The A to Z of Ceramics

Abrasives are materials used to wear down, smooth, or polish other surfaces through friction. In ceramics, abrasives are commonly used for tasks like sanding, polishing, and smoothing ceramic pieces, whether in the finishing or trimming stages.

Amorphous refers to a material that lacks a regular, crystalline structure. In ceramics, this term is often used to describe glasses or glassy phases that form when materials are melted and cooled quickly, preventing crystals from developing.

Apparent porosity refers to the percentage of open pores (void spaces) in a fired ceramic body that are accessible to water or gas. It tells you how absorbent or porous the material is.

In ceramics, an ark is a large mixing or storage container, made of fibreglass or steel, used in the clay preparation process and essential for storing and managing bulk materials efficiently.

An autoclave is a sealed, high-pressure vessel, usually made of steel, used for heating articles under pressure with steam or gas.

A ball mill is a machine used in ceramics (and other industries) to grind and mix materials into a fine powder. It's essential for preparing ceramic body materials. It consists of a lined cylinder rotating about it's horizontal axis and charged with flint pebbles or special ceramic grinding media plus the material to be ground. The mill may be operated dry or wet.

Batt wash is a protective refractory material coating applied to kiln shelves (and sometimes kiln furniture) to prevent glazed pottery from sticking during firing.

Bentonite is a general term for a montmorillonite clay. It is a highly plastic, fine-grained clay mineral that is often added to ceramic clay bodies and glazes in very small amounts to improve their workability and suspension properties.

Pottery bodies after the first firing stage. Known for their durability and porous nature, making them ideal for absorbing glazes in the final firing process.

Bloating is a ceramic defect where blisters or bubbles form under the surface of a clay body during firing, causing swollen, raised areas or even burst craters in the finished piece.

Blunging is the process of mixing clay or ceramic materials with water to create a smooth, uniform slurry or slip. It's usually done using a mechanical mixer called a blunger.

A body stain is a ceramic colourant specifically designed to be added to a clay body, rather than just the surface or glaze. It allows potters to colour the entire clay body consistently, creating coloured clays for decorative or functional use..

Bone china is a type of high-quality porcelain that contains bone ash typically made from calcined animal bones along with kaolin (china clay) and feldspar.

A bottle oven is a traditional type of kiln used primarily in the British pottery industry, especially in Stoke-on-Trent, during the 18th to mid-20th centuries. It's named for its distinctive bottle-shaped outer structure and firing was usually coal.

Calcination is the process of heating a material to a high temperature (below its melting point) in order to bring about a chemical or physical change. Usually to drive off volatile substances like water, carbon dioxide, or organic matter.

Casting in ceramics refers to the process of pouring liquid clay (called slip) into a plaster mould to form a shape. It’s a popular method for making hollow or detailed ceramic forms, especially in production and studio pottery.

Casting slip is a specially prepared liquid clay used in slip casting, where it’s poured into plaster moulds to form ceramic shapes. It’s made by mixing clay, water, and a deflocculant to create a smooth, fluid yet stable suspension of high specific gravity.

Derived from the Greek word 'Keramos' meaning 'earthen vessel'. Now applied to a whole field of products, i.e Bricks, Tiles, Pipes, Porcelain, China etc.

The Coefficient of Expansion (often called the Coefficient of Thermal Expansion or CTE) is a measure of how much a material expands or contracts when heated or cooled.

A colloid in ceramics refers to a mixture where very fine particles (typically less than 1 micron) are suspended in a liquid usually water without settling out quickly. These particles are small enough to stay dispersed, but not dissolved.

Comminution in ceramics refers to the process of breaking down raw materials (like clay, minerals, or rocks) into smaller particles or powders either for easier handling, improved mixing, or better firing performance.

Compressive strength in ceramics refers to the ability of a material to resist being crushed or squashed when a force is applied. It's a measure of how much compressive (pushing) force a ceramic body can withstand before it fails or breaks

Crackle is a decorative effect in ceramics where the glaze intentionally forms a network of fine surface cracks, similar to crazing but in this case, it’s done on purpose for visual interest.

