lid, plus pots on top of that, put too much strain on one side. I could make a new saggar from crank clay..but it's difficult to get the size exactly right, & good crank is very expensive..hey ho, back to the drawing board..

For the oxidised part of the firing (the part that was meant to be oxidised!), I experimented with an oribe glaze to which I'd added a little extra copper oxide to make it darker.

Sake cup, approx. 3 inches tall, cone 9

Sake cup, other side

Sake cup, underside & foot

I also tested a slip made with black iron oxide (rather than red), with hakeme brushwork over the top. The slip recipe was as follows:

50 kaolin
50 HVAR ball clay
6% black iron oxide
4% manganese dioxide
2% cobalt oxide

No blistering or bubbling of the slip layer this time, but the transparent glaze layer has thousands of tiny bubbles and pinholes in it, making it appear quite matt. I'm really not sure where the gases can be coming from as it isn't the white slip or the clay body..perhaps it's the manganese dioxide volatising or impurities in the iron oxide? It's possible that the glaze is applied a bit too thick, but more likely it needs extra flux to allow the gases to escape.

Bottle vase, approx 4 icnhes tall, cone 9

Two head cups came out okay. The one on the right (with shino glaze) should have been reduced but is mostly oxidised thanks to the cracked saggar. The one on the left is glazed inside and out with the matt ochre-ash was stood upside down on the tips of the horns, which worked really sticking to the shelf.

Head cups, approx 3 inches tall, cone 9.

The next piece was half reduced, half's the same shino glaze all over, but the top half was in the oxygenated part of the saggar. That's certainly a first for me!

Tea caddy, approx 6 inches tall, cone 9


  1. these look like they are carved out of rock, great surface angles, how do you get that cup to turn around in the video, cool.

    1. thanks for the kind words, Linda. The spinning cup is down to hand's a banding wheel with a throwing batt on top painted white. I turn the wheel from underneath, whilst trying not to get my arm in the video :)

    2. What a lovely tea caddy ! I like very much it's shapes and colors.
      Bravo for your work Mark.

    3. Thanks for your kind comment, it's much appreciated :)