Ceramic glazes seal and protect fresh pottery and make it beautiful. This vitreous substance can transform porous plates, cups, and bowls into food-safe and stain-resistant serving vessels. Glazing is a fun process in ceramic-making.
The magic happens when a glaze is applied to the piece and it is fired. However, firing pottery pieces isn’t all about hocus-pocus. It is possible to create fired pottery pieces with a basic understanding of glaze firing and glaze application.
Silica: The Glass Former
Silica (or industrial sand) is the essential ingredient in glass, raw pottery, and ceramic fused material for glazes. You can get silica from quartz, sandstone, and sand or flint. Or, you can make it as silica dioxide.
Products such as quartz, flant, and pure silicon can be used to make your glazes. Silica can form glass if it is heated enough. Silica’s melting points (approximately 3100 F, 1710 C) are higher than any ceramic kiln. Silica can not be used alone as a pottery sealer.
The Refractory – Alumina
Alumina (or aluminum oxide) is an important ingredient in almost all glazes. It acts as a stiffening agent. Alumina is essential for glazes to adhere to vertical surfaces. Without it, the glaze will slide off of the surface of any vertical pieces as they are applied. You can add alumina to your pottery as a clay (kaolin ball clay, fireclay, or fireclay), or as an alumina hydrate (a white manufactured dust), and the glaze sticks to the pottery’s surfaces without coming off.
Alumina helps stiffen glazes and disperse gas bubbles that may form during the firing process. Alumina also enhances the pink hues of the final pieces.
Flux Is The Melting Agent
Fluxes play a key role in lowering silica’s melting point to make it suitable for ceramic glazes. Fluxes promote vitrification, which is the transformation of silica to glass. Limestone and calcium oxides are the most popular fluxes used in ceramic glazes. These fluxes include soda feldspar as well as potash feldspar.
Each flux operates differently. Some fluxes can be very active and allow the glaze time to age at earthenware temperature. Others are less active and only useful when fired at mid- or high-range temperatures.
Important to know is that many of the fluxes made from metallic oxides can be inhaled even though they are still young. Take care when handling them and wear a dust mask. Make sure that the final cup, plate, or cup has reached full maturity to prevent any leaching into food.
The Beautifier Is Colorant
Silica melts and becomes transparent. This is why colorants are necessary to get the many hues that glazes offer. The ceramic colorants need to be able to withstand high temperatures and not burn off. Most are made of metallic oxides, which can also impact the glaze’s melting points.
Consider the color of your chosen colorant before firing. Do the math to determine the correct firing temperature. The color of raw metal oxides is often very different from the color they produce in the glaze. To get the right hue, it is crucial to understand what minerals make which colors.
Glazes may contain colorants as well as other modifiers. These can affect the glaze’s opacity or iridescence (or the glaze’s working quality) while it is raw (unfired).