The Complete Guide to High-Fire Glazes

1. Mastering the Fundamentals: Glazes, Oxides, and Materials

A glaze is a glass layer or coating that is fused to a clay piece during firing.

Glaze Components. Glazes are composed of three main components: glass-formers (primarily silica), fluxes (which assist in melting), and stabilizers (containing alumina). These elements, when combined in specific proportions, create a surface that is both visually appealing and functional. Understanding the properties of oxides and carbonates, the building blocks of glazes, is essential for creating successful glazes.

Material Properties. Each material used in glaze making has unique properties that affect the final result. These properties include melting point, mesh size, volatilization point, solubility, thermal expansion and contraction, surface tension, viscosity, and toxicity. Knowing these properties allows you to predict how materials will behave during firing and to adjust your recipes accordingly.

Oxide Properties. Oxides and colorants are the major components of glazes, each contributing specific qualities. For example, sodium oxide is a strong alkaline flux that creates brightly colored glazes, while alumina oxide is a stabilizer that helps to stiffen the glaze and keep it on the pot. Understanding the properties of individual oxides is crucial for formulating and controlling glaze effects.

2. The Art and Science of Mixing Glazes

This was a golden opportunity to test my recipes and at the same time see thousands of samples of student work.

Essential Equipment. Proper glaze-mixing equipment is essential for safety, accuracy, and consistency. This includes an OSHA-approved respirator, disposable gloves, accurate scales, stainless steel scoops, a hand mixer, a drill with a Jiffy blade, a rotary sieve, and clean buckets. Having the right tools makes the process easier and helps you achieve consistent results.

Mixing Process. The mixing process involves carefully weighing out each ingredient, adding them to water, and blunging the mixture thoroughly. It's important to write down the recipe, check off each ingredient as you add it, and ensure that the batch amounts are accurate. Sieving removes coarse materials and ensures even distribution of colorants.

Specific Gravity. The thickness of the glaze slurry, or specific gravity, is crucial to the success of glaze application. Measuring specific gravity with a hydrometer or triple-beam scale ensures that you get the proper amount of glaze on the pot. Glaze additives like bentonite, CMC, and Epsom salts can be used to adjust the suspension and consistency of the glaze.

3. Application Techniques: Achieving the Perfect Glaze Coat

Application is critical because a glaze coat that’s too thin or thick might come out of the kiln a different color, run off the pot, or crawl.

Bisque Firing. Bisque firing is a preliminary firing that strengthens the clay piece, drives off gaseous impurities, and allows the clay to remain porous for glaze absorption. Consistent bisque firing is essential for achieving consistent glaze results. The temperature at which you bisque-fire affects the rate at which the glaze is absorbed and the thickness of the final glaze coat.

Application Methods. Common methods of glaze application include dipping, pouring, spraying, and brushing. Each method has its advantages and disadvantages, and the choice depends on the size and shape of the piece, the type of glaze, and the desired effect. Regardless of the method, it's important to keep the glaze well stirred and to apply a consistent coat.

Resists. Resists, such as wax or latex, are used to keep glaze off specific areas of the pot. Wax is used to keep glaze off the foot of a pot or as a decorative resist material. Latex resist works differently from wax and must be removed once it dries to avoid leaving a rough area.

4. Firing Fundamentals: Heat, Kilns, and Atmosphere

The point of all this is to have successful pieces, and the way to do this is to control as many of the variables as possible.

Measuring Heat. Pyrometers and pyrometric cones are essential tools for measuring heat and determining when a firing is complete. Pyrometers measure the temperature inside the kiln, while cones indicate the time that the material has had to absorb the heat. Using both tools together provides a more accurate assessment of heat work.

Kiln Types. Fuel kilns burn fuel to produce heat and allow you to control the atmosphere. Electric kilns use coils to generate heat and have a relatively stable oxidizing atmosphere. The choice of kiln depends on the desired effects and the type of glazes you're using.

Firing Cycles. Understanding the firing cycle is crucial for reproducing glazes. The firing cycle includes the rate of heating, the peak temperature, the atmosphere (oxidation, neutral, or reduction), and the cooling cycle. Different glazes require different firing cycles to achieve the best results.

5. Troubleshooting Glaze Faults: Identifying and Correcting Problems

Random methods only produce random results, but glazing results are reproducible—the ceramics of the Chinese Sung Dynasty are proof of that!

Common Glaze Faults. Common glaze faults include bloating, carbon coring, crawling, crazing, dunting, leaching, and pinholes. Each fault has specific causes and requires different corrective measures. Understanding these faults and their causes is essential for achieving successful glaze results.

Crazing. Crazing happens when the glaze surface cracks after firing because the glaze contracts more than the clay body. To eliminate crazing, reduce the number or amount of oxides with high expansion/contraction rates, or increase the oxides that have low expansion/contraction rates in the glaze.

Pinholes. Pinholes are small holes on the surface of the fired glaze piece. They occur when a material burns out of the clay or glaze and comes to the surface as a gas. The bubbles leave craters that don’t heal over during the firing.

