Ceramic Arts Daily

Macaloid

Suspension agent/plasticizer similar to Bentonite and Veegum T. Up to 2% of dry-materials weight as plasticizer in high-kaolin claybodies. Up to 1/2 of 1% of dry-materials weight as suspension agent, brushing medium in glazes, and slips. Mix with water before adding other ingredients. Source: Clay: A Studio Handbook

macrocrystalline

Glaze effect featuring large patches of crystal development on glaze surface, as compared to microcrystalline effects. See crystaline glazes. Source: Clay: A Studio Handbook

magnesium carbonate

MgCO3—alkaline earth—HT flux, promotes mattness and opacity in LT glazes, smooth, hard, buttery surface in HT glazes—promotes purples or pinks with cobalt. High L.O.I., used to promote controlled crawl glaze effects. Source: Clay: A Studio Handbook

magnesium silicate; steatite; soapstone; talc

3MgO×4SiO2×H2O—HT alkaline earth flux in glaze, promotes smooth buttery surfaces, partial opacity—similar composition to clay, but in LT claybodies gives low shrinkage and high thermal shock resistance, as in standard 50/50 talc/ball clay whiteware body. Highly toxic in inhalation and ingestion. Source: Clay: A Studio Handbook

magnesium sulfate; epsom salts

MgSO4—water soluble, rarely used as magnesium source in glazes. Most often used as flocculant for slips and glazes. Often added to porcelain and porcelaineous stoneware bodies (1/2 of 1% of dry materials weight) to counteract deflocculating alkalinity released by kaolins or fluxes. Source: Clay: A Studio Handbook

maiolica

Earthenware, generally terracotta, with opaque white glaze (traditionally a lead glaze made opaque white with the addition of tin oxide) and colored overglaze decoration. Originated from Islamic/Moorish techniques used on the Spanish island of Mallorca, which exported these wares throughout the Mediterranean in the 15th and 16th centuries. The technique was adopted by Italy during… Read More »

majolica

Trade name for an historic English style of brightly glazed earthenware vessels made in the form of vegetables, animals, etc. Not to be confused with maiolica, which is defined as earthenware with opaque white glaze (traditionally a lead glaze made opaque white with the addition of tin oxide) and colored overglaze decoration. Source: Clay: A Studio… Read More »

manganese dioxide

MnO2—flexible colorant—with alkaline fluxes gives purple and red colors—by itself gives soft yellow-brown—with cobalt gives black. Used with iron to color basalt bodies. Concentrations of more than 5% may promote blistering. Toxic in inhalation and ingestion. Fumes from firing are especially toxic. Source: Clay: A Studio Handbook

manometer

Primitive gauge for measuring water column inches, consisting of a U-shaped glass tube partially filled with water, with the pressure source connected to one end. See water column inches. Source: Clay: A Studio Handbook

marble; chalk; whiting; calcium carbonate; limestone

CaCO3—alkaline earth, contributing calcium oxide to glaze—powerful AT flux—major HT flux for glazes—gives strong durable glass. Sometimes used in low-fire claybodies to extend firing range and give greater fired strength. Source: Clay: A Studio Handbook

matt glaze

Glaze featuring a dull, nonglossy surface. Source: Clay: A Studio Handbook

maturity; maturing point

Firing point at which a claybody or glaze reaches its desired condition of color, hardness, density, etc. Source: Clay: A Studio Handbook

memory

During drying and firing of clay, the phenomenon whereby a clay piece will “remember” the way it was formed and will often shrink specifically according to the forming method. If not accommodated, memory can aggravate a variety of drying and firing faults. Source: Clay: A Studio Handbook

mica

K2O×3Al2O2×6SiO2—an aluminosilicate with a fine sheet-lattice structure, closely related to clay and feldspar, and often found as minute iridescent flakes in some clays. Source: Clay: A Studio Handbook

microcrystalline

Crystal effects dependent on tiny crystals in the glaze surface, as compared to macrocrystalline effects. Source: Clay: A Studio Handbook

mid-range

Glaze-firing range usually including cone 4 to cone 7, very popular with electric kilns. Source: Clay: A Studio Handbook

