Understanding the Differences Between Glaze and Underglaze

Ceramic artists have a broad palette of color options at their disposal, including underglazes and glazes. While both adding color to clay surfaces, these two mediums have fundamental differences in their chemistry, appearance, intended uses, and firing requirements. Here, we delve into the comparison of these two art mediums.

Chemical Composition: Glaze vs Underglaze

At the core, the primary difference between glazes and underglazes lies in their unique chemical compositions:

Glazes

• Glazes primarily consist of silica, fluxes, and stabilizers, all of which collaborate to form a glass-like substance.
• They exist as an opaque, viscous medium.
• Glazes fuse to clay at high temperatures, creating a firm bond.

Underglazes

• Underglazes feature metal oxide pigments suspended in a flux-rich medium.
• They present as a more translucent, thinner medium compared to glazes.
• Unlike glazes, underglazes adhere to the surface at lower temperatures.

These fundamental chemical differences shape their distinct appearances, applications, and compatibility with various ceramic projects.

Visual Characteristics: Glaze vs Underglaze

Post-firing, glazes and underglazes exhibit stark visual variations:

Glazes

• Glazes impart a smooth, glassy surface to the ceramic piece.
• They can produce a glossy or matte finish, depending on the specific formula.
• The colors produced by glazes are usually opaque.

Underglazes

• In contrast, underglazes leave a matte, porous surface on the ceramic piece.
• They provide a satin sheen rather than a high gloss.
• The colors of underglazes are more translucent compared to glazes.

The final effect being that glazes encapsulate the clay, whereas underglazes meld into the surface, influencing the texture and aesthetic of the final piece.

Ideal Use Cases: Glaze vs Underglaze

Based on their distinct properties, glazes and underglazes are suited to different functional applications:

Glazes are ideal for:

• Creating waterproof, functional surfaces suitable for daily use.
• Achieving vibrant, opaque colors that stand out.
• Visually enhancing texture, amplifying the tactile experience.
• Developing depth and interest with layered effects.

Underglazes are best used for:

• Adding color while preserving the raw texture of the clay.
• Incorporating delicate details that aren't obscured by a glossy finish.
• Maintaining a porous clay body, which is critical for raku finishing.
• Creating colorful designs that can be topped with transparent glazes for added depth.

Firing and Compatibility Considerations

Proper firing is crucial for achieving the desired results with both glazes and underglazes:

Glazes:

• Glazes typically fire to maturity around cones 6-10.
• They require a careful match of glaze and clay thermal expansion to avoid cracking.
• Layering of glazes must be done with careful consideration of compatibility to prevent undesired effects.

Underglazes:

• Underglazes fire to maturity at a lower temperature, usually cone 06.
• They can be used on either raw or bisque surfaces.
• They are generally compatible with most glazes applied on top.

Understanding the distinct properties and ideal applications of glazes and underglazes can significantly enhance ceramic pieces' beauty and function. While both underglazes and glazes instill color, leveraging their strengths allows potters to create works of art that truly stand out.

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