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Where there's smoke, there's fun. Get the low-down on low-firing.
Put your pots in the pits -- discover Low-firing and Burnishing.
Put your pots in the pits -- discover Low-firing and Burnishing.
Glossary
Search Glossary
quartz; silica; silicon dioxide; flint
SiO2 —The primary glass-former in clay and glazes—vitrification, fluidity, transparency/opacity controlled by adding fluxes and/or refractories. Highly toxic in inhalation.
quartz inversion
Abrupt expansion in heating and corresponding contraction in cooling that occurs in silica crystals in all clay and glazes at around 1063° F. Greatest risk is in cooling high-fired wares and in refiring previously high-fired wares.
raku (japanese)
Traditional raku or Japanese raku is a low-fired glazed pottery by a direct process that involved putting the pots into and removing them from a red-hot kiln. The potter Chojiro is credited with being the first to produce raku ware in 1580. The term raku is translated as “enjoyment” or “ease.” From The Potters Dictionary of Materials and Techniques, by Frank and Janet Hamer. See raku (Western).
raku (western)
Inspired by traditional Japanese raku firing, Western (typically
referred to as American) raku is a relatively low-temperature firing
process using clay that is either under fired or otherwise formulated
to withstand the thermal shock of being removed from a kiln at top
temperature. Work is removed from the kiln at bright red heat and
subjected to post-firing reduction (or [...]
ramp
Profile of the firing of a kiln, including speed, duration, soaking
periods, etc. of both the heating and cooling cycle, as in firing ramp
and cooling ramp.
raw glazing; green glazing
Glazing leather-hard or bone-dry wares for single-firing.
recrystallization
During initial cooling, the formation of microcrystalline or macrocrystalline structure in the glaze surface.
red-figure style
In ancient Greek ceramics,
Classical stage of vase painting, featuring figures in red against
black background, with highlights painted in black and white.
Redart
Brick-red earthenware clay, produced by Cedar Heights Clay Company.
red lead; lead oxide
Pb3O4—Former source of lead for glazes. Highly
toxic in absorption, inhalation, and ingestion. Remove from studio and
dispose of in a responsible fashion.
reduce
To induce reduction.
reducing agent
Material such as silicon carbide used in clay or glaze to create localized reducing conditions in an oxidation firing.
reduction
Chemical reaction in which oxygen atoms are removed from a compound.
reduction cooling
In a fuel kiln, maintaining
slight reduction atmosphere during cooling cycle, from maturing
temperature down to approximately 1400° F, in order to minimize
reoxidation of clay and/or glazes. See reoxidation.
reduction firing
In fuel-burning kilns, firing atmosphere with insufficient oxygen to
completely combust fuel, introducing abundance of unoxidized carbon and
hydrogen, which extract oxygen molecules from surface of wares,
altering appearance of clay and glaze. See climbing reduction.
refractories
Any materials highly resistant to the effects of heat. In kiln
construction, high-temperature insulating materials, such as firebrick,
ceramic fiber, etc. In clay and glaze chemistry, the neutrals or
stabilizers that are resistant to melting and that combine with the
fluxes (bases) and glass-formers (acids). Primary refractory material
in ceramics is alumina, most often introduced in the form of clay
(aluminum [...]
refractory
Capable of withstanding very high temperatures.
registration marks; keys
In plaster slip-casting molds, small rounded depressions carved in each
parting face of a mold segment, so that when adjacent segments are cast
a corresponding bump will form. In use, the keys allow the mold
segments to be aligned and assembled securely after the original
prototype is removed.
reoxidation
In a reduction firing, oxidation that occurs after fuel is shut off at end of firing.
residual ash
In a wood kiln, the surface effects created by fly-ash settling upon
the wares. Effect is maximized by turbulence in firebox, considerable
draft through kiln, and/or active stirring of the coal bed. In short
wood firings ash doesn’t have time to melt, and leaves a rough, crusty
surface.
residual salt/soda
In a well-used salt/soda kiln, the light salt/soda effect that may be
obtained by firing wares in the kiln without introducing any additional
salt or soda, due to the revolatization of salt/soda from the walls of
the kiln.
