I usually only fire my work twice. A first bisque fire to bake the clay enough to glaze, then a final fire to melt the glaze into solid, shiny glass. But when a piece involves detailed painting, I fire it three times.
I rarely do this. An extra bisque uses extra electricity. Sometimes I even paint straight on greenware—unfired clay. But these pieces took so long—a second bisque sets the underglaze so I can apply glaze without risk of smudging. I’ve done this before. A finger brushed against underglaze that was once dry but was dampened by wet glaze. So frustrating!
A second fire also gives me a chance to correct mistakes. Like if the burgundy comes out too streaky, the way it did in my test tile, I can touch it up before sealing it forever under glaze.
Underglaze is strange. It's like watercolor but also not at all like watercolor. They share the same binder—gum arabic—so you can thin them and mix them the same way. You have to work quickly. Bisqueware absorbs water even faster than cotton rag, each brushstroke disappearing into the clay almost immediately. Wet on wet is next to impossible. And you layer colors the same way, light to dark. You can overpaint light on dark, but when it fires, the dark underpainting bleeds through no matter how many layers you put down. Because the particles move and bleed into the glaze during heating. Because science.
I boldly did a red imprimatura on one of my first ceramic frames, thinking it might show through a little. Oh no. It was a red mess with translucent scenery floating over it. You can see it in the photo below. I eventually put a gold luster rabbit on it and gave it to a friend. Lesson learned.
I use high-quality brushes for watercolor—brushes that hold water for long fluid strokes, brushes that keep a fine point. For underglaze I aim for the same, but the clay absorbs water so quickly there's no such thing as a long fluid stroke. And high-quality brushes are useless since the clay particles in the underglaze get trapped in the bristles and sabotage the fine point pretty quickly. I reach for smaller and smaller brushes so that even when they're splayed from clay, they're not too wide.
The biggest difference is how colors change during firing. With watercolor, what you see is what you get. With underglaze, the end result will be much darker. Or sometimes lighter. Sometimes it barely shows up at all. Your carefully planned color theory can fly out the window as soon as the kiln shuts. Science again.
I made test tiles. Little pieces of Standard 182 clay with one, two, and three coats of each color. I keep them on my table when I paint, a tiny taste of before and after. It gives me an idea of what to expect, which colors will be reasonably accurate and which will need more care.
We'll see.