Glaze is a thin, glass-like coating applied to clay pottery and ceramics. It starts as a liquid mixture of finely ground minerals suspended in water, and when fired in a kiln, those minerals melt together and fuse to the clay surface, creating a smooth, durable finish. Glaze serves two purposes: it makes pottery waterproof and it gives it color, texture, and visual depth.
What Glaze Is Made Of
Every ceramic glaze contains three core ingredients, each with a specific job. Silica is the glass former, the same compound found in sand and window glass. It melts at extremely high temperatures on its own, so a second ingredient called flux is added to lower that melting point to something a kiln can reach. Common fluxes include compounds of sodium, potassium, calcium, and other minerals. The more flux in a recipe, the lower the temperature needed to melt the glaze.
The third ingredient is alumina, which controls how the glaze behaves as it melts. Without alumina, molten glaze would run right off the pot. Alumina thickens the melt and helps the glaze shrink at a rate that matches the clay body underneath, so it fits snugly without cracking as it cools.
Beyond these three, potters add colorants and opacifiers to change the appearance. Iron oxide produces earthy browns, ambers, and even pale greens. Cobalt creates deep blues. Copper can yield reds or greens depending on the kiln atmosphere. These additions are small in proportion but dramatically change the finished look.
How Glaze Turns Into Glass
Raw glaze is a powder mixed with water to form a creamy liquid called a slurry. This slurry gets applied to pottery that has already been fired once (called bisqueware). During a second firing, the kiln heats the glaze until its crystalline mineral particles melt and rearrange into an amorphous, noncrystalline structure. That transformation from organized crystals to disordered glass is called vitrification.
The second firing runs at a temperature high enough to melt the glaze but lower than the first firing, so the clay body itself stays stable. As the kiln cools, the molten glaze solidifies into a permanent glass coating bonded to the clay surface. The result is a piece that’s harder, more durable, and resistant to water.
Glaze vs. Underglaze vs. Slip
These three materials all go onto clay, but they work differently. Glaze is the only one that melts into a glassy surface. Underglaze is mostly liquid clay with some glass-forming ingredients added. It’s used purely for decoration, applying color or designs that sit beneath a layer of clear glaze. Underglaze stays roughly the same color after firing, which makes it predictable for detailed work.
Slip is the simplest of the three: clay mixed with water. It contains more clay than underglaze and no real glass-forming components. Potters use slip as a kind of glue to join clay pieces together, to fill cracks, and to add texture through techniques like trailing thick lines across a surface. Unlike underglaze, slip can only be applied to clay that still contains moisture. It shrinks as it dries, so applying it to already-fired bisqueware causes it to peel off.
Common Types of Glaze
Glazes are often categorized by their surface finish. Glossy glazes are the most familiar: smooth, reflective, and easy to clean. Matte glazes have a soft, non-reflective surface created by a higher proportion of alumina or certain minerals that crystallize during cooling. Satin glazes fall between the two, with a gentle sheen.
Some glazes are defined by their tradition or visual effect:
- Celadon is a transparent glaze with a pale green or blue tint, achieved by adding small amounts of iron oxide and firing in a kiln with reduced oxygen. It has roots in Chinese porcelain.
- Shino glazes originated in 16th-century Japan and produce warm, earthy surfaces in orange, red, white, or black. They often feature smoky patterns caused by carbon trapped in the glaze during firing.
- Ash glazes use wood ash as a key ingredient, producing natural, flowing effects with subtle variation.
- Salt glazes form when salt is thrown into the kiln during firing. The salt vaporizes and reacts with silica in the clay itself, creating a thin, textured coating with an orange-peel quality.
- Crystal glazes encourage large, visible crystals to grow within the glaze during a carefully controlled cooling cycle, creating dramatic snowflake-like patterns.
- Oil spot glazes produce dark spots resembling oil on water, caused by excess iron bubbling to the surface during firing.
How Glaze Is Applied
The three main application methods are dipping, brushing, and spraying, and each works a little differently at the material level.
Dipping is the fastest approach. You submerge the bisqueware into a bucket of glaze and pull it out. The porous surface of the fired clay absorbs water from the slurry almost instantly, and as that water gets pulled in, glaze particles cling to the surface in a uniform layer. The coat dries in seconds. Dipping works best when your bisqueware has been fired consistently, because variations in temperature during that first firing change how porous the clay is, which changes how thick the glaze goes on.
Brushing is the most common method for beginners and anyone using commercial glazes from a bottle. Brushing glazes contain additives that slow down water absorption into the clay, giving you time to spread the glaze smoothly before it dries. You typically apply two or three coats to build up the right thickness. This method gives the most artistic control for painting designs, but it takes patience to get an even result.
Spraying uses a spray gun to atomize the glaze into a fine mist. It produces the most uniform coating and lets you control thickness precisely by adjusting distance and spray time. It’s ideal for large pieces or objects with complex shapes that are hard to dip evenly.
Food Safety and Lead in Glazes
Not all glazes are safe for eating or drinking from. The primary concern is lead, which has historically been used as a flux in glaze recipes. When a lead-containing glaze is fired at the correct temperature for the correct duration, the lead bonds into the glass matrix and stays there. But if the firing is insufficient, lead can leach into food and beverages stored in or served on the pottery.
The FDA warns that no amount of washing or boiling can remove lead from a poorly fired glaze. Cadmium is another concern, sometimes found alongside lead in brightly colored glazes, particularly those in orange, red, or yellow. If you’re evaluating pottery for food use, be cautious with pieces that are handmade with a crude or irregular appearance, visibly damaged or worn, purchased from flea markets or street vendors, or decorated in bright warm colors. Look for a stamp on the bottom. Pieces intended only for display sometimes carry a “Not for Food Use” warning.
Most commercial glazes sold in the U.S. today are formulated to be food-safe and are labeled according to ASTM D4236, a federal standard that requires manufacturers to have their products evaluated by toxicologists and clearly labeled for any chronic health hazards. If you’re buying glazes for functional pottery, choosing products labeled “food safe” or “dinnerware safe” from established manufacturers is the most straightforward way to avoid problems.
Common Glaze Defects
Even experienced potters run into glaze problems. Most defects come down to a mismatch between the glaze and the clay body, or issues during firing.
Crazing is the most common defect: a network of fine cracks in the glaze surface, like a cracked eggshell. It happens when the glaze contracts more than the clay as the piece cools, putting the glaze under tension until it fractures. The fix involves adjusting the glaze recipe so its expansion rate better matches the clay. Shivering is the opposite problem. The glaze contracts less than the clay and gets compressed until it flakes or chips off the surface.
Crawling looks like the glaze has pulled away from certain areas, leaving bare patches of clay. It’s caused by high surface tension in the melting glaze, which makes it bead up instead of flowing smoothly across the surface. Dust, oil, or moisture on the bisqueware before glazing can trigger it.
Pinholes and pitting are tiny holes in the glaze surface, often caused by gases escaping from the clay body during firing. Heavily textured clay bodies with coarse particles are especially prone to this. Blistering, which produces larger bubbles, can result from overfiring or from using certain soluble flux compounds in the glaze recipe.