A Guide to Ceramic Glaze and Firing

When choosing a ceramic tableware item, many people are drawn to its exquisite appearance. The intricate patterns you see may come from advanced decal techniques or from the unpredictable effects of kiln-transformation glaze crafted by master artisans. In this article, we will focus on what glaze is, what high-fire glaze is, and what kiln-transformation glaze is.

1. What Is Glaze?

Glaze is a colorless or colored glassy layer that covers the surface of ceramic products. It is made by blending mineral materials (such as feldspar, quartz, talc, kaolin, etc.) in certain proportions, then grinding them into a glaze slurry, which is applied to the clay body and fired at a specific temperature. Glaze can significantly improve the mechanical strength, thermal stability, and dielectric strength of the product, while also making it more attractive and easier to clean (source: Ceramic Craft Handbook).

2. Main Components of Glaze

• Silicate Minerals
  Provide the fundamental glassy framework of the glaze. Common raw materials include feldspar, kaolin, and quartz.
• Fluxes
  Lower the melting point of the glaze, enabling it to melt at lower temperatures. Common fluxes include sodium oxide (Na₂O), potassium oxide (K₂O), lead oxide (PbO), and boron oxide (B₂O₃).
• Stabilizers
  Enhance the glaze's chemical corrosion resistance and mechanical strength. Common stabilizers include calcium oxide (CaO) and aluminum oxide (Al₂O₃).
• Pigments and Colorants
  Provide vibrant colors to ceramics. Common colorants include cobalt oxide, copper oxide, iron oxide, and manganese oxide (source: wiki).

3. Types of Glaze

3.1 Classified by Appearance

• Transparent Glaze: The glaze layer is transparent (e.g., blue and white porcelain).
• Opaque Glaze: The glaze layer is semi-transparent or opaque.
• Colored Glaze: Various metal oxides produce different colors (e.g., red glaze, blue glaze).
• Gloss Glaze: The surface is smooth and glossy.
• Matte Glaze: The surface lacks gloss (e.g., crackle glaze).
• Crystalline Glaze: Crystals precipitate within the glaze, forming unique patterns.

3.2 Classified by Composition

• Lime Glaze: Mainly composed of limestone.
• Feldspar Glaze: Mainly composed of feldspar.
• Lead Glaze: Contains lead components.
• Lead-Free Glaze: Does not contain lead.
• Boron Glaze: Contains boron components (source: Ceramic Classification Data).

4. Low-Fire, Mid-Fire, and High-Fire Glazes

High-Fire Glaze

• Firing temperature: ~1250°C–1350°C (2282°F–2462°F).
• High-temperature firing fuses the glaze more tightly with the clay body, making it stronger.
• Often used for everyday ceramics and high-end art pieces.
• Cheralle’s coffee cup is fired at 2408°F (source: Cheralle brand info).

Mid-Fire Glaze

• Firing temperature: ~1100°C–1250°C (2012°F–2282°F).
• Combines the strengths of both high-fire and low-fire glazes, featuring good gloss and hardness.
• Widely used in the production of daily-use ceramics.

Low-Fire Glaze

• Firing temperature: ~800°C–1100°C (1472°F–2012°F).
• The glaze layer and the clay body bond less tightly, giving a softer finish.
• Suitable for artistic or decorative ceramics, but less durable than mid- or high-fire ceramics.

5. What Is Kiln-Transformation Glaze?

Kiln-transformation glaze develops during the ceramic firing process, when changes in temperature or kiln atmosphere lead to unexpected variations in the glaze, producing unique color effects. Each piece is one-of-a-kind due to this “accidental” phenomenon, making it highly valued for its artistic merit. The colors arise from multiple chromogenic elements (copper, iron, manganese, cobalt, titanium, etc.) that bond naturally under high temperature to create random hues, patterns, or speckles. Cheralle’s handmade coffee cups use kiln-transformation glaze, where precise control of temperature and a reducing atmosphere allows the glaze to flow across the surface, resulting in a wave-like, organic texture.

6. Is Kiln-Transformation Glaze Toxic?

Many people wonder whether the bright colors in ceramic glazes could be harmful. Generally, ceramic decoration methods fall into three main categories:

• Overglaze Painting (On-Glaze):
  Applied to the glazed and fired ceramic surface, then fired at a lower temperature. It can yield bright colors but is more prone to wear and fading, and has a slightly raised texture.
• Underglaze Painting:
  Painted onto the unfired (or bisque) body, followed by a coat of transparent or light-colored glaze and a single high-temperature firing. The design lies below the glaze layer, making it resistant to fading, with a smooth surface.
• In-Glaze Painting:
  Glaze is first applied to the clay body, then painted with color, followed by another layer of glaze and a fast high-temperature firing. This creates dual-layer protection, giving colors a soft, warm look that’s more resistant to abrasion.

Cheralle’s handmade coffee cups are not painted on the body but instead have glaze sprayed onto the clay body, then fired at 2408°F classifying them as underglaze products. Additionally, our products have undergone FDA (food-contact) testing by professional agencies and meet first-grade national export standards, so you can use them with confidence (source: Cheralle product info).

7. Oxidation vs. Reduction Atmospheres

Ceramic firing typically involves two main atmospheres: oxidation and reduction.

• Oxidation Atmosphere:
  The kiln is well-supplied with oxygen, producing bright, vivid, and high-saturation glaze colors. It is often used for lower-temperature firing (700°C–1000°C or 1292°F–1832°F), making it suitable for mass-produced commercial ceramics, though with less variation in results.
• Reduction Atmosphere:
  In the latter stage of firing, oxygen supply is reduced, and carbon in the fuel reacts with oxygen in the clay and glaze. This yields deeper, more layered colors with subtle surface changes—ideal for unique kiln-change effects. Typically used at higher temperatures above 1200°C (2192°F), requiring more stringent control (source: Firing Process Data).

Conclusion

Ceramic glaze is at the core of ceramic artistry and craftsmanship. It not only enhances the physical properties of ceramics but also imparts distinctive aesthetic qualities. Different glaze formulas, firing temperatures, and kiln atmospheres result in a wide range of visual and tactile experiences. For those who admire ceramics or plan to purchase quality coffee cups, understanding how glaze is made and the various classifications can help in selecting pieces that suit personal taste, while also revealing the skill and wonder behind the craft.