Friday, 10 February 2012

Experiments with shino, oribe and under-glaze slips

Well, I just about managed to get a firing ready last weekend, despite the sub-zero temperatures. I tried out several new approaches to decorating and glazing, some of which worked really well, others much less so. For the shino-glazed pots I was using a new saggar, the rim of which turned out to be slightly uneven. Some air must have leaked in between the rim and the kiln shelf (which I use as a lid), making a few of the pots oxidise and the glazes bubble and blister very badly. I'll have to go back to sealing the gap with kiln fibre.

The two pots below have crawled quite alot, but I still like them:

Sake cup, approx. 2.5 inches tall

Chawan, approx 4 inches wide

Chawan with clam shell marks
Another experiment was to apply oribe glaze over an iron oxide slip. I wanted to leave some areas of the clay body exposed, so on this small jug I brushed the glaze on:

Jug, oxidation firing, cone 9

I'm very pleased with the outcome and will use the technique again. I think maybe the glaze needs some extra copper oxide to make the green darker, and so it will flux and run a bit more..just need to be careful it doesn't end up all over the kiln shelf! With the sake cup below, I masked sections with a polyurethane varnish (instead of using wax), then dipped the pot in the glaze as normal. This worked very well, except when the pot was fired, the varnish burned, peeled off and took some of the (unfired) iron oxide slip with it. Note to self: always apply the iron oxide before the bisque firing.

Sake cup, oxidation firing to cone 9

Finally, I made a few vases and coated them with black and then white slip while at the leatherhard stage. After bisque firing, a transparent ash glaze was applied. These pieces really have me puzzled since one came out perfect..

Bud vase, approx. 4 inches tall
Oxidation firing to cone 9

..while two other pieces were covered in tiny blisters under the white slip, like the bloating one would get in over-fired earthenware. I figure that the bubbles were caused by gases being expelled from the black slip layer, and the white slip and viscous glaze have formed an impermeable barrier. Hmmm, that's my theory anyway..and I'm not sure how to solve the problem, except by experimenting with different formulations of black slip.

I was wondering if using black iron oxide (Fe0) would help as it won't decompose, releasing oxygen as red iron oxide does. If anyone has any advice or thoughts on this one, I'd be really interested to hear them.

 Never give up..the best is yet to come!


  1. That little bud vase is stunning! The proportions, the glaze, it's perfect.

    Have you managed to sort out your problems with the FeO yet? I hope so - it'd be nice to see more work like this.

    (Are you firing in oxidation in electric or in a gas kiln? If it's in gas, oxidation is really "oxidation" so you may be still getting some possible volatilization of the FeO through mild reduction, resulting in pinholes or blisters.)

  2. Jack, thanks for the nice comment! I'm still firing electric but starting to do some experimental firings with gas, but not using black iron yet. I have fired black slip under a shino glaze in a wood kiln and the heavy reduction seemed to stop the shino from bubbling up. The problem I found with Fe0 is that what they sell here isn't actually Fe0, it's magnetite!