UPMSP Vocational Ceramics flashcards

34 cards. Read the term and the definition here. The deck at the top of this page is optional.

Kaolin

A primary clay mineral (kaolinite, Al2O3·2SiO2·2H2O) with low plasticity, high refractoriness and white firing colour; it is the main component of porcelain and a key ingredient in most clay bodies.

Ball clay

A fine-grained, highly plastic sedimentary clay with high shrinkage and organic content; added to bodies to improve workability and green strength, but it darkens the fired colour and increases drying shrinkage.

Grog

Fired clay that has been crushed and added to a clay body to reduce shrinkage and drying cracks, improve thermal shock resistance and open the body for even drying.

Plasticity

The property of damp clay that allows it to be deformed without cracking and to hold its shape. It depends on particle size, water film and clay mineral type; bentonite is far more plastic than kaolin.

Blunging

Mixing clay and water in a blunger to produce a slip, usually with a deflocculant such as sodium silicate, so that the slip has low viscosity at a high solids content.

De-airing pug mill

A machine that extrudes clay under vacuum to remove air pockets, producing dense, homogeneous, plastic clay that throws better and shrinks less than non-de-aired clay.

Filter press

Equipment that presses water out of slip between filter cloths to produce filter cake of about 20-25 % moisture, which is then pugged or wedged into a workable body.

Ageing clay

Storing damp clay for weeks so that bacteria and organic matter develop, which improves plasticity and workability, especially in ball-clay-rich bodies.

Wedging

Working clay by spiral kneading or cut-and-stack methods to remove air pockets, equalise moisture and homogenise the body before forming.

Leather-hard

The stage of drying when clay is stiff enough to hold its shape but still damp enough to be trimmed, joined with slip, or carved without deforming.

Jiggering

A production method in which a clay ball is pressed onto a rotating plaster or metal mould by a profile tool to form flatware such as plates and saucers.

Extrusion

Forming clay by forcing it through a die in a pug mill or extruder to produce a constant cross-section, such as tiles, tubes or handles.

Quartz inversion

The reversible change between beta-quartz and alpha-quartz at about 573 °C, accompanied by roughly 0.8 % volume change; kilns must be heated and cooled slowly through this temperature.

Bisque firing

The first firing of unfired ware, typically to 950-1050 °C, which burns out carbon and chemically combined water and gives the body enough strength for glazing.

Pyrometric cone

A small cone of ceramic material that bends at a specified temperature for a given heating rate, used to monitor heat work inside a kiln (e.g., cone 06 ≈ 999 °C).

Black coring

A firing fault in which carbon is trapped inside the body because the early firing was too fast or insufficiently oxidising; the core turns black and the ware may blister.

Reduction firing

Firing with insufficient air so that carbon monoxide forms; it produces celadons, copper reds and iron speckling, and is controlled by damper and air intake settings.

Crazing

A network of fine cracks in a glaze caused by the glaze being in tension, usually because its thermal expansion is higher than that of the body.

Sgraffito

A decoration technique in which a design is scratched through a layer of slip or glaze to reveal the contrasting colour of the body underneath.

Slip density

The mass per unit volume of a slip, typically 1.4-1.7 g/cm3 for decorating slip and 1.7-1.8 g/cm3 for casting slip; it is checked with a hydrometer or by weighing a known volume.

Tin oxide opacifier

A white opacifier used at about 5-10 % in glaze to make it opaque and to produce bright whites and clean colours, though it is expensive.

Centring

The first step of wheel throwing, in which a ball of clay is pressed into a true rotation on the wheel head using even hand pressure, so the walls can later be thrown thin and even.

Trimming

Turning a leather-hard pot on the wheel to refine its shape and cut a foot ring, using loop tools while the pot is centred and held in place.

Coil building

A hand-building method in which rolls of clay are stacked and joined with slip and scoring, then blended with a rib to build hollow forms without a mould.

Plaster to water ratio

The proportion of plaster of Paris to water by weight, typically 100:70 for strong moulds and 100:80 for more porous slip-casting moulds; more water gives a weaker, more absorbent mould.

Slip casting

Pouring deflocculated slip into a dry plaster mould; the mould absorbs water and builds a wall, and the excess slip is poured out to leave a hollow cast.

Two-piece mould

A plaster mould made in two or more parts with a parting line, allowing complex shapes with undercuts to be removed without damage; parts are keyed and held together during casting.

Shrinkage allowance

The extra size added to a mould or drawing to compensate for the 8-15 % total shrinkage of clay during drying and firing, so the fired piece reaches the intended dimensions.

Orthographic projection

A technical drawing method that shows an object in separate front, side and top views, with dimensions in millimetres, used to communicate ceramic product designs for production.

Silica dust hazard

Respirable crystalline silica from dry clay and glaze materials causes silicosis; controls include wet methods, local exhaust ventilation, respirators and wet cleaning instead of dry sweeping.

Break-even quantity

The number of units that must be sold for total revenue to equal total cost, calculated as fixed costs divided by the difference between selling price and variable cost per unit.

Cost-plus pricing

Setting a selling price by adding a fixed markup percentage to the total unit cost, which includes materials, labour, firing, overheads and packaging.

Settling tank

A tank used to hold clay and glaze wastewater so that solids settle out; the clear water is decanted and the sludge is dried and disposed of as solid waste rather than poured down drains.

Lead-free glaze

A glaze formulated without lead compounds for food-contact ware, avoiding lead leaching into food; lead glazes are restricted or banned for tableware in many countries.