By Lucy Alexander
If you’ve spent any time in the glaze room at either of our studios, you’ve probably seen some jars labeled underglaze — and you may have wondered what is underglaze and how is it different from glaze? They look similar, they both add color, and they both get fired… so what is underglaze for and when should it be used vs glaze?
This post breaks down what underglazes and glazes actually are, how they behave at cone 5, when to combine them, and when you don’t need to. Whether you're new to clay or years into your practice, understanding the difference will dramatically expand your options for surface design and creative expression.
What Is Underglaze?
Underglaze is essentially colored clay, made from pigments or stains, clay, water, and a very small amount of flux. Because of this clay-based structure, underglaze stays exactly where you put it — even at cone 5. This makes it perfect for precise, detailed surface work such as illustration, linework, sgraffito, or painting.
Underglazes fire to a matte surface on their own, and they do not melt or become glassy without a clear glaze over them.
Why Use Underglazes?
Underglazes offer a level of control, stability, and versatility that glazes simply can’t match. One of their biggest advantages is that they can be applied at two different stages: on greenware (before the bisque firing) or on bisqueware (after the bisque firing). Glazes, by contrast, can only be applied to bisqueware.
Because underglazes are essentially colored clay, they are extremely stable in the kiln. They do not run, melt, shift, or move during firing, which gives you reliable, predictable results even with delicate surface designs.
Underglazes can also be mixed like acrylic or oil paints, allowing you to blend custom hues and create your own palette. You can lighten, darken, tint, or tone colors in ways that aren’t possible with glazes.
This makes underglazes ideal for:
painting fine lines
detailed illustration
lettering
subtle shading
delicate, painterly surfaces
sgraffito and mishima
What Is Glaze?
Glaze is essentially a thin layer of melted glass that fuses to the clay body during firing. It’s made from a blend of silica (the glass former), fluxes (which lower the melting point), alumina (which adds stability and prevents the glaze from running too much), and various colorants or opacifiers to achieve different surfaces and effects.
When fired to cone 5, glaze melts into a smooth, continuous coating that:
seals the pores of the clay body
creates a more water-tight, impervious surface
can be glossy, satin, or matte depending on the formula
increases durability and makes functional ware easier to clean
How Underglaze and Glaze Work Together
If you want your underglaze design to be shiny, apply a clear glaze after bisque firing. The clear coat brightens color, adds durability, and creates a sealed surface.
However:
✔ Underglaze without clear glaze is still functional
It remains slightly porous and matte, but is still safe to use — many potters intentionally leave the outside of their pieces unglazed for a soft, tactile texture.
✔ Underglaze with clear glaze becomes sealed and glossy
This is ideal for any surface that will come into contact with food, water, or regular wear.
Studio Recommendation
Always glaze the inside of functional ware used for food or drink
Always glaze the inside of vases or anything meant to hold water
The outside of pieces can be underglaze-only and still perfectly usable
Important Note for Our Studios
When using Amaco Velvet Underglazes, we recommend the
Zinc-Free Brushing Clear instead of our studio-made dipping clear.
The zinc-free formula produces more consistent, predictable results and prevents color shifts or unexpected textures on Velvets.
How Underglaze Behaves at Cone 5
Most major underglaze brands hold up beautifully at cone 5. You can expect:
strong, vibrant color
crisp lines
excellent stability
minimal smudging under clear glaze
very little movement during firing
A few colors (especially pinks, purples, and bright reds) may shift depending on pigment chemistry, but overall underglazes are highly dependable at mid-range.
Why Underglaze Sometimes Smudges Before Firing
If your underglaze smears when applying clear glaze, don’t panic — the smudge usually disappears after firing. Smudging is more likely to happen when:
the underglaze was applied very thick
it hasn’t fully dried
the bisque surface is especially smooth
the clear glaze brush is overly wet or aggressive
To reduce smudging:
Let underglaze dry thoroughly and lightly dampen the bisque before glazing to help the brush glide smoothly.
Why and When to Use Glaze Instead of Underglaze
Use glaze when you want:
a sealed, non-porous surface
a shiny or satin finish
a functional interior surface
melted, glassy texture
Use underglaze when you want:
precision
linework
illustration
mixed colors
painterly marks
sgraffito or mishima
stable, non-moving color
Many artists use both — glaze for function, underglaze for expression.
Final Thoughts
Underglaze and glaze each bring something different to the ceramic process. Underglazes give you precision, mixable color, and painterly control. Glazes give you shine, sealing, and the glassy finish associated with functional pottery.
Understanding how both materials behave at cone 5 — and how to combine them effectively — opens up a world of creative possibilities. If you ever want help choosing materials or exploring new techniques, we’re always here to support your process.