mishima

East Asian method of creating an inlaid effect by applying contrasting slip into a design incised in leather-hard clay. When the slip stiffens, the excess is scraped off. Source: Clay: A Studio Handbook

modifiers

Glaze materials that are added to a base glaze to modify surface qualities such as color, gloss, mattness, opacity, crystal development, etc. Source: Clay: A Studio Handbook

mold-release compounds

In making plaster molds, compounds that are applied to all surfaces except damp clay, in order to prevent plaster from sticking. Liquid hand soap works great. Paint it on and let it dry before pouring plaster. Never use oil-base release compounds on plaster molds for press-molding or slip-casting. Source: Clay: A Studio Handbook

molecule

A group of atoms chemically bonded together to form a compound. Source: Clay: A Studio Handbook

montmorillonite

Clays such as bentonite, resulting from the breakdown of airborne volcanic ash. The finest particle size of all clays. Very high shrinkage, and generally used only as an additive to claybodies or glazes. See bentonite. Source: Clay: A Studio Handbook

mouse-hole

In some wood kilns, a small air-port and passage that admits air directly beneath the coal bed. Source: Clay: A Studio Handbook

MSDS

MATERIAL SAFETY DATA SHEET, available from your supplier, giving toxicity information on each of the ceramic materials you use. Source: Clay: A Studio Handbook

muffle kiln

A fuel-burning kiln in which flames and combustion gases are ducted through sealed refractory flues within the ware chamber, heating the wares while maintaining a clean oxidizing atmosphere. Source: Clay: A Studio Handbook

mullite

3Al2O3×2SiO2—calcined Kyanite, but is also found naturally. Very desirable interlocking needlelike aluminum silicate crystals form above 1800° F and greatly add to the strength of high-fired vitrified claybodies and glazes. In well-formulated high-fired wares, clay-glaze interface is extremely strong partially due to interlocking mullite crystals. Source: Clay: A Studio Handbook

multifired

Wares where more than one glaze firing is required to achieve particular surface effects, as in lusters and china paints. Source: Clay: A Studio Handbook

natural draft

Firing system in a fuel-burning kiln that uses natural convection currents in kiln (updraft) or in chimney (downdraft), without any mechanical blowers, to draw in secondary air, circulate heat and atmosphere through kiln, and exhaust combustion gases. Source: Clay: A Studio Handbook

NC-4 feldspar

Na2O×Al2O3×6SiO2—soda feldspar, interchangeable with Kona F-4. Toxic in inhalation. Source: Clay: A Studio Handbook

necking-in; collaring

Process of reducing the upper diameter of a thrown form by working the walls of the rotating form inwards with fingers or rib, as in a bottle shape. Source: Clay: A Studio Handbook

nepheline syenite

K2O×3Na2O×4AlO3×9SiO2—a common feldspathic flux, high in both soda and potash, used in claybodies and glazes. Less silica than soda feldspars, and therefore more powerful. Increases firing range of low-fire and mid-range glazes. Toxic in inhalation. Source: Clay: A Studio Handbook

Neuman red clay

Refractory red stoneware clay, used to obtain rich red and red-brown colors in high-fire claybodies. Source: Clay: A Studio Handbook

neutral atmosphere

n fuel-burning kiln, atmosphere that is neither oxidizing nor reducing—most efficient atmosphere for efficient climb and even temperature. Source: Clay: A Studio Handbook

neutrals

In glaze chemistry, the refractories or stabilizers that are combined with bases (fluxes) and acids (glass-formers). Source: Clay: A Studio Handbook

nickel carbonate, green

Ni2CO3—weaker nickel colorant, reduces to green nickel oxide in the firing. Toxic in ingestion and inhalation. Source: Clay: A Studio Handbook

nickel oxide, black

Ni2O3—reduces to green nickel oxide early in firing, and produces similar effects. Toxic in ingestion and inhalation. Source: Clay: A Studio Handbook

nickel oxide, green

NiO—colorant or modifier—can give blues, tan, browns, greens, grays, dependent on fluxes present. Often used to mute the effects of cobalt, copper, and other colorants. Toxic in ingestion and inhalation. Source: Clay: A Studio Handbook

noborigama

Translates as “step-climbing kiln”—traditional Japanese multichamber downdraft/crossdraft climbing kiln, with initial firebox at bottom, and secondary fireboxes in each chamber. As kiln is fired, each chamber preheats the next. See climbing kiln. Source: Clay: A Studio Handbook

nuka

An often iridescent milky white Japanese glaze containing rice-hull ash. Source: Clay: A Studio Handbook

