Friday, July 9, 2010

Underglazing with Concepts

As I mentioned in the previous post, the newer types of underglazes (Concepts, Stroke & Coats, SuperStrokes) are often problematic for painting realistic horses. They are popular at what are known in the ceramic trade as "Contemporary Pottery Studios" (also sometimes called Ceramic Cafes) where people can come and paint bisqueware. That's because they are designed to have the clear glaze applied while still raw, so that the piece is completed in one glaze firing. Traditional underglazes are bisque fired first, and then covered with clear glaze and fired again.

For the sake of simplicity, I'll use the brand name Concepts in this post to refer generally to this type of glaze.

Concepts can be bisque fired like normal underglazes, but they have some properties that make working with them a little different. The most important of these is how they interact with liquid latex. Latex masking fluid is commonly used on ceramic horses to mask off white markings and patterns. I've painted on the bisque tile above in a giant "N" for the world "no" because latex and these kinds of underglaze do not mix.

These two tests have been sprayed with a mixture of Concepts colors. With the "Inspire" tile, I've added directional shading in (traditional) transparent underglaze colors and then completely cleaned the latex from the edges. On the "N" tile, I only sprayed the Concept mixture. Afterwards, I pulled most of the latex off from the letter, but left it on the border. I'll fire them both at Cone 04.

(I should mention that traditional underglaze and Concepts can be intermixed just fine, but the resulting underglaze almost always behaves like a Concept underglaze.)

Here is what happens after the tile is fired. With ordinary underglazes, latex usually burns away or at most leaves flakes of color that can be brushed off easily. With Concepts, the latex melds with the underglaze to form heavy, raised edges of dense color. These will not come off. The only way to remove them is to break away the underlying bisque.

Although it is hard to see in these photos, the rest of the surface is semi-glossy. That makes it hard to add details with transparent underglaze because it will bead along the surface. It also has a slightly pebbled texture that, while it usually fires away once the true glaze is added and fired, makes added detail even more difficult.

That pebbled texture is the other reason that Concepts aren't well suited to body color on horses. The texture goes away with firing, but in many cases it has enough dimension when raw to catch the directional spray. This close-up of the green background on the "Inspire" tile shows this really well. As the darker green was sprayed to catch the edges of the text and border, it also caught the undersides of the "pebbles".

Most of the time these do even out during the final glaze firing. All underglazes tend to diffuse and blend a bit during firing, so the speckled effect usually disappears or is at least toned down. It doesn't always, though. That's not a big deal with a background, where it can look like an artsy treatment, but it can ruin an otherwise nicely-done horse.

The other aspect of the Concepts is that after bisque firing, the finish is as hard as nails. That is a problem if more etching or erasing is needed. But it's the one trait that makes them really useful for medallions. In the next few posts I'll show in more detail the two different approaches to coloring backgrounds.


Marge Para said...

One problem with Concepts, at least with what I have experienced, is the some colors will run if you apply additional gloss glaze on top of it.. especially in high fire pottery... as the gloss glaze moves, it drags the color with it.

Lesli Kathman said...

That's good to know, Marge! I hadn't done any high fire with them, so I didn't know.