6. Exploring White and Clear Glazes: The Foundation of Color

The goal is to get the glaze color and texture you want, and accurate mixing, bisque firing, careful application, glaze firing, and record keeping will help you to be successful with high-fire glazes.

Base Glazes. Clear glazes are the foundation for creating a wide range of colors and effects. Studying clear glazes is an excellent way to learn about the interaction of basic glaze materials. Common base recipes include the Leach 4/3/2/1, Hamada 5/3/2, and Classic 25/25/25/25 bases.

White Glazes. White glazes are created by adding opacifiers to a clear glaze recipe. Primary opacifiers include tin oxide, zirconium oxide, titanium dioxide, and bone ash. Each opacifier imparts a different shade of white and affects the glaze's surface texture.

Surface Effects. Clear and white glazes can exhibit a variety of surface effects, including crazing, crackling, and matte finishes. These effects can be controlled by adjusting the glaze recipe, firing cycle, and application technique.

7. Unlocking Color: Iron, Copper, and Beyond

Ceramics is often described as the study of oxides and carbonates.

Iron Oxide. Iron oxide is a strong fluxing colorant that produces a wide range of colors and effects in glazes, including browns, yellows, greens, olives, blues, oil spots, and metallic colors. It's extremely sensitive to oxidation and reduction atmospheres.

Copper Oxide. Copper oxide is a very active flux that disperses well in a glaze. It can produce many colors, including red, pink, green, blue, black, and metallic lusters. Copper oxide is extremely sensitive to oxidation or reduction atmospheres in the kiln.

Other Colorants. Other colorants used in ceramics include chromium oxide, cobalt oxide, manganese dioxide, nickel oxide, praseodymium oxide, rutile, and vanadium oxide. Each colorant has unique properties and produces different effects in glazes.

8. The Allure of Shino: A Journey into Texture and Tone

I want to ensure that this book’s approach to the subject is as accessible to the novice as to the experienced ceramist, albeit not necessarily for the same reasons.

Shino Characteristics. Shino glazes are high in feldspar and clay, both of which are relatively high in alumina and silica. It’s the firing, and the proportions of feldspar and clay with their naturally occurring impurities, that give shinos their varied and unique look. To simplify the discussion of such a broad range of glazes, shinos may be classed as belonging to one of several subtypes: traditional, carbon trap, or high alumina.

Traditional Shino. Traditional or basic shinos are usually white, opaque, satin-to-matte glazes when applied thickly. Thin application on stoneware creates rusty areas of fire color. This type of shino is often fired in a wood-burning kiln in a reduction atmosphere.

Carbon Trap Shino. The addition of 3 to 17 percent soda ash is the defining feature of this type of shino glaze. Because soda ash is soluble, as water evaporates from the glaze coat it deposits the soda ash on the surface of the pot as a white powder. The migration of soda ash to the surface of the pot creates the possibility for its distinctive carbon trapping.

9. Embracing the Unexpected: Wood, Salt, and Soda Firing

We learned to be open to the unexpected actions of the kiln and not have rigid expectations about the outcome.

Wood Firing. Wood firing is the oldest method of firing pottery and often involves the use of bare clay or a combination of flashing slips, colored slips, natural fly ash, and the action of the fire to decorate the pots. Wood kilns can produce a variety of effects, depending on the design, the type of wood used, and the firing cycle.

Salt Firing. Salt firing occurs when salt (NaCl) is introduced into the kiln at high temperatures. The salt vaporizes and flows throughout the kiln, coating everything with a thin layer of sodium oxide, which appears as a thin sheen of soda glass on the clay surfaces. A salt-fired orange peel surface, characteristic of traditional German salt jugs, is a typical example of this method of firing, which uses no applied glazes at all.

Soda Firing. In soda firing, sodium carbonate (Na2CO3) is introduced into the kiln. Sodium carbonate gives a much more subtle effect because there is no action created by the volatile chlorides that are in a salt kiln. Also, the orange peel texture will be minimized—or even non-existent—and the glazes will be only lightly affected.

10. Safety First: Protecting Your Health in the Ceramics Studio

Reducing the level of your exposure to all ceramic materials whenever possible is the most prudent course.

Material Safety Data Sheets. Safety begins with reading and understanding the Material Safety Data Sheet (MSDS) for each material used in your studio. Manufacturers are required to provide these sheets, which describe handling procedures and safety hazards associated their products, to every customer.

Silica Hazards. Inhaled silica is a known carcinogen and causes silicosis, a progressive, untreatable lung disease. Take steps to reduce your exposure to this ubiquitous material, especially if you intend to make a career in pottery. To insure your safety, purchase and use an OSHA-approved respirator.

Kiln Emissions. A kiln in the studio is very dangerous, particularly if it isn’t vented. All kilns should be outdoors in well-ventilated areas because many ceramic materials volatilize during firing. Be sure to follow the manufacturer’s recommendations for installing kilns vents and situate kilns so that their emissions can’t enter nearby buildings.

Last updated: April 6, 2025