Crawling is a glaze defect where the glaze pulls away from areas of the ceramic surface during firing, leaving bare, unglazed patches, often with uneven, raised edges. This is usually due either to dust or grease on the surface or to the production of collodial material.

Crazing is the development of fine cracks in the glaze on a ceramic surface caused by excessive contraction of a glaze or enamel. It usually appears after firing and can be either immediately visible or develop over time.

Cryptocrystalline refers to a material made up of extremely fine crystals that are so small they can’t be seen even under a standard microscope.

Cut glaze refers to areas on a ceramic piece where the glaze has been intentionally removed before firing either for decorative effect or to prevent glaze from being in unwanted places (like the base of a pot).

The removal of air from clay, plaster etc. Various means are used but the most common with clay is via the de-airing chamber of the pugmill.

The dispersion of clay slip or glaze by the addition of an electrolyte for example, sodium silicate or soda ash etc.

Devitrification in ceramics refers to the process where a glassy surface (like a glaze) begins to crystallize during or after firing. Instead of staying smooth and glossy, the glaze develops a dull, cloudy, or matte appearance due to the formation of tiny crystals within the glass.

Dilatancy is a property of certain clay or slip mixtures where they become thicker and more resistant to flow when agitated, stirred, or shaken. It’s also known as shear thickening.

The application of a glaze by immersion, allowing the excess to drain off.

Absorption of glaze by an unglazed surface nearby during firing.

Globules of glaze found on open glost ware, usually from glaze accumulations on the roof of the kiln.

Dunting is a type of cracking in ceramic ware that happens due to silica inversion and sudden temperature changes, usually during cooling (or sometimes heating) in the kiln. It’s a thermal shock issue, and the cracks can be sharp, stress-like fractures, often running deep through the piece.

A moderately porous pottery formed from a body containing ball clay, china clay, flint and Cornish stone, which is fired to a temperature somewhat below that required to produce a vitreous article.

An edge runner mill is a type of grinding or mixing machine used in ceramics (and other industries) to crush, mix, or refine materials like clays, glazes, pigments, and raw minerals.

Efflorescence is a phenomenon in ceramics where white, powdery deposits appear on the surface of a fired or unfired clay piece. It’s caused by soluble salts in the clay body or water that migrate to the surface and crystallize as the water evaporates.

Made for use as an electrical insulating material. Typical composition per cent is ball clay 28, china clay 22, quartz 25, felspar 25.

The separation of particles according to their size of specific gravity by a controlled velocity water stream. The basis of a method for the determination of fineness of materials.

In ceramics, enamel refers to a glassy, decorative coating that is applied over a glaze, often in the form of painted patterns or designs, and then fired at a low temperature to fuse it to the surface.

Enamel firing is a low-temperature firing process used to melt enamels which are finely ground glass onto the surface of ceramics (or metals), creating a smooth, glossy, and often colourful decorative finish.

An engobe is a liquid clay slip that contains added colorants and sometimes fluxes, used to coat the surface of a ceramic piece usually to change its colour, texture, or decoration.

In ceramics, a eutectic refers to a specific mixture of materials that melts at a lower temperature than any of the individual components on their own.

Extrusion in ceramics is the process of forcing clay through a shaped die to create long, continuous forms.

Fettling is the process of cleaning up and refining a clay piece, especially after it has been cast or formed. It typically involves removing seams, rough edges, or excess clay with a knife or sponge to prepare the piece for firing.

Fillers in ceramics are materials added to clay bodies (and sometimes glazes) that do not melt during firing. They serve to alter texture, reduce shrinkage, and improve the clay’s workability, strength, or firing characteristics.

A filter cloth in ceramics is a fabric used in filter presses to separate water from clay slip during the dewatering process. It's designed to let water pass through while trapping fine clay particles, resulting in a solid, workable clay.

Flocculation in ceramics refers to the process of causing the tiny particles in a clay slip or glaze suspension to clump loosely together rather than staying evenly dispersed to give a proper consistency for dipping, casting, thickening.

