The principle of underglaze painting for ceramic tableware

Under-Glaze Decoration in Ceramic Tableware: Core Principles and Technical Processes

Under-glaze decoration is a traditional ceramic technique that involves applying pigments beneath a transparent or colored glaze layer, resulting in durable, vibrant designs that are protected from wear and chemical exposure. This method is widely used in ceramic tableware for its aesthetic appeal and practical longevity. Below, we explore the scientific foundations and step-by-step execution of this工艺 (process).

1. Raw Material Selection and Preparation

The foundation of under-glaze decoration lies in the choice of pigments and clay bodies. Under-glaze colors are typically formulated from metal oxides and ceramic stains that can withstand high firing temperatures without fading. Common pigments include cobalt oxide (blue), copper oxide (green), and iron oxide (brown or red). These materials are ground into fine powders and mixed with water or a binding agent to create a paintable slurry.

The clay body used for tableware must also be compatible with the firing process. Porcelain and stoneware are popular choices due to their ability to vitrify at high temperatures, creating a dense, non-porous surface that enhances the durability of the under-glaze design.

2. Application of Under-Glaze Designs

Once the pigments are prepared, artists apply them to the bisque-fired ceramic piece. Bisque firing transforms raw clay into a hard, porous material, allowing it to absorb the pigment without smudging. Techniques for applying under-glaze designs vary widely:

• Brushing: Artists use fine brushes to paint intricate patterns or images directly onto the ceramic surface. This method allows for detailed control but requires steady hands and skill.

• Screen Printing: For repetitive designs, screen printing ensures consistency. A stencil transfers the pigment onto the ceramic, creating sharp, uniform lines.

• Decals: Pre-printed under-glaze decals can be applied to the surface and then fired, offering a quick way to add complex imagery without manual painting.

Regardless of the method, the pigment must adhere evenly to the ceramic to prevent uneven firing results.

3. Glazing and Firing Processes

After the under-glaze design dries, a transparent or colored glaze is applied over it. This glaze serves as a protective layer and enhances the visual depth of the colors beneath. The type of glaze chosen affects the final appearance:

• Transparent Glaze: Reveals the full vibrancy of the under-glaze colors, creating a glossy, polished finish.

• Matte Glaze: Softens the colors slightly, giving the piece a more subdued, textured look.

The glazed piece then undergoes a final firing at a temperature higher than the initial bisque firing (typically between 1,200°C and 1,300°C). During this stage, the glaze melts and fuses with the ceramic body, while the under-glaze pigments bond permanently to the surface. The high heat ensures that the design remains intact even after repeated use and washing.

4. Chemical Reactions During Firing

The success of under-glaze decoration hinges on precise control of firing conditions. As the kiln heats up, several chemical changes occur:

• Pigment Stabilization: Metal oxides in the under-glaze react with oxygen in the kiln, forming stable compounds that lock in the color. For example, cobalt oxide transforms into cobalt aluminate, producing a deep blue hue.

• Glaze Maturation: The glaze materials melt and flow, creating a smooth, glassy coating. If the firing temperature is too low, the glaze may remain rough; if too high, it could become overly runny, distorting the design.

• Clay Vitrification: The ceramic body itself undergoes vitrification, becoming harder and less porous. This process ensures that the tableware is strong enough for daily use.

Mastering these reactions requires careful calibration of firing schedules, including ramp-up rates, holding times, and cooling phases.

5. Advantages of Under-Glaze Over Other Techniques

Under-glaze decoration offers several benefits compared to over-glaze or in-glaze methods:

• Durability: Since the design is sealed beneath the glaze, it is resistant to scratches, fading, and chemical damage from acidic foods or detergents.

• Food Safety: The non-porous glaze layer prevents pigments from coming into direct contact with food, making under-glaze tableware safe for everyday use.

• Aesthetic Versatility: Artists can achieve a wide range of effects, from delicate watercolor-like washes to bold, graphic patterns, all protected by the glaze.

These qualities make under-glaze decoration a preferred choice for functional ceramic pieces that demand both beauty and resilience.

By understanding the interplay of materials, application techniques, and firing processes, artisans can create under-glaze ceramic tableware that combines artistic expression with practical performance. This time-honored method continues to evolve as new pigments and firing technologies emerge, ensuring its relevance in modern ceramics.