Ocmulgee

Rich golden-brown stoneware clay, no longer being mined. See Lizella clay. Source: Clay: A Studio Handbook

OM-4 (old mine #4)

A well known Kentucky ball clay. Source: Clay: A Studio Handbook

opacifier

In glaze formulation, a material that produces inert inclusions or minute crystals in glaze, causing it to become opaque. Most common are tin oxide and zirconium silicate. Source: Clay: A Studio Handbook

Opax

Zircon opacifier. See ziconium silicate. Toxic in inhalation. Source: Clay: A Studio Handbook

orifice

In a gas or oil burner, the restricted opening through which a jet of fuel emerges. Source: Clay: A Studio Handbook

outgassing

The escape of gases from clay and glazes during firing. Carbonates, sulfates, nitrates, organic contaminants, and chemically combined water volatize between 600 and 1200° F. Other compounds, especially fluxes, volatize later during glaze-melt. See carbon coring, oxidation cleanup, water-smoking stage. Source: Clay: A Studio Handbook

overglaze

Any surface decoration applied over the glaze surface, either as an oxide wash applied over raw glaze surface before glaze-firing, or as a lower-temperature medium fired onto a previously higher-fired glaze surface, as in china paints and lusters. Source: Clay: A Studio Handbook

overspray

In spraying glazes or other mediums, the small droplets and/or dust that do not settle on the object being sprayed. An adequate spray booth is essential in order to exhaust all overspray. Source: Clay: A Studio Handbook

Oxford feldspar

Potash feldspar no longer mined. Substitute Custer or G-200. Source: Clay: A Studio Handbook

oxidation

Any chemical reaction in which atoms or molecules combine with oxygen atoms. The combustion of fuel is an oxidation reaction. Source: Clay: A Studio Handbook

oxidation cleanup; oxidation soak

After glaze reduction, a short period of oxidation to allow outgassing to stop and glazes to heal. Source: Clay: A Studio Handbook

oxidation firing

Any kiln atmosphere with an abundance of oxygen to combust the fuel and oxidize the ceramic materials. Includes all electric firings and any gas firing with adequate air to insure complete combustion of the fuel close to the burner. Source: Clay: A Studio Handbook

oxide

A molecule combining any element with oxygen. Source: Clay: A Studio Handbook

oxide stain

A mixture of coloring oxide and water, sometimes including a little flux, used as an overall patina (often on unglazed work) or for overglaze brushwork. See patina. Source: Clay: A Studio Handbook

oxidize

To subject a material to a high-oxygen atmosphere, encouraging oxidation reactions. Source: Clay: A Studio Handbook

paddle and anvil

Forming technique where a soft leather-hard form may be shaped and thinned with a wood or bisqued clay paddle on the outside and a rounded “anvil” of wood, stone, or bisqued clay applying corresponding pressure on the inside. Source: Clay: A Studio Handbook

paddling

Technique of shaping a soft or medium leather-hard piece by gently hitting with a wooden paddle (sometimes textured) to create flat facets or to resolve irregularities in the surface. Source: Clay: A Studio Handbook

paper clay

Technique popularized by Rosette Gault, utilizing a claybody or slip containing paper pulp, which reduces shrinkage in drying stage, and encourages extremely strong joinery, allowing unconventional joinery such as wet to dry. Source: Clay: A Studio Handbook

paper resist

Decoration technique where strips of moist or adhesive paper are adhered to the surface to resist application of slip or glaze. Source: Clay: A Studio Handbook

