Pottery is made of clay, water, and heat. At its core, every piece of ceramic starts as a naturally occurring material called clay, which is composed of fine mineral particles, primarily hydrated aluminum silicates. What transforms that soft, wet earth into a hard, permanent object is firing it in a kiln at temperatures ranging from roughly 900°C to over 1300°C. The specific type of clay, any additives mixed in, and the glaze coating on the surface all determine the final character of the piece.
The Minerals Inside Clay
The most important mineral in pottery clay is kaolinite, a layered silicate made of aluminum, silicon, oxygen, and hydrogen. It forms naturally when feldspar and other common rocks break down over geological time through weathering. Kaolinite’s layered structure is what gives wet clay its plasticity: thin sheets of mineral slide over each other when you add water, letting you shape the material by hand or on a wheel.
Pure kaolinite alone doesn’t make great pottery, though. It’s too stiff and difficult to work with, and it fires at very high temperatures. That’s why potters use blended clays that combine kaolinite with other minerals and organic matter. Ball clay, for example, contains kaolinite along with fine quartz and mica particles that make it extremely plastic and easy to shape. Most commercial pottery bodies are recipes: a potter or manufacturer mixes several clay types with added minerals like silica (ground quartz) and feldspar to control how the final piece shrinks, how strong it becomes, and at what temperature it matures.
Types of Clay Bodies
The three broad families of pottery clay are earthenware, stoneware, and porcelain. They differ mainly in mineral purity, particle size, and the temperature needed to fire them.
- Earthenware is the oldest and most common type. It contains iron and other mineral impurities that give it a red, brown, or buff color. It fires at relatively low temperatures and remains slightly porous unless glazed. Terra cotta flower pots are a familiar example.
- Stoneware fires at higher temperatures and becomes dense and waterproof even without a glaze. It typically contains a mix of ball clays, feldspar, and silica, and it ranges in color from grey to brown.
- Porcelain uses the purest form of clay, primarily kaolin, combined with feldspar and silica. It fires at the highest temperatures and produces a white, translucent, glass-like body. Its purity makes it harder to shape, which is why porcelain has historically been considered the most technically demanding ceramic to produce.
A fourth category worth noting is bone china, which replaces a large portion of the clay body with cattle bone ash. About 50% of the material in bone china is calcined bovine bone, a calcium phosphate powder. This high bone content is what gives bone china its characteristic translucency and strength, making it generally more durable than standard kaolin-based porcelain despite feeling thinner and more delicate.
What Happens During Firing
Shaping clay is only half the process. Firing is what turns a fragile, dried-out form into a permanent ceramic. The transformation happens in stages as the temperature climbs inside the kiln.
Below about 600°C, any remaining water trapped between mineral layers burns off. The clay is now chemically altered and can never be reconstituted with water. As temperatures rise further, organic matter combusts, and the mineral structure begins to shift. This first firing, called a bisque firing, typically reaches somewhere between 900°C and 1060°C depending on the clay type. The result is a porous, chalky body that’s hard enough to handle but still absorbent enough to accept a glaze.
The second firing, often called the glaze firing, pushes temperatures higher. For stoneware and porcelain, this can reach 1200°C to 1300°C. At these temperatures, a process called vitrification begins: the feldspar and silica in the clay body start to melt and form a glassy matrix that fills in the spaces between the remaining solid particles. The clay becomes denser, stronger, and less porous. Research on natural clays shows that most vitrification happens not during the temperature climb itself but during the “soak,” the period when the kiln holds at peak temperature. The longer the soak, the more glass forms within the clay body.
Low-fire earthenware never fully vitrifies, which is why it stays porous and needs a watertight glaze for functional use. High-fire stoneware and porcelain vitrify almost completely, producing a body that’s waterproof on its own.
What Glazes Are Made Of
A glaze is essentially a thin layer of glass fused to the surface of the clay. It serves two purposes: it seals the surface, and it provides color, texture, and visual finish. Glazes are made from three core components.
The glass-former is silica, the same silicon dioxide that makes up window glass. Silica gives the glaze its hard, glassy surface. The problem is that pure silica doesn’t melt until around 1700°C, far too hot for a pottery kiln. That’s where fluxes come in. Fluxes are minerals that lower the melting point of silica so the glaze can mature at a usable temperature. Low-fire glazes traditionally use compounds containing lead, boron, or commercially prepared mixtures called frits. High-fire glazes rely on calcium carbonate (limestone), dolomite, or even wood ash as their flux. The third ingredient, alumina, acts as a stabilizer. It keeps the melted glaze from running off the pot during firing and helps it shrink at the same rate as the clay underneath. Alumina is typically introduced through kaolin or ball clay mixed into the glaze recipe.
Color comes from small additions of metal oxides. Iron produces earthy browns and greens, cobalt creates blue, copper yields green or red depending on the firing atmosphere, and manganese gives purple-brown tones. The same oxide can produce dramatically different colors depending on whether the kiln atmosphere is oxygen-rich or oxygen-starved during firing.
Additives and Non-Clay Ingredients
Beyond the basic clay minerals and glazes, potters and manufacturers add a range of materials to fine-tune how clay performs. Grog, which is pre-fired clay ground into granules, gets mixed into the body to reduce shrinkage and cracking. It’s especially common in large sculptural pieces and raku pottery, where thermal shock resistance matters. Sand and fine silica serve a similar structural role.
Paper clay blends cellulose fiber into the wet clay body. The paper fibers burn away during firing but leave behind a network of tiny channels that make the greenware (unfired clay) stronger and easier to repair. Some potters add volcanic ash,iteite, or even recycled glass to achieve specific textures or lower the firing temperature.
Safety in Functional Pottery
If pottery will hold food or drink, the materials in the glaze become a health concern. Lead and cadmium, both historically common in bright-colored glazes, can leach into food and liquids. The FDA sets specific limits on how much lead a finished piece of dinnerware can release. Cups and mugs have the strictest threshold at 0.5 micrograms per milliliter of leaching solution, while flatware like plates is allowed up to 3.0 micrograms per milliliter. Pitchers are held to the same tight standard as mugs.
Commercially produced dinnerware sold in the U.S. is tested against these limits. Handmade pottery from markets, travel souvenirs, or antique pieces may not meet current standards. Brightly colored glazes, especially in orange, red, and yellow, are more likely to contain lead. If you’re unsure about a piece, home lead-testing kits are available, though lab testing is more reliable. Using questionable pieces for decoration rather than food service eliminates the risk entirely.