The phenomenon of glaze turning yellow can be perplexing for ceramic enthusiasts and artists. Several factors contribute to this color shift, each shedding light on the complex chemistry and firing processes involved in ceramics.
1. **Firing Conditions**: Glazes can turn yellow if they are fired in conditions that promote the development of specific crystal formations. Certain firing atmospheres and temperatures can cause colorants within the glaze to react and produce a yellow tint.
2. **Metallic Oxidation**: The presence of certain metallic elements like iron and sulfur can lead to yellowing when exposed to high temperatures during firing. These elements can form compounds that contribute to the yellow coloration.
3. **Incomplete Combustion**: If the kiln firing process involves incomplete combustion, it can introduce carbon or other residues onto the surface of the glaze, leading to discoloration, including yellowing.
4. **Interaction with Other Glazes**: When multiple glazes are applied to the same piece and interact during firing, unexpected color changes can occur. Chemical reactions between glazes can result in color shifts, including yellowing.
5. **Underlying Clay Body**: The type of clay used as the base for the ceramic piece can influence the color of the glaze. Some clays contain minerals that can interact with glaze components and cause color changes.
6. **Firing Atmosphere**: Different firing atmospheres, such as reduction or oxidation, can alter the appearance of glazes. Yellowing might occur if glazes are fired in an atmosphere that encourages the development of specific coloration.
7. **Raw Materials and Impurities**: The quality and purity of glaze raw materials play a significant role. Impurities or variations in raw materials can lead to unexpected color shifts during firing.
Understanding the chemistry and variables involved in glaze formulation and firing is key to addressing yellowing issues. By carefully controlling firing conditions, using high-quality materials, and conducting test firings, ceramicists can work towards achieving the desired glaze colors while minimizing the risk of unwanted yellowing.