In ceramics, a flux is a material that lowers the melting point of other substances especially silica in glazes and clay bodies. Fluxes help glazes melt, flow, and form a glassy surface during firing.

Frit is a pre-melted, ground glass used in ceramic glazes. It’s made by melting raw glaze materials together (like silica, fluxes, and colorants), rapidly cooling the molten mix (often by quenching it in water), and then grinding it into a fine powder.

A thin glassy layer formed on the surface of a ceramic product by exposure of the dipped article to a high temperature. Usually applied in the form of a suspension of ground glaze in water, to the clay or biscuit surface of the ware.

Glaze–body fit refers to how well a glaze adheres to and interacts with the clay body during firing, especially in terms of thermal expansion and contraction. A good fit means the glaze and the clay shrink and expand at compatible rates as they cool, preventing defects. This avoids the crazing of the glaze due to the tensile stresses in it.

A glaze stain is a pre-made, stable ceramic colorant used to colour glazes. Unlike raw oxides, glaze stains are specially formulated to produce consistent, reliable colours in ceramics.

Glost (also spelled glost firing) refers to the firing of glazed pottery. It is the stage in which the glaze is melted onto the bisque-fired ware in the kiln, creating a glassy surface.

Greenware refers to unfired clay that is, pottery or ceramic work that has been shaped but not yet bisque fired in the kiln. It's still in a fragile state and can be easily broken or rehydrated.

Grog is a type of fired and ground-up ceramic material that is added to clay bodies to improve their working and firing properties. It’s usually made from crushed fired clay (like bricks or previously fired pots), and it comes in a range of particle sizes from fine to coarse and is inert. Grog helps to decrease shrinkage of a clay body when firing.

A process of heating decorated biscuit ware to a temperature of approx 650-700 degrees celsius in order to burn out the organic media of the decoration and in order to fix the colour prior to glazing.

Hot pressing is a manufacturing process in which a material is heated and pressed simultaneously under high pressure to form a dense, solid shape. The process involves applying both heat and pressure to the material, causing it to bond and form a uniform, strong, and compact structure.

Jiggering is a pottery forming technique where a clay body is shaped by pressing it onto a shaped mold using a jigger tool. The tool consists of a spindle or rotating arm with a profile that shapes the clay as it spins, creating symmetrical pieces. Used mainly for flatware.

Kaolin is a fine, white clay that is highly valued in ceramics for its purity and ability to withstand high firing temperatures. It was first discovered in China, specifically in the kao-ling region which is why it's often referred to as china clay. The name "kaolin" comes from "kao-ling", the name of the hill where it was first found.

A kiln is a high-temperature oven or furnace used to fire ceramics. It heats materials to a specific temperature to bring about chemical changes that harden, glaze, or melt them.

Kiln furniture refers to the various supports and structures used inside a kiln to hold and organize ceramics during the firing process. It helps maximise the available space in the kiln and ensures that the pieces are properly positioned for even heat distribution.

Laminations in ceramics refer to thin layers or sheets of unfired or fired body often formed intentionally during the manufacturing process.

A fine mesh sieve, usually made from phosphor bronze or stainless steel and supported by a strong frame. Used in ceramics for sifting or screening materials like clay, glaze, or other powdered substances.

Leather-hard refers to a stage in the drying process of clay where the clay is partially dry but still retains some moisture. At this stage, the clay is firm enough to hold its shape, but it’s not fully dry or fired yet. It’s called "leather-hard" because the clay feels similar to the texture of leather—firm, but still somewhat flexible. It is the ideal stage for turning, fettling or sponging.

Lithography is a printing process that involves creating an image or design on a flat surface, typically a stone or metal plate, using a combination of water and oil-based inks. The term comes from the Greek words "lithos" (stone) and "grapho" (to write or draw).

Loss on ignition (LOI) is a laboratory process used to determine the amount of volatile material in a sample, usually a ceramic material, clay, or mineral, by measuring the weight loss that occurs when the sample is heated to a high temperature, typically between 900–1000°C.

Low solubility refers to the limited ability of a substance to dissolve in a particular solvent, usually water, at a given temperature and pressure. If a substance has low solubility, it means that only a small amount of it can be dissolved before the solution becomes saturated.

