Glaze is a thin layer of glass fused onto the surface of a ceramic piece during kiln firing. It starts as a liquid or powder mixture applied to unfired or partially fired clay, then melts in the kiln to form a smooth, glassy coating that is permanently bonded to the surface. Glaze serves two purposes at once: it makes pottery waterproof and durable enough for daily use, and it provides color, texture, and visual depth that raw clay alone cannot achieve.
The Three Core Ingredients
Every ceramic glaze, from a simple clear coat to an elaborate crystalline surface, is built from three chemical components working together. Silica is the glass former. It is the same material as sand or quartz, and its chemical name is silicon dioxide. On its own, silica melts at an extremely high temperature, far beyond what most kilns can reach practically. That’s where the second ingredient comes in.
Flux is any chemical added to lower silica’s melting point so the glaze can actually melt inside a kiln. Common fluxes include compounds containing calcium, sodium, potassium, and lithium. Different fluxes melt at different temperatures, which is why some glazes are designed for low-fire kilns and others for high-fire kilns.
The third ingredient is alumina, typically introduced through clay. Alumina controls the glaze’s viscosity so it doesn’t run off the pot during firing. Just as importantly, it helps the glaze shrink at the same rate as the clay body underneath as both cool down. Without alumina, the glaze would crack or peel away from the surface.
What Happens Inside the Kiln
When a glazed piece enters the kiln, the powdered coating gradually melts into a liquid glass that flows across the clay surface. As the kiln cools, this liquid solidifies into a continuous glassy layer permanently fused to the clay beneath it. The glaze doesn’t simply sit on top of the clay like paint. At the boundary between the two, molecules from the glaze and the clay intermingle, creating a bond that makes them inseparable.
This bond is also why compatibility between glaze and clay body matters so much. Both materials expand when heated and contract when cooled, and they need to expand and contract at nearly the same rate. When they don’t match, problems show up either immediately or over time through repeated heating and cooling, like running a mug through the dishwasher hundreds of times.
Why Potters Glaze Their Work
The most practical reason to glaze pottery is waterproofing. Unglazed clay is porous, meaning it absorbs liquids. A glazed surface is non-porous, which makes a piece capable of holding water, coffee, soup, or anything else without leaking or staining. This is what makes a ceramic mug or bowl genuinely functional for food and drink.
Glaze also creates a surface that is easy to clean and sanitize, since bacteria and food particles can’t penetrate the smooth glass layer. Beyond hygiene, the glassy coating adds scratch resistance and protects the clay body from chipping and general wear over years of use.
How Glaze Gets Its Color
Color in ceramic glaze comes from metal oxides, tiny amounts of metallic compounds mixed into a clear base glaze. The results can be surprisingly vivid from very small additions. Just 1.5% cobalt oxide turns a glaze deep blue. Adding 3% chrome oxide produces green. Blend the two together and you get blue-green.
Iron oxide is one of the most versatile colorants. Depending on how much you add, it can produce anything from pale amber to straw yellow to golden orange to deep brown-black. Copper oxide yields turquoise or green under normal conditions but turns a striking blood red under special firing conditions. Manganese oxide creates purples and eggplant browns in some firings, honey browns in others.
What makes ceramic color especially fascinating is that the same metal oxide can produce completely different colors depending on the atmosphere inside the kiln during firing.
Oxidation vs. Reduction Firing
The kiln atmosphere has a dramatic effect on glaze color and behavior. In oxidation firing, plenty of oxygen circulates inside the kiln. Metal oxides keep their oxygen atoms and produce one set of colors. In reduction firing, the oxygen supply is restricted, and the metals in the glaze are forced to give up some of their oxygen. This converts them to a more metallic state, which changes their color entirely.
Copper is the classic example. In an oxidation kiln, copper oxide fires green or turquoise. In a reduction kiln, it fires red. Iron oxide behaves differently too. In oxidation, iron produces stable amber-to-brown tones and doesn’t melt very aggressively. In reduction, iron becomes a powerful melting agent that creates a wide range of intense earth tones, from the pale jade green of celadon glazes to the deep brown-black of traditional tenmoku glazes. This is why a potter can take the same glaze recipe and get strikingly different results just by changing how the kiln is fired.
Glossy, Matte, and Satin Surfaces
Not all glazes are shiny. The surface texture of a fired glaze depends on its chemistry and how the kiln cools. Glossy glazes cool into a smooth, uniform glass with no internal structure to scatter light. Matte glazes develop tiny crystals within the glass during cooling, and these crystals break up the surface just enough to eliminate the shine.
The most common way to create a matte glaze is through a process called devitrification, where certain oxides in the glaze encourage crystal growth as the kiln temperature drops. Slower cooling gives crystals more time to form, producing a drier, flatter surface. Faster cooling can keep the same glaze glossy. Different matte-producing oxides also create distinctly different textures. Magnesium-based mattes often feel smooth and buttery, sometimes described as having a “butterfat” quality. Barium and strontium mattes tend to feel drier to the touch but can produce unusually bright colors. Satin mattes fall roughly halfway between a dry matte and a full gloss, offering a soft sheen without the reflectivity of a glossy surface.
When Glaze and Clay Don’t Match
The most common glaze defect is crazing: a network of fine cracks across the glaze surface, sometimes visible immediately after firing, sometimes appearing weeks or months later. Crazing happens when the glaze contracts more than the clay body as they cool. The glaze is essentially too tight for the surface it’s bonded to, so it fractures to relieve the tension.
The opposite problem is shivering, where the glaze is under compression and actually flakes or peels off the clay. Both defects come down to the same root cause: a thermal expansion mismatch between glaze and clay. As a piece goes through heating and cooling cycles during normal use, an incompatible glaze will eventually fail one way or the other. The fix isn’t cosmetic. It requires adjusting the glaze chemistry or switching to a clay body that expands and contracts at a compatible rate.
Crazing is more than a visual flaw on functional ware. Those tiny cracks break the waterproof seal, allowing liquids and bacteria into the porous clay underneath. For a decorative piece it may be acceptable or even intentional (some potters create deliberate crackle effects), but on a coffee mug or dinner plate, it compromises both hygiene and durability.
Food Safety and Lead in Glazes
Historically, many ceramic glazes contained lead as a flux because it melts at low temperatures and produces a brilliant, smooth surface. Lead-containing glazes can leach into food and drinks, especially acidic ones like coffee, juice, or tomato sauce. The FDA regulates ceramicware intended for food use and tests for extractable lead. If a piece exceeds the agency’s action levels, it can be considered adulterated and pulled from the market.
Ceramicware labeled “Lead Free” must contain no extractable lead when tested under the FDA’s prescribed methods. Decorative pieces that look like they could be used for food but aren’t safe for it must carry a conspicuous permanent warning label stating they are not for food use. If you’re buying handmade pottery for your kitchen, knowing whether it was glazed with food-safe materials and properly fired matters. Underfired glazes, even those made from safe ingredients, may not fully melt into a stable glass, leaving a surface that can leach chemicals it otherwise wouldn’t.