What Is Glaze?

After you shape and bisque fire a clay piece, you usually coat it in glaze before firing again. Glaze is essentially a specialized glass formula engineered to melt and fuse onto clay at certain temperatures. It makes ceramic ware waterproof, more durable, and visually appealing. Without glaze, many ceramic pieces would be prone to leakage, staining, or rapid wear.

Though humans have made simple, unglazed ceramics for more than ten thousand years, we’ve only been glazing our pottery for a few thousand. Early potters discovered that materials like wood ash, plant ash, soda, lead, and alkaline minerals could melt at kiln temperatures, forming the first rudimentary glaze coatings. Civilizations in the Middle East, China, and Egypt recognized that ash from wood or plants, as well as other naturally occurring fluxes, would help transform clay vessels into watertight and often beautifully colored objects. Over centuries, as kiln technologies advanced, cultures around the world refined these discoveries.

In ancient China and elsewhere, alkaline/ash glazes arose from the fluxing oxides naturally found in wood ash, which helped silica melt into a stable glass coating at high temperatures. Lead glazes, meanwhile, were once very common in low-fire traditions but are no longer advised for functional ware due to toxicity. By refining fluxes and strengthening the clay body itself, potters in China eventually perfected high-fire stoneware and porcelain glazes, which greatly influenced later ceramic traditions worldwide.

Since the late 19th century, ceramicists have increasingly turned to scientific methods to understand how glazes work, relying on measurable oxide formulas and tailored firing schedules. This union of craft and chemistry has expanded glazing from a simple decorative finish to a vast field of creative exploration, enabling ceramicists to control qualities like color, texture, gloss, and fit in highly precise ways.

The Composition of a Glaze

Before firing, glaze ingredients usually appear as a dull, powdery layer on top of the clay body. Yet in the intense heat of the kiln, these materials melt and fuse into a thin, glassy coating.

Most glazes can be understood by grouping their oxide components into three key categories: the glass former, the flux, and the stabilizer. Silica (SiO₂) is a common glass former, but it requires very high heat—beyond 3100 °F (1700+ °C)—to melt on its own. To lower this melting point to a more practical range, ceramicists add fluxes containing oxides like sodium, potassium, calcium, or boron. Meanwhile, stabilizers (often introduced through clay minerals containing alumina like Kaolin) help control the melt’s viscosity so that the glaze neither runs off the pot nor becomes too brittle once cooled.

Although these three groups—glass former, flux, and stabilizer—are the heart of every glaze, many other materials also come into play. Colorants such as iron, cobalt, and copper oxides shift the glaze’s hue or create special visual effects, while opacifiers like tin oxide and zircon introduce opacity. In modern pottery, glazes can be designed in a seemingly infinite variety of ways, sometimes with as few as two ingredients or as many as a dozen or more.

Eutectics: How Oxides Interact

Eutectics are crucial in glaze making because they allow mixtures of oxides to melt at lower temperatures than any single oxide alone. What makes eutectics especially important is that the melting behavior isn’t linear—adding more of a flux like whiting (CaO) doesn’t simply lower the melting point proportionally. Instead, there are specific ratios where oxides interact to create dramatic drops in melting temperature. This “sweet spot” is the eutectic point, where the combination is most effective. Understanding these relationships helps ceramicists formulate glazes efficiently, using optimal proportions rather than simply increasing flux content.

Phase Diagrams help us understand how different oxide combinations behave at various temperatures. These visual maps show melting points, temperature ranges, and most importantly, the eutectic point where mixtures melt most efficiently. Ceramicists use these diagrams (which might show two oxides like SiO₂-CaO in a binary diagram or three oxides like SiO₂-Al₂O₃-CaO in a ternary diagram) as guides to develop stable, predictable glazes without excessive trial and error. Though they may look technical at first glance, phase diagrams are essentially road maps that help potters create better glazes with fewer unexpected results.

Melting & Fusing

When fluxes interact with silica and alumina at high temperatures, the mixture becomes a molten glass that flows and can combine at the surface of the clay. The proportions of silica, alumina, and flux determine how easily the glaze melts, how fluid it becomes in its molten state, and how durable or glossy it remains after cooling. Some fluxes, such as sodium and potassium, encourage higher thermal expansion in the finished glaze, while materials like dolomite (which supplies magnesium) can produce a lower-expansion, potentially more matte surface through crystalline growth.

There are several firing stages where the glaze undergoes key transformations. At lower temperatures, moisture and other organics burn off, making the clay and glaze safe from steam explosions or smoke. As the kiln continues to heat, the glaze particles begin to sinter—meaning they partially melt and fuse together—until they become fully molten at peak temperature. At this stage, the glaze actively interacts with the clay surface, often forming an interfacial layer that locks the glaze onto the body. Cooling then solidifies the molten glass. If under-fired, the glaze won’t fully melt and may remain rough or matte in unintended ways. Conversely, over-firing can lead to defects like blistering or pinholing, and if the clay body itself is overheated, it can deform or even begin to fuse.

Bisque and Glaze Application

Many ceramicists prefer bisque firing first, which turns the clay into a porous yet firm body that readily absorbs water from the glaze slurry. This absorption helps the powdery glaze ingredients adhere evenly. In contrast, a technique called raw glazing or “once-firing” applies glaze to unfired clay. Once-firing can save time and energy but demands precise control of moisture levels to prevent the glaze or the body from failing during the single firing.

