What Is Underglazing (and When Do You Need to Do It)?

Underglaze is a color product applied to pottery before a clear or transparent top glaze is added. Unlike regular glaze, underglaze stays where you put it, fires with crisp, predictable color, and can be applied at multiple stages of the pottery process. If you’ve ever wanted to paint a design on a ceramic piece and have it actually survive the kiln, underglaze is how you do it.

If you’ve spent any time browsing pottery supplies, you’ve probably noticed that underglaze and glaze are sold separately, described differently, and used in different ways. For a beginner, the distinction isn’t always obvious. Both come in jars, both go on bisqueware, and both get fired in the kiln. So what exactly is underglaze, how does it differ from regular glaze, and when does your project actually call for it?

The answers are simpler than you might expect, and once you understand how underglaze woks, it opens up a whole category of decorative possibilities that regular glaze alone can’t deliver.

What Is Underglaze?

Underglaze is a commercially prepared decorating product made from finely ground clays, frits, and metal oxides or stains that give it its color. The name comes from where it sits in the finished piece: underneath the top glaze layer, fused into the surface of the clay. When you fire a piece that has been underglazes and then top-glazed, the underglaze becomes a permanent part of the ceramic surface, protected beneath a glass-like coating.

The key property that distinguishes underglaze from regular glaze is that it doesn’t flow or move during firing. Regular glazes are formulated to melt and become fluid in the kiln, which is part of what gives them their rich, glassy quality, and also part of what makes them unpredictable around edges and fine details. Underglaze, by contrast, is formulated to stay put. It fires to a matte or near-matte finish on its own, holding the shape of every brushstroke, stamp, stencil, or drawn line right where you left it.

That stability is the whole point. Underglaze gives you control that top glaze simply can’t offer when you need precise color in a specific location.

How Is Underglaze Different from Regular Glaze?

The difference comes down to chemistry and purpose. Regular glazes are glass-forming: they contain silica and flux in amounts that cause them to fully melt at kiln temperature, producing that characteristic shiny, smooth, sealed surface. This is what makes a glazed mug hold coffee without leaking or staining.

Underglaze contains color-producing materials but much less of the glass-forming content. It partially fuses with the clay body during firing without fully melting, which is why it holds its shape and doesn’t bleed into surrounding areas. On its own, unfired underglaze is porous and relatively fragile. A clear or transparent top glaze applied over it provides the protective, sealed surface that makes the piece functional and durable.

There’s another practical difference worth knowing: all underglazes are mixable like paint. You can blend two underglaze colors together to create a custom shade, thin them with water for a wash effect, or layer them wet-on-dry to build depth. Regular glazes are sometimes mixable, but often are not, and mixing them carelessly can produce unpredictable or poor results in the kiln. With underglaze, the behavior is much more like working with paint, which is part of why beginners and classroom settings find them so approachable.

When Can Underglaze Be Applied?

One of underglaze’s most useful qualities is its flexibility in terms of when it can be applied during the pottery process. Unlike regular glaze, which is applied after bisque firing only, underglaze can go on at three different stages.

Leather-hard clay is the stage where the piece has dried enough to hold its shape but is still cool and slightly damp to the touch. Applying underglaze at this stage allows it to bond closely with the clay as both dry together. This is a good option for sgraffito work, where you apply underglaze and then carve through it to reveal the clay beneath, or for designs that benefit from the slight texture of working on raw clay.

Bone-dry greenware is fully dried, unfired clay. Underglaze can be applied here as well, though greenware is fragile and must be handled carefully. Some potters prefer bone-dry application because the clay is more stable than leather-hard for detailed brushwork.

Bisqueware is the most common stage for underglaze application and the approach most beginners start with. After the first kiln firing, bisqueware is firm, easy to handle, and porous enough to absorb underglaze readily. It’s the most forgiving surface to work on and gives you the most control over the decorating process.

Regardless of when you apply underglaze, the piece goes through the glaze fire only once, after the top glaze has been applied over the underglaze. If you apply underglaze to greenware or leather-hard clay, the bisque fire that follows will set the underglaze before you add the top coat.

How Many Coats Do You Need?

Most underglazes are applied in two to three coats, allowing each coat to dry fully before adding the next. This is especially important for achieving true, saturated color. A single thin coat of underglaze often fires lighter than expected, because the underglaze layer is thin enough for the clay body color to show through.

