Saturday, October 11, 2008

Combining underglaze and art glaze

One of the technical problems that has obsessed me for the last few years has been colored backgrounds. To get an even tone, underglazes must be applied with an airbrush. Underglaze also soaks into the porous surface of the earthenware, making it impossible to completely remove overspray. That means any surface not being painted must be masked.

This might seem like a time-consuming, but not especially limiting, requirement. But there is a catch. Once underglaze is applied to an area, it cannot be masked again. The surface is too delicate to touch, never mind have latex applied and then peeled off again. (That is, if latex actually peeled off an underglazed surface. In my experience the two become permanently wed!)

This made complex colors difficult enough, but the real challenge was putting a realistic horse on a colored background. Eventually I found that I could use some of the underglazes used at those you-paint-it coffeehouse type ceramic shops. They contain just a bit of glaze, so when they fire the surface has a sheen. If I fired that as the background, I could then paint the horse and wipe off the excess on the background. Some overspray remained, but if I chose rich colors they would sink below.

What I really wanted, though, was to use art glaze on the background. Art glazes are designed to concentrate in the recesses, which is great for bringing out texture. Because I design medallions and tiles with art glaze in mind, I use a lot of texture - particularly on the backgrounds. In an ideal world, I could have a realistic underglazed horse with an art glaze background. The only problem is that glazes move. That allows them to flow into the recesses... or flow over onto the horse.

So adding an art glazed background was a really good way to muck up a nicely glazed horse. Still, I could see such a pretty end result in my head that I kept messing up medallions in search of an answer. What I found was that I could add the art glaze if I completely finished the horse first.

Here are my two Clinky Classic awards with the horse design completed. The foals were painted with underglaze and fired, then gloss glaze was added and the medallion fired just like any other piece. The background is bare earthenware bisque.

At this point, I can add the art glaze to the background. Any excess is easy enough to remove with a sponge since the foal is now impervious under the gloss. I then trim the border between the gloss and the art glaze with a #11 blade. This tends to pile the art glaze ever so slightly into a ridge along the edge. I suspect this is what helps contain the art glaze on its side of the design. I also fire this second glaze at a slightly lower cone (07 instead of 06), and that seems to help, too.

I wasn't sure with this design if the technique might work. I make my own designs flush against the background, but this little guy has a recessed border around him. I wasn't sure if gravity might not flow the glaze down into that recess. I was happy to see that it didn't.

I was unsure enough that I only glazed the tobiano and left the overo, figuring I could at least salvage the one if it didn't work. I did get really clean borders, but I ended up with a paler background than I wanted. That has been the real challenge with art glaze backgrounds - depth of color (and sometimes type of color!) is really unpredictable. Getting a combination that flatters the underglaze work requires a bit of luck.

I am pretty sure that while the light green doesn't have enough contrast for the tobiano, it's probably right for the darker overo so I'm using it again. I use make-up sponges to apply the glaze, and in this photo I haven't cleaned the excess off yet. I've also added another layer on the tobiano ornament, hoping to deepen the color on that one. These will both go in my next glaze firing, along with a handful of my own ornaments. Those all use the other (Concepts underglaze) background method, so I should be able to show the difference between the two approaches once they are all done.

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