partial reduction; climbing reduction

In a fuel kiln, atmosphere that is slightly reducing, but still allows increase in temperature. See reduction firing. Source: Clay: A Studio Handbook

pate-sur-pate

Paste-on-paste—decoration in which sculptural relief decoration is built up with contrasting slip on surface of leather-hard ware. Source: Clay: A Studio Handbook

patina

An overall thin wash of glaze or oxide stain, allowing the color and texture of the claybody to show through. Source: Clay: A Studio Handbook

pearl ash; potassium carbonate

K2CO3-HT alkaline potash flux, but soluble. Occasionally used as AT color modifier to soften effects of coloring oxides. Source: Clay: A Studio Handbook

peephole; spyhole

Small holes in kiln door or wall, allowing viewing of cones, flames, wares, and/or atmosphere during firing. Source: Clay: A Studio Handbook

periodic kiln; intermittent kiln

A kiln that is loaded cold, brought to temperature, cooled and unloaded. See continuous kiln. Source: Clay: A Studio Handbook

periodic table of the elements

A table listing all of the chemical elements (those substances that can theoretically be reduced to individual atoms), with the smallest and simplest across the top and the heaviest and most complex at the bottom and grouped by similar physical/chemical characteristics. Source: Clay: A Studio Handbook

perlite

Porous expanded granular silica, useful in making insulating refractories. Source: Clay: A Studio Handbook

petalite

Li2O×Al2O3×8SiO2—lithium feldspar—HT alkaline flux—good for reducing thermal expansion, increasing thermal-shock resistance. Toxic in inhalation. Source: Clay: A Studio Handbook

petuntse

Feldspathic rock, similar to Cornwall stone, found in China. Traditional Chinese flux for HT glazes and claybodies. Source: Clay: A Studio Handbook

pinching

Handbuilding method where clay objects are formed by pinching repeatedly between thumb and fingers or between fingers of one hand and palm of opposing hand. Source: Clay: A Studio Handbook

pinholing

Glaze defect characterized by fine pinholes in the surface—often caused by pinholes already present in dry unfired glaze coating. Can also be caused by burst bubbles in glaze surface that are not given opportunity to “heal” at end of firing. Source: Clay: A Studio Handbook

Pioneer kaolin

Al2O3×2SiO2×2H2O—plastic secondary kaolin—wider particle distribution gives greater plasticity, green strength. Source: Clay: A Studio Handbook

pit-firing

A type of bonfire-firing where wares are buried in sawdust in a pit in the ground and a bonfire is built on top, so that the fire and coals slowly burn away the sawdust and fire the wares. Not to be confused with sawdust smoking. Source: Clay: A Studio Handbook

plastic vitrox

K2O/Na2O/CaO×Al2O3×10SiO2—a plastic high-potash feldspathic clay, similar in structure to Cornwall stone—used in place of potash feldspar in some porcelain bodies to increase plasticity. Toxic in inhalation. Source: Clay: A Studio Handbook

plasticity

Quality of moldable flexibility in damp clay—superior plasticity depends on smaller clay particle size, slight acidity, less nonplastic additives, aging of damp claybody, adequate water content, and/or addition of accessory plasticizers, such as Veegum T or Macaloid. Source: Clay: A Studio Handbook

plasticizers

Materials added to some claybodies, especially those high in kaolins, to increase plasticity and dry strength—includes bentonite, Macaloid, Veegum-T. Source: Clay: A Studio Handbook

platelets

Flat, thin crystals that make up clay. When wet they become sticky and slippery, creating the phenomenon we call plasticity. Source: Clay: A Studio Handbook

polishing

As compared to burnishing, the act of creating a shiny surface on terra sigillata (or any clay or slip) by rubbing with soft cloth, a soft brush, or a piece of plastic film. Source: Clay: A Studio Handbook

porcelain

High-fired vitreous claybody containing kaolin, silica, fluxes, and often ball clay to increase plasticity, with total clay component not more than 50%. Usually pure white or “eggshell” in color; some porcelains may fire translucent where thin. Source: Clay: A Studio Handbook

porcelaneous

White-firing stoneware claybodies closely related to porcelain. Source: Clay: A Studio Handbook