Lustres are metallic or iridescent coatings applied to ceramics, typically after the piece has been glazed and fired. These coatings create a shiny, reflective surface that can have various colour effects, often with a rainbow-like or metallic finish.

Majolica is a type of ceramic ware that is characterized by its brightly coloured, low fired glaze. The technique was originally developed in Italy during the Renaissance and became highly popular in Spain and Portugal as well.

The original or prototype of the piece to be made. Usually in clay, occasionally in plaster or other material.

The Modulus of Elasticity, also known as Young's Modulus, is a measure of the stiffness or rigidity of a material. It defines the relationship between stress (force applied) and strain (deformation or displacement) within the elastic range of the material meaning before the material begins to permanently deform.

Modulus of Rupture (MOR) is a measure of the flexural strength or the ability of a material to resist breaking when subjected to bending or flexural stress. It refers to the maximum stress a material can withstand before it fractures or ruptures under bending forces.

Moisture expansion refers to the increase in volume that occurs when a material absorbs water or moisture. This property is particularly important in ceramics, as some clays or clay bodies can expand when they absorb moisture during the forming, drying, or firing processes.

A muffle kiln is a type of furnace used in ceramics where the material being heated is isolated from the direct flame or heat source by a protective chamber. This allows for uniform heating without direct exposure to the combustion gases, which can be important for maintaining the quality of the work being fired.

An opacifier is a substance that is added to glazes or clays to make them opaque, meaning they prevent light from passing through and create a non-transparent surface. Opacifiers are commonly used to achieve a matte or opaque finish in ceramic pieces, as well as to cover up underlying surfaces or designs.

An optical pyrometer is a type of non-contact thermometer used to measure the temperature of an object by detecting the intensity of the light (usually infrared radiation) emitted by the object. This device is particularly useful for measuring the temperature of objects that are too hot to touch or difficult to access, such as those in furnaces or kilns.

Particle size distribution (PSD) refers to the measurement and analysis of the range and proportion of different particle sizes present in a given material. In ceramics, this typically applies to raw materials like clays, feldspars, silica, and grogs.

Peeling is a ceramic surface defect where layers of glaze or slip flake off or detach from the clay body, usually during drying or firing and usually due to high compressive stresses in the layer.

Pinholes are tiny holes or craters that appear in the surface of a glaze after firing. Pinholes are the result of trapped air erupting through the body or glaze during maturation in the kiln.

Pitchers are waste pottery, typically pieces that have been rejected during or after firing due to flaws. These discarded items are often crushed back into grog, or fillers.

Plasticity in ceramics refers to the workability of clay. its ability to be shaped and hold form without cracking, especially when wet and when the force that is shaping the clay is removed.

Plucked ware is a ceramic defect where small bits of the glaze or surface of the clay body are pulled away or “plucked off” when a pot is removed from the kiln shelf after firing.

Porcelain is a high-fired, fine-grained ceramic material known for its whiteness, strength, translucency, and smooth surface.

Porosity in ceramics refers to the amount of empty space (pores) within a fired clay body that can absorb water or air.

Pugging is the process of mixing, de-airing, and compressing clay into a smooth, workable form using a machine called a pugmill.

Pyrometric cones are small, cone-shaped tools made from ceramic materials that are designed to bend or melt at specific temperatures but more importantly, they measure heat work, which is a combination of time and temperature in the kiln.

Quenching in ceramics refers to the process of rapidly cooling a hot ceramic piece, usually by plunging it into water or exposing it to air to achieve specific effects.

Raw glaze is a type of glaze that is applied directly to unfired (bone-dry) greenware, rather than onto bisque-fired pottery.

A reduction atmosphere in ceramics is a kiln environment where there is limited oxygen, causing the fuel (like gas, wood, or even carbon) to pull oxygen out of the clay and glaze materials during firing.

Refractory refers to a material's ability to withstand very high temperatures without melting, deforming, or breaking down. In ceramics, it usually describes materials used in kilns, kiln furniture, or clay bodies that must endure extreme heat.