Glaze Firing Stages

1. Burnoff (Low Temp): Residual moisture and organics combust.
2. Sintering & Melting: The glaze particles start to fuse. By peak temperature, the glaze is fully molten and can interact with the clay surface.
3. Bonding: A thin interface layer forms between clay and glaze, improving adhesion.
4. Cooling & Solidification: As the kiln cools, the molten layer solidifies into glass.

Each of these phases influences the final outcome, and minute adjustments in time, temperature, and atmosphere can change a glaze’s color, texture, or durability dramatically.

Cone Firing Temperatures

Ceramicists measure heatwork using pyrometric cones, which melt and bend once a specific balance of temperature and time is achieved. Ceramic wares fall broadly into three popular firing ranges:

• Low-Fire (Earthenware): ~Cone 06–02 (~1800–2050 °F / 980–1120 °C). Glazes here often rely on high flux content to melt at lower temps.
• Mid-Fire (Stoneware): ~Cone 5–6 (~2160–2230 °F / 1180–1220 °C). Popular for functional ware; offers broad color possibilities.
• High-Fire (Stoneware & Porcelain): ~Cone 8–11 (~2300–2380 °F / 1260–1300 °C). Produces very durable, sometimes more subtle, glazes that can vary dramatically in reduction firings.

It is essential to match the glaze’s firing range to the clay body’s maturation temperature. If the body is designed to mature at cone 6–8, for instance, it usually pairs well with a cone 6 glaze recipe. Mismatch here can lead to problems like bloating, incomplete vitrification, crazing, or shivering.

Firing Atmosphere & Cooling

• Oxidation: Plenty of oxygen (typical of electric kilns). Metallic colorants stay in higher oxide states, often resulting in bright, consistent colors. Electric kiln firings are typically oxidation.
• Reduction: Oxygen-starved environment (common with gas or wood kilns) changes the oxide chemistry of colorants like iron or copper, yielding unique colors (e.g., copper red, celadon). This process can also affect how fluid a glaze becomes and how it crystallizes. Gas kilns are often fired in reduction.
• Neutral: Balanced atmosphere that’s neither oxidizing nor reducing.
• Salt & Soda: Vapor glazing where salt or soda ash is introduced into the kiln, creating distinctive rivulets and orange-peel textures.
• Wood: Using wood as fuel creates natural ash deposits that interact with the glaze.
• Raku: Dramatic process involving removal from the hot kiln and rapid cooling.
• Luster: Special metallic or iridescent effects created by applying metallic compounds (like silver, copper, or bismuth) as an overglaze and firing at a lower temperature.

Kiln Cooling

Even a single glaze can appear very different in oxidation vs. reduction, or under varying cooling schedules. Once the maximum temperature is reached, how quickly the kiln cools can dramatically affect the final surface.

• Fast Cooling: Tends to “freeze” the molten glaze quickly, preserving a bright, glassy surface.
• Slow Cooling (or Soaks): Allows crystals to form and grow, which can convert a once-glossy glaze into a satin or matte finish. Some glazes specifically require slow cooling or an extended soak to develop surface crystals or special color effects.

Glaze Characteristics

Glazes can be categorized in a variety of ways, but two major factors are transparency and surface finish.

Transparency

Glazes range from fully transparent (like a clear window) to heavily opaque (e.g., white tin glaze). Opacity can come from additives (tin oxide, zircon) or from crystalline inclusions that scatter light.

Surface

Glazes also differ in how much they reflect light.

• Glossy: Highly reflective, smooth. Often easy to clean.
• Semi-matte or Satin: A subdued sheen partway between gloss and matte.
• Matte: Dull surface, sometimes slightly textured at the microscopic level due to devitrification or crystal growth. Always verify matte glazes for food-safety if they contain unusual ingredients.

Glaze Fit and Thermal Expansion

When the piece cools, both glaze and clay body cool and shrink. If their rates of thermal expansion and contraction are mismatched, the glaze may craze (forming a network of fine cracks) if it shrinks too much relative to the body, or it may shiver and flake off if it doesn’t shrink enough. This relationship is sometimes measured using a calculated thermal expansion (CTE). Having a glaze and clay body in harmony—often called “good glaze fit”—is essential for producing sturdy, long-lasting pottery without defects like crazing or shivering.

References

• Ceramic Glaze (Wikipedia) 
• Ceramic Glaze (Digitalfire) 
• Dave Finkelnburg, “How Glazes Melt” (NCECA 2012)
• “UMF Phase Diagrams: Guidemaps for Ceramic Glaze Development” by Howard Sawhill 
• “Phase and Eutectics” by Linda Bloomfield 

Books you can borrow online:

• The ceramic spectrum by Robin Hopper 
• Chinese glazes : their origins, chemistry and re-creation by Nigel Wood 
• The craft and art of clay by Susan Peterson 
• Clay and glazes for the potter by Daniel Rhodes 
• Cooper’s book of glaze recipes by Emmanuel Cooper 
• Glazes for the potter by Emmanuel Cooper 
• The potter’s book of glaze recipes by Emmanuel Cooper 
• Electric kiln ceramics by Richard Zakin 
• Glazes cone 6 : 1240⁰c/2264⁰f by Michael Bailey 
• A handbook of pottery glazes by David Green 
• Salt-Glaze Ceramics by Rosemary Cochrane 
• Mastering raku by Steven Branfman 
• Celadon Blues by Robert Tichane 
• Chinese stoneware glazes by Joseph Grebanier 
• Dry Glazes by Jeremy Jernegan 
• Cone 5-6 glazes : materials and recipes 
• The ceramic glaze handbook : materials, techniques, formulas by Mark Burleson