Saturday, 26 November 2011

Some experiments with black underglaze

Most of the glazes I applied for the last firing were successful, but I was a little disappointed with the black slip I used for the underglaze decoration (brushed onto bisqued clay). I tried out several different clear glazes, glossy and matt, and most of them "ate" the black slip to some extent. I was hoping that by adding 15% Mason's black stain to the slip recipe I was already using, I would obtain a denser, more solid, line. The colour certainly turned out much blacker, but on some pieces the decoration was still too faded under the glaze. Of course, it's possible that the slip was slightly too diluted..

One surprise was that the Leach 4321 glaze with 2% tin oxide came out almost clear..just slightly milky in the thicker areas, but the underglaze painting has remained reasonably well defined on this bottle-vase:

Bottle-vase. Approx 6 inches tall. Cone 9 oxidation.

I read yesterday that on traditional oribe ware, Japanese potters paint brown iron oxide (a kind of burnt umber? We can't get brown iron oxide here) on top of a very thin transparent glaze, and that this glaze is made by adding a little wood ash glaze to feldspar. Unfortunately, I don't yet have a recipe, but perhaps painting onto the glaze helps prevent it fusing and being absorbed. But I find it hard to believe that all oribe work is done this way, because I've watched Goro Suzuki painting directly onto raw clay..also, many of the pictures I've seen on the web seem to show the iron decoration under patches of thicker, cloudy glaze. If anyone can enlighten me as to the exact process, please drop me a line! Painting directly onto unfired glaze would also be problematic if the glaze surface is loose and powdery (as it is with Hamada clear glaze), which may explain why they use a form of feldspar to help harden the surface. Having said that, I imagine that most oribe potters will be raw glazing (applying glaze to leatherhard work), which could make a huge difference to the qualities of the glaze surface.

Most clear glaze recipes I come across, contain some form of calcium oxide, which in large quantities can have a bleaching effect on iron oxide. So, I'm guessing that oribe potters use a glaze which avoids using whiting (calcium carbonate), for example.

The bottle vase shown below was glazed with a low-clay shino, and the decoration has stayed very clear and solid under the glaze, with no fuzzy edges. I was really pleased with the finish achieved here and will be using this combination again.

Bottle-vase, approx 3 inches tall. Cone 9 oxidation.

Bottle-vase, approx 3 inches tall. Cone 9 oxidation

Another approach might be to spray wood ash onto the bisqued clay, before decorating with a gas kiln, it gives an attractive flashing effect on the clay, but I don't know if it would work in an electric or whether it would harm the elements (in the same way that soda ash does).

The next step is to try applying the clear glaze a little thinner and see how different iron pigments behave on top. (Just thinking out loud..any craters and pinholes in the glaze surface are also not going to help with fine brushwork..). I'll mix up a few combinations of yellow, red and black iron oxides and also combine them with a clay slip to see how it affects brushability etc.

And tomorrow, I really must tidy up the studio a little..experimenting with new glazes makes plenty of mess!

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


  1. Thank you, and for subscribing to my blog. Your paintings are wonderful..very interesting work!