Jargon

Stages of Clay

slip — a mixture of water and clay that acts like clay glue. I like mine at a consistency like thick pancake mix.

wet/fresh/plastic — clay straight from the bag (assuming you didn’t leave it open); clay that can be worked with great flexibility

leather hard — clay that has dried some, usually a handful of hours to a few days depending on the clay and size of the work, but still has some moisture; this is the ideal phase for trimming

bone dry — clay that has completely dried out but hasn’t yet been through a kiln

greenware — unfired pottery

bisque/bisque ware — unglazed fired pottery

glaze ware — glazed and fired pottery; finished pottery

ceramic — clay that has been fired and chemically changed by water particles being removed, clay particles being fused together, and becoming vitrified (becoming glass-like)

firing

cone — a measure of heatwork (temperature + time). Similar to the difference between “baked at 400°F” and “medium-rare”—one describes just the heat, the other describes how done it actually is.

bisque firing — the first firing that turns clay into ceramic, when it can no longer be reversed to workable clay

glaze firing — the second firing that turns glazes into a glass-like shell around the pottery; there are sometimes additional firings to lock in specialty details like lusters

oxidation firing — oxygen is consistent and plentiful in the firing, resulting in stable and predictable outcomes in glazes and clay; common in electric kilns

reduction firing — oxygen is removed or decreased in the firing, causing compounds in the glazes and clay bodies to be consumed to keep the temperature high, resulting in unpredictable and unique outcomes; common in gas kilns or open-fire firing, and raku

raku — pottery is removed from the kiln while it’s red-hot and put into a pit or barrel filled with combustibles (paper, leaves, etc); the resulting smoke and flames create a reduction firing atmosphere and moody, spectacular effects in raku-appropriate glazes and clays.

cone 6 — mid-range firing around ~2200–2250°F / ~1200–1230°C; a common level of heat work for electric kilns known for predictable glaze and vitrification results

cone 10 — high-fire around ~2340°F / ~1280°C; a common level of heat work in gas kilns or reduction firings known for extra strong, extra vitrified clay and complex glaze results

Glazes + Finishes + Defects

underglaze — can be brushed or stamped onto greenware (the only kind of glaze that can be put on greenware!) or bisque before firing. These glazes are very stable, consistent, and don’t run. This is likely the kind of glaze you’ve painted on mugs or plates at one of those paint-your-own-ceramic stores!

speckles — a popular element in clays and glazes that look like freckles or confetti; comes in various sizes, colors, and “runniness” from various minerals like iron or manganese; runnier speckles look like dabs of paint after firing, while less runny, traditional speckles look more conventionally freckled

overglaze — colored or metallic decoration applied on top of an already-fired glaze. Most non-metallic overglazes are food-safe if fired properly; metallic overglazes (like lusters) are decorative only.

luster — a type of overglaze that results in a metallic or iridescent finish applied over a fired glaze; usually is decorative rather than functional.

crystalline glaze — a special glaze that grows shiny, flower-like crystals during a controlled firing cycle

breaking — where a glaze becomes lighter or changes color or texture on a curve, edge, or raised area of clay after firing; like “waves breaking,” but the waves is color!

beading — glaze that beads up as it fires resulting in colored bumps on the finished clay. Warning for those with trypophobia (fear of/repulsion by clusters of holes or bumps)!

crazing/crackling — tiny, random spiderweb-like cracks in the glaze surface that happens when the glaze shrinks more than the clay body as it cools; can be intentional for aesthetic effect or unwanted for the inside of functional ware

crawling — glaze pulls away from the clay surface, leaving bare patches; Often caused by impurities or too much moisture on the clay surface, too thick of glaze application, glaze/clay shrinkage mismatch. Can be intentional for aesthetic effect or unwanted for the inside of functional ware.

pin-holing — tiny holes in the glaze surface usually caused by air bubbles or gases escaping the clay or glaze during firing

dimpling — small indentations or depressions in the glaze surface; usually caused by uneven glaze thickness, surface tension, or trapped air under a thick glaze. Can look similar to pin-holing but less like a hole and more like a dent

shivering — glaze flaking or chipping off because of a mismatch in glaze and clay compatibility, resulting in the glaze shrinking and cooling faster than the clay;

