UPMSP Vocational Ceramics key terms

The ability of a moist clay body to be shaped and to hold a new form without cracking or sagging. It depends on particle size, water content and the type of clay minerals present.

A prepared mixture of clays plus non-plastic additions such as silica, feldspar, grog or chamotte, formulated to give a workable, dryable and fireable material with chosen colour and shrinkage behaviour.

Pre-fired clay that has been crushed and graded into coarse particles and added to a body to reduce shrinkage, open the texture, improve drying safety and give tooth for hand building.

The progressive melting and densification of a fired body as glassy phase forms, closing open porosity. Complete vitrification gives low absorption and higher strength but raises the risk of warping and over-firing.

Soaking dry clay lumps or powder in water until they break down into a soft, workable mass. Slaking is gentler than mechanical mixing and is used when preparing clay from dry material.

Mechanical agitation of clay and water in a tank or mixer to produce a smooth, uniform slip. It is used for casting slip and for reconstituting scrap clay in a production workshop.

Kneading and folding a clay mass by hand or with a wire cutter to remove air pockets, equalise moisture and homogenise the body before forming. It reduces uneven shrinkage and firing faults.

A wedging pattern in which the clay is folded and pressed in a spiral or ram's-head motion so that trapped air is worked out and the moisture is distributed evenly through the mass.

A machine that mixes and compresses clay under vacuum while extruding it as a dense, air-free bar. The vacuum removes air pockets that cause pinholing, lamination and blow-outs during firing.

A fault in which layers of clay fail to bond, leaving planes of weakness that open up as cracks or blisters during drying and firing. It comes from poor wedging or careless joining.

The stage at which clay has stiffened enough to hold its shape but still feels cool and damp and can be trimmed, joined, stamped or bent without cracking. Most joining and decoration is done at this stage.

The condition of clay with virtually all free water evaporated, appearing pale and feeling warm and dry to the touch. Ware must reach this state before bisque firing to avoid steam explosions.

The reduction in size of a clay piece, first as water leaves during drying and again as the body densifies in firing. Total shrinkage must be measured so moulds and components are sized correctly.

The first firing of ware, usually to a lower temperature than the final glaze firing, which burns out organic matter and hardens the body into a porous state suitable for glazing.

A small tapered test piece of calibrated ceramic material that bends at a known temperature, integrating the effect of temperature and time. Cones are used to judge heat work inside the kiln.

A kiln atmosphere with sufficient oxygen for carbon and sulphur in the clay and glaze to burn away cleanly. Inadequate oxidation causes black coring, carbon trapping and dull or blistered surfaces.

A firing atmosphere starved of oxygen, produced by restricting air or adding fuel, which causes oxides to give up oxygen. It shifts iron and copper colours and affects clay body colour and glaze texture.

The temperature range at which a glaze or body has melted and developed its intended surface and strength without under-firing or running off the pot. It is normally expressed in cone numbers.

A thin, glassy coating melted onto fired clay, made from silica with a flux and a stabiliser plus colouring oxides. It makes ware impermeable and food-safe and provides the decorative surface.

An oxide, such as lead, sodium, potassium, calcium or boron, that lowers the melting point of a glaze so it fuses at kiln temperatures. Flux choice controls gloss, fit and durability.

Clay mixed with water to a creamy liquid consistency, used for casting in moulds, for joining clay parts with a brush, and for decorating or engobing a surface.

An additive such as sodium silicate or sodium carbonate that makes slip particles repel each other, so the slip flows freely with less water. This reduces casting shrinkage and gives a denser cast.

A network of fine cracks in a glaze caused by the glaze shrinking more than the body on cooling. It can be a decorative effect but on tableware it allows liquid into the body.

The standard hand-building joint: the surfaces to be joined are roughened with a fork or needle tool, brushed with slip and pressed together so the seam fuses rather than breaking apart.

A hand-building method in which long rolls of clay are laid one above another, each joined to the previous coil, and the wall is then smoothed, giving a strong, even-walled hollow form.

Constructing forms from flat sheets of clay rolled or cut to a uniform thickness, which are then joined at the edges. Even thickness and firm leather-hard clay prevent sagging and cracking.

The first step of throwing, in which a spinning ball of clay is pressed and guided until it sits exactly on the wheel axis with no wobble, so the walls can be raised evenly.

Forming hollow ware on a rotating wheel by opening a centred ball of clay and drawing the wall upward between wet fingers, using the wheel's momentum to shape a symmetrical vessel.

A shaped wooden or plastic tool used at the wheel to compress, smooth and refine a thrown wall, and to remove slurry. Compression with a rib strengthens the wall and reduces cracking.

A shaped hollow or case, usually of plaster, used to reproduce a form by casting slip inside it or pressing clay into it. The mould absorbs water and releases the shape as it dries.

A deflocculated, free-flowing clay suspension poured into a plaster mould. The mould draws water out of the slip, building a firm wall while the excess is poured away or left to set solid.

Calcined gypsum powder that sets hard when mixed with water, used for moulds, bats and models. The water-to-plaster ratio controls setting time, strength and absorbency of the finished mould.

A thin coating such as soft soap, oil or a parting dust applied to a mould or model so that cast or pressed clay and plaster separates cleanly without sticking or tearing.

Fine crystalline particles released when dry clay, grog or fired ware is cut, sanded or swept. Inhaling it causes silicosis, so wet methods, extraction and approved respirators are required.

Worn protection such as an apron, goggles, gloves and a dust respirator, selected to match the hazard. PPE is the last line of defence and does not replace dust control or safe working methods.

The controlled extraction of fumes, carbon monoxide and water vapour from a kiln and its room. It protects operators from toxic gases and helps maintain a clean oxidising atmosphere during firing.

Working out the true cost of a ceramic item: materials, fuel and power, labour time, mould and tool wear, breakage and overheads. Correct costing sets the minimum price a product can be sold at profitably.

The amount added to unit cost to arrive at a selling price, usually expressed as a percentage. It must cover overheads and profit and is distinct from the profit margin on the sale price.

The sales volume at which total income equals total costs, so neither profit nor loss is made. Knowing it helps a workshop decide output targets, pricing and whether to accept a bulk order.