Underglaze for Beginners: Brushing, Layering and Firing

Underglaze is colored clay slip designed to go under a clear glaze. It lets you paint, stencil and draw on a pot the way you would on paper, with colors that stay where you put them.

Unlike glaze, underglaze does not run, which makes it ideal for detailed designs.

What underglaze is, and is not

Underglaze is a color. It is not a glaze. Most underglazes need a clear or transparent glaze over the top to make the surface smooth, shiny and waterproof, especially on anything meant for food. Some are fired and left matte as decoration.

Read the label for the cone range and whether it is safe for your project. Our lesson on what an underglaze is covers the basics.

When to apply it

You can apply underglaze at different stages.

• Leather hard. The color sticks well and you can carve through it later, as in sgraffito.
• Bone dry. Works, but the porous surface drinks the color quickly.
• Bisque. The most common stage for brushing designs, because the surface is hard and holds detail.

Check what your underglaze brand recommends. Many are formulated to work best at one stage.

Brushing basics

1. Wipe the bisque with a barely damp sponge to remove dust.
2. Shake the jar well. Pigments settle.
3. Use soft brushes. A flat brush for solid areas, a round brush for lines.
4. Apply 2 to 3 coats for solid color. Let each coat dry until it is no longer shiny.
5. Brush in alternating directions to avoid stripes.

Thin coats beat thick ones. A thick coat can flake, crack or look streaky.

Practice on a flat tile first, as in paint a solid underglaze coat on a test tile.

Layering colors

Underglazes can be layered like paint. Let the first layer dry before adding the next. If you want a crisp edge, stencil or use masking tape on bone-dry or bisque surfaces.

Layered colors can mix or shift when fired, so test your combinations. Lighter colors usually need more coats to cover darker ones. See layering underglaze colors.

Clear glaze over the top

Apply clear glaze after the underglaze is dry and the bisque is clean. Dip or brush it according to your glaze instructions and avoid thick applications that can blur your design.

If you fire the underglaze first and then glaze, you are doing a three-firing process (bisque, underglaze, glaze). Most potters skip the middle one and glaze over the underglaze before the final firing, as long as the underglaze and glaze are compatible.

Colors change in the kiln

Underglaze colors often look different wet, dry and fired. Greens, blues and browns can shift, and reds are tricky. Make a test tile for every color with the exact clear glaze and cone you plan to use. A tile board of your colors saves a lot of disappointment. Our post on glaze test tiles shows a labeling system.

Common mistakes

• Applying to dusty or oily surfaces. The color crawls or peels.
• Putting on thick coats. They flake and shiver.
• Skipping the clear glaze on food surfaces.
• Not testing. The fired color surprises you.

Keep a color library

Photograph each tile and record the brand, color name, number of coats, clear glaze and cone. In Claybook you can save surfaces with photos and layers, so you can find the colors you like in seconds.