Rheopexy is a rare and interesting property of certain fluids or suspensions, like some ceramic slips, where the material becomes thicker or more viscous over time when kept at rest after being stirred or shaken. Rheopexy is the opposite of Thixotropy.

Saggars are protective containers made of refractory clay used in kilns to shield ceramic pieces from direct flame, ash, gases, or debris during firing. They were commonly stacked in old bottle ovens.

Salt glaze (or salt glazing) is a traditional ceramic surface treatment used commonly in the 17th and 18th century for the glazing of domestic ware. Common salt (sodium chloride) is introduced into a hot kiln during the high-temperature firing usually around 1,200–1,300°C (Cone 10). The ensuing vapour reacts with the clay to form a sodium aluminosilicate glaze.

Sedimentation in ceramics refers to the settling of particles in a suspension like a clay slip or glaze based on their size, shape, and weight. Used for determination of fineness in ceramic materials.

Sgraffito (pronounced skrah-FEE-toh) is a decorative technique in ceramics where a surface layer, usually coloured slip or underglaze, is scratched through to reveal the contrasting clay body or layer beneath.

Sintering is the process in ceramics where clay particles begin to bond together during firing before the body becomes fully vitrified or melted.

Slip is a liquid mixture of clay and water, used in ceramics for a variety of purposes from decoration to construction.

Silk screen (or screen printing) in ceramics is a method of transferring detailed designs onto pottery using a fine mesh screen, usually made of silk or synthetic fabric, stretched over a frame.

Soak refers to a period during kiln firing when the temperature is held steady either at the peak temperature or during a specific stage for a set amount of time.

Spalling in ceramics is a defect where small flakes or chunks break off the surface of a fired ceramic piece usually due to stress from sudden temperature changes or impurities.

Specific surface area (SSA) in ceramics refers to the total surface area of particles per unit of mass or volume, usually measured in square meters per gram (m²/g).

Spitout is a ceramic defect where tiny pellets or bursts of material are ejected from the surface of a fired piece, leaving behind small craters, blisters, or pits almost as if the pot has "spit" during firing.

Spray drying is a method used in ceramics (especially in industrial production) to convert a liquid slurry into a fine, dry powder by spraying it into a chamber through which hot air is passed.

Stoneware is a type of ceramic clay body and finished ware that is dense, durable, and non-porous when fired typically at mid to high temperatures (around 1,200–1,300°C / Cone 6–10). It is a vitreous opaque ceramic that contains a naturally vitrifying clay.

Surface tension in ceramics refers to the tendency of a liquid (like molten glaze) to resist spreading or breaking apart, due to cohesive forces between its molecules.

Tailings are the waste materials left over after the processing of raw materials. In Tailings usually come from the extraction and refining of clay, feldspar, quartz, or other minerals used in ceramic production.

Tenmoku (also spelled Temmoku) is a type of dark, glossy glaze usually deep brown to black often with reddish, rust, or oil-spot highlights. It's traditionally used on stoneware and has its origins in Chinese and Japanese pottery.

Tensile strength in ceramics refers to the material’s ability to resist being pulled apart that is, how much stretching force it can withstand before breaking.

Thermal conductivity is a measure of how well a material can conduct heat. It is the rate of which heat passes through a material as measured by it's rise in temperature.

Thermal shock is the stress and potential damage that occurs in ceramics when a piece experiences a sudden change in temperature.

A thermocouple is a temperature-sensing device used in kilns to measure and monitor heat during firing. It measures the voltage generated when two different metal conductors are heated.

Thixotropy is a property of certain ceramic materials, like slips, glazes, and clay slurries, where they become more fluid when stirred or agitated, and then thicken again when left at rest. Thixotropy is the opposite of Rheopexy.

Throwing is the process of shaping clay on a potter’s wheel.

True porosity in ceramics refers to the actual volume of open pores within a fired clay body relative to its total volume including the pores that are accessible to liquids or gases.

Underglaze is a type of ceramic decoration applied to the surface of pottery before it is glazed. It sits underneath the clear or translucent glaze layer, which seals and enhances the decoration during firing.