resist
Material used in glazing and decorating that can be applied to surface to prevent adhesion of slip or glaze.
rheostat
Variable electric switch often found on forced-air burners to control
blower speed. Small inexpensive rheostats used on most power burners
are imprecise, and adjustments should always be made on basis of flame
at burner tip, preferably with an air- shutter on the blower-intake for
fine adjustments.
rib
Wide, flat handheld tool used to shape, smooth, and/or scrape clay
surfaces; usually wood, rubber, plastic, or metal, either rigid or
flexible, with straight, curved, or profiled edge.
rolled edge
Beveled edge obtained by rolling the outer edge of the foot of a soft leather-hard pot at an angle against a hard flat surface.
Roman arch
Kiln arch that forms a perfect half circle, with the arch beginning and ending on a horizontal surface, using no skew bricks.
roulette; coggle
Small stamp wheel with raised pattern around the rim, which when rolled
along a plastic clay surface leaves a band of relief pattern. Usually
formed with damp or dry clay and bisque-fired.
rutile
Titanium ore, used as source of titanium dioxide, contains iron, other
trace minerals—gives tan color, promotes crystallization giving mottled
multicolor effects in some HT glazes, or in overglaze stain (very
refractory, use sparingly). Gives rich mottled medium blue in some HT
glazes. Dark rutile contains higher percentage of iron.
s-cracks
S-shaped cracks that occasionally appear in the bottoms of wheel-thrown pots, resulting from inadequate compression of the bottom and/or excessive water left in bottom. Occur most often in fine-grain gritless claybodies, especially thrown off the hump.
saggar
A refractory clay enclosure in which wares are fired. Originally developed to protect wares from ash-slagging and flame-flashing in wood firings. In contemporary use, with clean-burning gas firings, process used in exactly opposite way.
saggar firing
Contemporary firing process in which wares are placed within refractory saggars, along with chemicals and combustibles, in order to achieve certain surface effects.
salt; sodium chloride
NaCl—table salt, rock salt—used in salt-firing. Skin irritant.
salt firing; salt glazing
Vapor-glazing process where salt (sodium chloride) is introduced into kiln firebox at high temperature. Salt vaporizes, and sodium vapor combines with silica in clay surface, forming extremely hard sodium-silicate glaze. See soda firing.
sand
Granular silica. Major grit besides grog used to give clay bodies structure for throwing and hand building, high shrinkage in high-temperature clay bodies, but gives smoother fired surface than grog. Toxic in inhalation.
scoring
Process of incising surface of wet or leather-hard clay in crosshatch pattern before applying slurry and joining pieces.
secondary air
In fuel-burning kilns, air that enters after initial combustion and feeds flames as they leave primary combustion source. In gas kiln, secondary air enters burner port around burner tip. In wood kiln, secondary air enters above grates in conventional firebox, and below grates or hobs in Bourry-box. See primary air.
secondary clays; deposited clays; sedimentary clays
Clays that have been transported away from their point of geologic origins by wind or water. Finer particle-size gives greater plasticity—ball clays, stoneware clays, fireclays, etc.
secondary kaolins
Kaolins that have been transported some distance from the parent rock and are still very pure, but much more plastic than primary kaolins.
sedimentary clays; secondary clays; deposited clays
Clays that have been transported away from their point of geologic origins by wind or water. Finer particle-size gives greater plasticity—ball clays, stoneware clays, fireclays, etc.
seger formula; unity formula
A formula generated by glaze calculation, which lists all glaze oxides present in separate columns for fluxes, refractories, and glass-formers.
sequential firing
Firing in a sloped tube kiln or noborigama, where as each chamber or zone reaches maturity, the fire is moved up to the next, and each one preheats subsequent chambers or zones.
set
To place wares in a kiln. Or, in a loaded kiln, the entire structure of shelves, furniture, and wares.