Safety

vitrification — when the clay partially melts and re-fuses more densely, and becomes strong, glass-like and nearly un-absorbent; when clay is “fully baked.” Different clay bodies become vitrified at different cones, hence “low fire,” “mid fire,” and “high fire” clay types. Note: clay shouldn’t be fired higher than its intended range because it can melt, bloat, warp, etc; and if fired too low will be under-fired and not vitrified, therefore very likely not food safe.

food safe vs dinnerware safe — all dinnerware safe items are food safe, but not all food safe items are dinnerware safe; the main difference is durability. For example, a mug is food safe but not dinnerware safe, because if you were to use a spoon or other eating utensil against the interior surface of a mug as often as you would a dinner plate, that glaze would degrade faster to the point of it becoming un-food-safe; but it’s perfectly fine to use the same mug for its intended purpose of holding your favorite hot beverage. All pottery becomes un-food-safe if the glaze or ceramic becomes cracked or chipped where food or drink would come into contact, due to the inability to competently clean in these crevices, allowing bacteria to grow.

silicosis — a serious lung disease, also known as “potter’s rot,” that results in scar tissue in the lungs from breathing microscopic silica dust particles over a long duration of time and/or in great quantities. It’s not curable, and treatment is only symptom management. This is why potters avoid sweeping dry dust and instead clean with water or use proper ventilation and respirator masks.

Steps/Techniques/Methods

wedging — kneading clay to make it as homogenized as possible in terms of moisture and particle alignment, and eliminating air bubbles

hand-building/slab-building — making pottery without a wheel, by just using your hands!

throwing — making pottery using a pottery wheel; the term gets the name because you throw the ball of wedged clay onto the wheel before starting, to suction it to the wheel-head so it doesn’t move when the wheel is moving

centering — the process of getting the mass of clay balanced on the center of the pottery wheel, to ensure even and consistent distribution of manipulation across the clay

coning — part of centering; the process of repeatedly shaping clay into a cone and pushing it back down on the wheel to center it and make it uniform, like wedging.

pulling — shaping the walls of a pot by stretching or squeezing the clay upward when on a pottery wheel

trimming — the satisfying process of cleaning up and refining the shape of the pot when it’s leather hard by removing excess clay

sgraffito — the technique of applying an underglaze to greenware, then carving a design into the piece leaving negative space behind in the glaze after firing, showing the color of the naked clay. A clear glaze may be applied to the piece after its initial bisque firing.

Mishima — the technique of carving a design into leather hard clay and then filling that space with a different colored clay slip; the excess is then scrapped off to create a smooth two-toned ceramic

Nerikomi or neriage — a form of hand building that shapes, stacks, layers different colored clays together then sliced to create unexpected patterns; kind of like those slice-and-bake cookies made of different colored doughs to create iconic ghosts, pumpkins, and black cats when sliced.

Kintsugi — a form of mending broken pottery or ceramic with gold lacquer to emphasize the beauty in its imperfections rather than throwing out a sentimental piece or disguising the damage

clay Types

reclaimed — previously used but unfired clay that has been rehydrated to a workable moisture and extruded and/or re-wedged to a homogenous chunk of ready-again-to-use clay

earthenware — a type of clay that stays porous and is fired at relatively low heat work (e.g. terra cotta); there is debate about whether earthenware can be food safe without glaze

stoneware — a type of clay that is fired at very high heat work that becomes glass-like when fully vitrified and may have slight porosity; it has a higher proportion of natural impurities that gives it varying colors from clay body to clay body. While it is more durable than earthenware to chips, staining, and scratches, stoneware isn’t as resilient as porcelain.

porcelain — a type of clay with very few mineral impurities that is fired at extremely high heat work and is very durable to scratching, staining, and chipping, making it the ideal material for heavy-use dinnerware; produces a beautiful, well, porcelain white ceramic that is sometimes semi-translucent and iridescent, which is why it is sometimes used in chandeliers.

air-dry “clay”* — a water-based paper-pulp and earth mixture; cannot produce true ceramic and should never be used for anything coming into contact with food or drink

polymer “clay”* — an oil, dye, glue, and plastic mixture; also cannot produce true ceramic and definitely should never be used for food ware or drink ware!

~philosophical~

wabi-sabi — appreciation for the beauty in imperfections and impermanence, and acceptance that these are parts of the human experience