Portland cement

Calcium silicate aluminate—air-setting cement, often added in small quantities to homemade castable refractor mix in order to introduce air-setting qualities. Toxic in inhalation. Source: Clay: A Studio Handbook

post-firing smoking

See raku. Source: Clay: A Studio Handbook

potash feldspar

See G-200; Custer feldspar. Source: Clay: A Studio Handbook

potassium carbonate; pearl ash

K2CO3-HT alkaline potash flux, but soluble. Occasionally used as AT color modifier to soften effects of coloring oxides. Source: Clay: A Studio Handbook

potassium dichromate

K2Cr2O7—Occasionally used as acid-green colorant in raku glazes, but is highly toxic in absorption, ingestion, and inhalation. Source: Clay: A Studio Handbook

pottery wheel; potter’s wheel

Machine with a horizontally spinning wheel head upon which potters form and trim clay vessels by hand. Source: Clay: A Studio Handbook

potter’s finger; jug finger

A long wooden tool with a knob on the end, often covered with chamois, which is inserted in a bottle or jug on the wheel after the top has been necked in and is used to broaden and refine the shape. Source: Clay: A Studio Handbook

power burner

Any burner in which a mechanical blower or other pressurized air source entrains the primary air for combustion. Source: Clay: A Studio Handbook

press-mold

A mold, usually plaster, into which moist clay is pressed to crease multiples. Source: Clay: A Studio Handbook

pressing; press molding

Forming of multiples by pressing plastic clay into a plaster press-mold. Source: Clay: A Studio Handbook

primary air

In a fuel-burning kiln, air that feeds initial combustion; in a wood-burning kiln, air that feeds base of flames; in a gas kiln air that enters back end of burner tube in an atmospheric burner, or via blower in a power burner. See secondary air. Source: Clay: A Studio Handbook

primary clays; residual clays

Clays that remains at the site of the parent rock, without being transported by wind or water—tend to be free of contaminants but have coarse particle size and therefore low plasticity. Includes the purest kaolins. Source: Clay: A Studio Handbook

pugmill

A machine similar to an oversized meat grinder, used to homogenize plastic claybodies. Deairing pugmills have a vacuum pump attachment, which effectively removes all air from clay, eliminating need for hand wedging. See carpal tunnel syndrome. Source: Clay: A Studio Handbook

pumice; volcanic ash

Fine volcanic particulate expelled from surface vents in a volcanic eruption. HT alkaline flux, similar in composition to potash feldspar, but higher in silica, with at least 1% iron. May be substituted for 7 parts potash spar, 3 parts flint. Toxic in inhalation. Source: Clay: A Studio Handbook

Pyrax HS

A manufactured pyrophyllite product, promotes plasticity more than other pyrophyllite products. Product of the H.T. Vanderbilt Company. Source: Clay: A Studio Handbook

pyrometer

Temperature gauge connected to a thermocouple, which indicates temperature within kiln. Pyrometers provide good general reference, but only respond to temperature, whereas clay and glazes (and cones) are affected by temperature, duration, and atmosphere of firing. See pyrometric cones. Source: Clay: A Studio Handbook

pyrometric cones

Small slender pyramidal-shaped indicators made of ceramic material formulated to bend at a specific temperature—standard method for determining maturing temperature of firing. Like clay and glazes, cones respond to temperature, duration, and atmosphere of firing far more accurately than mechanical measurement. Source: Clay: A Studio Handbook

pyrophyllite

Al2O3×4SiO2×H2O—used in HT claybodies (20% or less) to reduce thermal expansion, increase thermal shock resistance, reduce shrinkage, give stronger vitreous bodies. May reduce plasticity. Source: Clay: A Studio Handbook

pyroplastic

Flexibility of clay or glaze when heated sufficiently, as a result of developing glassy-phase. Source: Clay: A Studio Handbook

Pyrotrol

A manufactured pyrophyllite product, made by the Resco Company. See Pyrophylite. Source: Clay: A Studio Handbook

Glossary

Glossary

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