Glaze chemistry is a fascinating field that combines art and science, allowing us to create stunning surfaces on our ceramic pieces. At its core, glaze chemistry involves understanding how different materials interact at high temperatures to form a glassy layer. Silica, or silicon dioxide (SiO₂), is the primary glass-forming oxide in ceramic glazes, typically making up more than 60% of most glazes. It acts as the backbone of the glaze, providing hardness, durability, and chemical resistance. In its pure form, silica has a high melting point of approximately 1,720°C (3,128°F). To lower this melting point to a more manageable range for ceramic firing, we introduce fluxes like sodium oxide (Na₂O), potassium oxide (K₂O), and calcium oxide (CaO). These fluxes help the silica melt and form a glassy matrix that coats the ceramic surface. Common ways to introduce silica into the glaze are feldspar, clay, and wollastonite.
However, achieving the perfect glaze isn’t always smooth sailing. Common glaze defects can significantly impact the aesthetic and functional qualities of our pottery. One frequent issue is pinholing, where tiny holes appear on the glazed surface due to gases escaping during firing. This can often be resolved by increasing the firing duration or ensuring the clay body is free of impurities. Blisters, raised bubbles on the glaze surface, can result from overfiring or rapid firing, which doesn’t allow the glaze to mature properly. Crazing, those fine cracks on the glaze surface, often occur due to a mismatch between the clay body and glaze expansion rates. Shivering, where the glaze peels off, happens when the glaze is under too much compression. Crawling, where the glaze pulls away, leaving bare patches, is often caused by dust or grease on the bisque ware surface. Dunting is cracking due to thermal shock during firing or cooling, and warping is distortion of the piece’s shape during firing. Peeling is the glaze lifting away from the body, mainly occurring on the edges of pots such as cup rims and handles.
Glaze defects can be caused by many different factors, but with experience and careful observation, you can often pinpoint and address the cause.
This video from Craftsy is a great introduction to glazing pottery, covering the basics of the process and materials involved:
Firing temperature plays a crucial role in glaze chemistry. Cone 5 and Cone 6 glazes, both in the mid-range firing spectrum, differ slightly in temperature: Cone 5 is around 2118°F (1159°C), while Cone 6 is approximately 2165°F (1185°C). This seemingly small difference can significantly impact glaze maturation. Cone 6 glazes might not fully mature at Cone 5, requiring adjustments to the flux content. Boron is a key flux in mid-range glazes, especially at Cone 5, due to its low melting point. When applying glazes, ensure the bisque-fired pottery is clean and free of dust. Mix the glaze to a smooth consistency and apply it in thin, even coats using methods like dipping, brushing, or pouring. For more information, refer to this guide on unlocking glaze chemistry. Also, consider exploring glazing techniques for potters.
Safety is paramount when working with glaze materials. Glazes often contain toxic substances like silica, lead, cadmium, barium, and chromium. Always wear a NIOSH-approved respirator, nitrile or latex gloves, and safety glasses to protect yourself. Work in well-ventilated areas and use wet methods to reduce dust. Familiarize yourself with the Safety Data Sheets (SDS) for each material and label containers clearly. Opt for lead-free glazes and avoid carcinogenic colorants whenever possible. With careful attention to safety and a willingness to experiment, you can unlock the endless possibilities of glaze chemistry and create truly unique and beautiful ceramic art.