Building up coats produces more consistent, opaque color. For lighter colors like yellow, white, or pale pink, three coats are typically recommended to avoid patchiness. Darker colors like cobalt blue or black are more opaque and may cover well in two coats, though three is rarely a mistake.

Some manufacturers offer single-coat underglaze formulations that are thicker and more heavily pigmented, designed to provide full coverage with one application. These are especially practical for classrooms and production settings where speed matters. Clay-King carries one-coat products that are often fluxed to produce a slight low sheen rather than a completely matte surface.

Choosing an Underglaze: What to Look For

Commercial underglazes vary in quality, color range, and performance, but several product lines have earned reputations as reliable, beginner-friendly choices.

Amaco Velvet Underglazes are among the most widely used underglazes in studio and classroom settings. They offer a broad color range, excellent coverage, and consistent results across firing temperatures. The Velvet line is known for producing rich, true color that doesn’t shift dramatically during firing, and the colors are mixable with each other to create custom shades.

Mayco Stroke & Coat is technically a glaze rather than a traditional underglaze, but it behaves like one in many contexts: it can be applied to greenware or bisqueware, it holds brushstroke detail well, and it fires to a glossy, colorful surface without requiring a separate top glaze. It’s a popular choice in paint-your-own pottery studios for exactly that reason. It also mixes like paint, which makes it intuitive to use straight out of the jar.

Amaco Teacher’s Palette Series is formulated with classroom use in mind: easy application, reliable color, and predictable results across a wide range of student skill levels. Like other Amaco underglazes, colors in this series are fully mixable.

When shopping for underglaze, check that the product’s firing range matches your clay body and kiln. Most commercial underglazes are compatible with a broad range of temperatures from low fire through cone 6, but high-fire applications can sometimes shift or mute colors. Reading the manufacturer’s specifications before committing to a product for a new project is worth the few minutes it takes.

Do You Always Need a Top Glaze?

Most underglazes require a clear or transparent top glaze applied over them to seal the surface, bring out the true color, and make the piece functional. Without a top glaze, the underglaze surface is matte, slightly porous, and not food-safe for functional ware like mugs or bowls.

That said, there are situations where leaving underglaze unglazed is an intentional choice. Sculptural and decorative pieces that won’t hold food or liquid can be fired with underglaze only, producing a matte, painterly surface that some potters prefer aesthetically. Some underglazes are also formulated to produce a slight self-gloss at certain temperatures, giving a usable surface without a separate top coat.

For functional ware intended to hold food or drink, always apply a compatible clear or transparent glaze over your underglaze and confirm that the combination is food-safe before putting a piece into regular use.

Underglaze vs. Slip: A Common Point of Confusion

One more distinction worth clearing up: underglaze and slip are sometimes confused, particularly by beginners who encounter both in the context of surface decoration. They are not the same thing and are not interchangeable.

Slip is a liquid form of clay, essentially clay mixed with enough water to make it fluid. It can be used for casting in molds, as an adhesive for joining clay pieces, or as a surface coating. When used decoratively, slip can only be applied to leather-hard clay. If you try to apply slip to bisqueware, it won’t adhere properly because the clay body can no longer absorb water in the same way.

Underglaze, by contrast, can be applied to leather-hard clay, bone-dry greenware, or bisqueware. It’s a commercial product formulated specifically for decorating, not a clay-based material. The two have different properties, different application windows, and different finished effects.

Getting Started with Underglaze

For most beginners, the straightforward approach is to work with bisqueware, apply two to three coats of a commercial underglaze, add a compatible clear glaze on top, and fire according to the manufacturer’s specifications. That process is reliable, the materials are widely available, and the results are repeatable, which gives you a clear baseline to experiment from.

From there, the possibilities expand considerably. Stencils, stamps, sgraffito through underglaze on leather-hard clay, layered colors, wax resist used in combination with underglaze: each of these techniques builds on the same foundational material, just used in a different way.

Call us at 888-838-3625 or email info@clay-king.com to get started.

Still figuring out where to start? The Clay-King team is here to help. We’ve spent a lot of time thinking about glazes, and we love matching potters to the right products for their projects and skill levels. Give us a call at 888-838-3625 or send us an email at info@clay-king.com and we’ll be happy to point you in the right direction.

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