Dissecting a Glaze to Explore the Functions of Raw Materials in a Glaze Jeff Zamek

Functions of Raw Materials in a Glaze 

Each raw material used in a glaze contains one or more oxides or their carbonate forms. Some raw materials contain multiple oxides such as Minspar 200 feldspar having sodium, potassium, alumina, and silica components. When possible it is always best to choose raw materials that have multiple oxide components as long as they fulfill the requirements of the glaze formula as their oxides are integrated more efficiently in nature as compared to using only single oxides. However, in some instances, a single oxide is needed to complete the glaze formula. Potters have many raw material choices to fulfill a glaze formula. Additionally, the same glaze can be constructed using a different set of raw materials as long as the oxide requirements are met in the formula. Let’s take apart a glaze and see how each material functions. 

Minspar 200 — Na₂O K₂O SiO₂ Al₂O₃ The major flux in this glaze is feldspar. This sodium-based feldspar (the predominate oxide is sodium with lesser amounts of potassium, both are strong alkalis) also contains silica and alumina. Feldspars are an efficient way to introduce alkali oxides into a glaze in relatively insoluble forms. At cone 9 (2300°F/1260°C) Minspar 200 contains all of the above oxides in near-ideal ratios to form a glaze almost by itself. Minspar 200, aside from being a strong flux at this temperature, also brings other glaze materials into an active melt. 

Silica — SiO₂ Silica will not melt by itself at the temperatures reached in potter’s kilns; however, combining it with other glaze materials lowers its melting point. For most glazes, 325-mesh silica is used. However, finer 400-mesh or coarser 200-mesh sizes are available from most ceramics suppliers. As a general rule, the finer the mesh the more complete the melt as additional surface area is exposed in the heating process. 

Wollastonite — CaO SiO₂ This common glaze material contains calcium and silica and is an ideal way to incorporate both materials into a glaze formula. In many glazes, wollastonite can be used in place of whiting (calcium carbonate) as it does not release carbon dioxide when going into a melt. The release of this gas can cause bubbles in the fired glaze. Either the glaze can be recalculated for the addition of silica contained in wollastonite or, in some instances, the extra silica will not significantly change the glaze. 

EPK — Al₂O₃ 2SiO₂ 2H₂O EPK is one of many kaolins which are a group of primary clays formed on site. They are relatively non-plastic and white firing. Both its silica and alumina content are refractory stiffening the molten glaze and keeping it on vertical surfaces. An addition of any type of clay in the glaze helps to suspend the liquid glaze in storage. 

Gerstley Borate — Na₂O 2CaO 5B₂O₃ 16H₂O 2CaO 3B₂O₃ 5H₂O Na₂O 2CaO 5B₂O₃ 10H₂O Gerstley borate is a popular glaze material with a complex chemical composition of ulexite, colemanite, and probertite. Gerstley borate is an unrefined, hygroscopic (can take on water in the atmosphere in storage), soluble ore with a chemical history that can vary. While these are characteristics that do not lead to reliability, Gerstley borate is found in many past and current glaze formulas bringing other glaze materials into a melt.¹ Gerstley borate is soluble and can leach into the water system of a glaze altering the actual glaze formula. Soluble materials in Gerstley borate can also migrate to the outer surfaces of the pottery during glaze application causing blisters and rough areas in the fired glaze. However, Gerstley borate can promote a variegated glaze surface which is one reason for its use in glazes. Gerstley borate is no longer being mined but still remains in potters’ raw material bins. Before formulating any glaze make sure all materials are still currently available. 

Zinc Oxide — ZnO While not a strong flux in small amounts it reacts with other glaze materials causing fusion. Zinc oxide also helps prevent crazing (a fine network of lines in the fired glaze due to glaze tension upon cooling) and has an intensifying effect on cobalt blue colors. Zinc oxide contributes durability and hardness, and promotes a craze-resistant glaze. 

Cobalt Carbonate — CoCO₃ Cobalt carbonate and the more concentrated larger particle size, cobalt oxide is one of the strongest metallic coloring agents in either its oxide or carbonate form. One part of cobalt in 100,000 parts of white glaze will have a tinting effect. Cobalt can be a strong flux in glazes and dissolves efficiently in high alkaline and boron-based glazes.³ 

Batch Weight Limits 

The batch weight limit formulas are, in part, based on the unity molecular formulas, which detail the specific parts of molecules of the oxides in the glaze. The unity molecular formula is often referred to as the Skeleton Formula as many glazes are based on this calculation and are then turned into batch weight formulas ready for use. 

Based in part on the unity formula batch weight limits for each glaze, materials can be determined. There are no precise cut-off points to the minimum and maximum amounts of materials. In most instances, when a minimum or maximum limit has been reached, as in silica 5% to 25%, the glaze will not be noticeably different—even if 4% or 26% of silica, or possibly greater percentages, is used. 

100% Batch Glaze 

The individual glaze materials when added should total 100% in the glaze batch. Gums, suspension agents, dyes, opacifiers, metallic coloring oxides, and stains are listed after the 100% batch weight.⁴ 

the author Jeff Zamek started his career 57 years ago. He obtained BFA/MFA degrees in ceramics from Alfred University, College of Ceramics, New York. In 1980, he started Ceramics Consulting Services, a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the US. His books, The Potter’s Studio Clay & Glaze Handbook, What Every Potter Should Know, Safety in the Ceramics Studio, and The Potters Health & Safety Questionnaire are available from Jeff Zamek/ Ceramics Consulting Services. For technical information, visit www.jeffzamek.com. 

1 Jeff Zamek, Ceramics Technical #39 November 2014, March 2015, A Simple Glaze.
2 Val Cushing Handbook, 3rd edition and raw material notes from Alfred University, College of Ceramics 1972–73.
3 Frank and Janet Hamer, The Potter’s Dictionary of Materials and Techniques, A& C Black, University of Pennsylvania Press, Philadelphia, 1986, page 65. 
4 Alfred University, College of Ceramics raw material notes 1973. 
Acknowledgments: The following sources were used for additional technical information: 
Ian J. Mc Colm. Dictionary of Ceramic Science and Engineering, second edition, Plenum Press, NY 
Tony Hanson’s Digital Fire Insight Program Limit Formula (Ron Roy Limits).