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Glossary
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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.
macrocrystalline
Glaze effect featuring large patches of crystal development on glaze surface, as compared to microcrystalline effects. See crystaline glazes.
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.
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.
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.
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 the
Renaissance,
which is when the term [...]
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.
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.
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.
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.
matt glaze
Glaze featuring a dull, nonglossy surface.
maturity; maturing point
Firing point at which a claybody or glaze reaches its desired condition of color, hardness, density, etc.
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.
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.
microcrystalline
Crystal effects dependent on tiny crystals in the glaze surface, as compared to macrocrystalline effects.
mid-range
Glaze-firing range usually including cone 4 to cone 7, very popular with electric kilns.
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.
modifiers
Glaze materials that are added to a base glaze to modify surface
qualities such as color, gloss, mattness, opacity, crystal development,
etc.
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.
molecule
A group of atoms chemically bonded together to form a compound.
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.
mouse-hole
In some wood kilns, a small air-port and passage that admits air directly beneath the coal bed.
MSDS
MATERIAL SAFETY DATA SHEET, available from your supplier, giving toxicity information on each of the ceramic materials you use.
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.
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.
multifired
Wares where more than one glaze firing is required to achieve particular surface effects, as in lusters and china paints.
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.
NC-4 feldspar
Na2O×Al2O3×6SiO2—soda feldspar, interchangeable with Kona F-4. Toxic in inhalation.
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.
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.
Neuman red clay
Refractory red stoneware clay, used to obtain rich red and red-brown colors in high-fire claybodies.
neutral atmosphere
n fuel-burning kiln, atmosphere that is neither oxidizing nor reducing—most efficient atmosphere for efficient climb and even temperature.
neutrals
In glaze chemistry, the refractories or stabilizers that are combined with bases (fluxes) and acids (glass-formers).
nickel carbonate, green
Ni2CO3—weaker nickel colorant, reduces to green nickel oxide in the firing. Toxic in ingestion and inhalation.
nickel oxide, black
Ni2O3—reduces to green nickel oxide early in firing, and produces similar effects. Toxic in ingestion and inhalation.
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.
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.
nuka
An often iridescent milky white Japanese glaze containing rice-hull ash.
Ocmulgee
Rich golden-brown stoneware clay, no longer being mined. See Lizella clay.
OM-4 (old mine #4)
A well known Kentucky ball clay.
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.
Opax
Zircon opacifier. See ziconium silicate. Toxic in inhalation.
orifice
In a gas or oil burner, the restricted opening through which a jet of fuel emerges.
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.
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.
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.
Oxford feldspar
Potash feldspar no longer mined. Substitute Custer or G-200.
oxidation
Any chemical reaction in which atoms or molecules combine with oxygen atoms. The combustion of fuel is an oxidation reaction.
oxidation cleanup; oxidation soak
After glaze reduction, a short period of oxidation to allow outgassing to stop and glazes to heal.
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.
oxide
A molecule combining any element with oxygen.
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.
oxidize
To subject a material to a high-oxygen atmosphere, encouraging oxidation reactions.
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.
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.
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.
paper resist
Decoration technique where strips of moist or adhesive paper are adhered to the surface to resist application of slip or glaze.
partial reduction; climbing reduction
In a fuel kiln, atmosphere that is slightly reducing, but still allows increase in temperature. See reduction firing.
pate-sur-pate
Paste-on-paste—decoration in which sculptural relief decoration is built up with contrasting slip on surface of leather-hard ware.
patina
An overall thin wash of glaze or oxide stain, allowing the color and texture of the claybody to show through.
pearl ash; potassium carbonate
K2CO3-HT alkaline potash flux, but soluble. Occasionally used as AT color modifier to soften effects of coloring oxides.
peephole; spyhole
Small holes in kiln door or wall, allowing viewing of cones, flames, wares, and/or atmosphere during firing.
periodic kiln; intermittent kiln
A kiln that is loaded cold, brought to temperature, cooled and unloaded. See continuous kiln.
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.
perlite
Porous expanded granular silica, useful in making insulating refractories.
petalite
Li2O×Al2O3×8SiO2—lithium feldspar—HT alkaline flux—good for reducing thermal expansion, increasing thermal-shock resistance. Toxic in inhalation.
petuntse
Feldspathic rock, similar to Cornwall stone, found in China. Traditional Chinese flux for HT glazes and claybodies.
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.
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.
Pioneer kaolin
Al2O3×2SiO2×2H2O—plastic secondary kaolin—wider particle distribution gives greater plasticity, green strength.
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.
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.
plasticizers
Materials added to some claybodies, especially those high in kaolins, to increase plasticity and dry strength—includes bentonite, Macaloid, Veegum-T.
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.
platelets
Flat, thin crystals that make up clay. When wet they become sticky and slippery, creating the phenomenon we call plasticity.
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.
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.
porcelaneous
White-firing stoneware claybodies closely related to porcelain.
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.
post-firing smoking
See raku.
potash feldspar
See G-200; Custer feldspar.
potassium carbonate; pearl ash
K2CO3-HT alkaline potash flux, but soluble. Occasionally used as AT color modifier to soften effects of coloring oxides.
potassium dichromate
K2Cr2O7—Occasionally used as acid-green colorant in raku glazes, but is highly toxic in absorption, ingestion, and inhalation.
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.
pottery wheel; potter’s wheel
Machine with a horizontally spinning wheel head upon which potters form and trim clay vessels by hand.
power burner
Any burner in which a mechanical blower or other pressurized air source entrains the primary air for combustion.
press-mold
A mold, usually plaster, into which moist clay is pressed to crease multiples.
pressing; press molding
Forming of multiples by pressing plastic clay into a plaster press-mold.
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.
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.
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.
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.
Pyrax HS
A manufactured pyrophyllite product, promotes plasticity more than other pyrophyllite products. Product of the H.T. Vanderbilt Company.
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.
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.
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.
pyroplastic
Flexibility of clay or glaze when heated sufficiently, as a result of developing glassy-phase.
Pyrotrol
A manufactured pyrophyllite product, made by the Resco Company. See Pyrophylite.