Also, an aqueous solution’s ability to cling to a vertical surface without running, as in a wet, raw glaze on a pot.
setting
Process of loading a kiln, or technically, of placing the set. See set.
sgraffito
Decorating technique achieved by scratching or carving through a layer of slip or glaze (helps to apply wax-resist over glaze before carving) before firing to expose contrasting claybody beneath.
shard
A broken fragment of pottery.
shino
Classic Japanese glaze ranging from gray to white to orange, composed primarily of feldspar. The orange color is achieved with thinner glaze coating when fluxes in the glaze activate iron content in the claybody. In the West, shino glazes often contain spodumene or other source of lithium, and/or nepheline syenite. Many potters seek shinos that [...]
shivering
Serious and dangerous glaze defect where excessive glaze compression causes small razor-sharp chips of glaze to pop off along outer edges, corners, and rims. All wares showing shivering must be destroyed. The remedy is to slightly increase flux and/or decrease the silica in the glaze.
short
Clay with insufficient plasticity—tends to fragment during forming.
shrinkage
Permanent contraction of the clay in both drying and firing stages. Overall may be as much as 18%.
shuttle kiln
Kiln where kiln floor and often the door are mounted on a car that may be rolled in and out of kiln on tracks. Sometimes has two cars, with doors in both ends of the kiln. See car kiln.
sieve
A container with fine-mesh wire screen in the bottom, available in different mesh sizes, used for straining slips and glazes in order to achieve complete mixing of raw materials and removal of large particles or contaminants.
sieving
The process of working a glaze or slip through a wire-mesh sieve to strain out impurities and to break up clumped raw materials.
silica; silicon dioxide; flint; quartz
SiO2—The primary glass-former in clay and glazes, with vitrification, fluidity, transparency and opacity controlled by adding fluxes and/or refractory materials. Highly toxic in inhalation.
silicon carbide
SiC — Extremely refractory material used to form kiln shelves. Highly resistant to corrosive atmospheres, and therefore suitable for salt, soda, and wood firing. Silicon carbide kiln shelves conduct electricity and should never be used in electric kilns. Occasionally used in powdered form as local reducing agent in oxidation firings, used in very small quantities [...]
single-firing; green firing
Process of firing glazed greenware without a bisque firing.
sintering
In heating clays and glazes, a solid-state reaction where particles stick together permanently, and mass can be considered fired. With increasing temperature, fluxes and glass-formers begin to interact, increasing the strength of bond between refractory particles, still leaving open, porous structure, as in bisque-fired and low-fired wares, which are sintered but not vitrified.
skew bricks
Special angled bricks used to support the first course of arch bricks on either side of a sprung arch.
slabroller
A mechanized or manually operated device for rolling out large uniform slabs of clay.
slaking; slake down
The process of returning dry unfired clay to a slip by soaking in water.
slip
Clay suspended in water, usually the consistency of thick cream. May be colored and used to decorate surfaces, or may be cast into plaster molds to create ceramic forms.
slip-clay
A naturally occurring clay that contains a high enough component of flux to form a glaze at high-fire temperatures.
slip-glaze
A glaze with slip-clay or earthenware clay contributing the primary flux.
slip-resist
Decorating technique where resist materials are applied to prevent slip from adhering to some areas.
slip casting
The creation of ceramic forms by casting slip in plaster molds.
slip trailing
Application of decoration to wet or leather-hard clay by flowing on lines of slip with a fine pointed dispenser, such as a rubber syringe.
slumping
Process of creating ceramic wares with a slump-mold. Firing defect where glassy-phase begins to dissolve sintered structure in clay, causing it to sag and deform.
slump mold
A typically shallow frame or mold into which a slab of clay is allowed to fall or settle in order to form a vessel. These can be frames with no bottom.
soaking
During firing or cooling ramp, the act of holding kiln at steady temperature for a period of time to allow proper formation or maturation of certain clay and glaze effects.
soap
A specialized hardbrick shape, half the width of a standard brick. Often used for posts in salt and wood firings.
soapstone; talc; magnesium silicate; steatite
3MgO×4SiO2×H2O High-temperature alkaline earth flux in glaze that contibute magnesia and silica. Promotes smooth buttery surfaces, partial opacity. Similar composition to clay, but in low-temperature clay bodies gives low shrinkage and high thermal-shock resistance, as in standard 50/50 talc/ball clay whiteware body. Highly toxic in inhalation and ingestion.
soda ash; sodium carbonate
Na2CO3—soluble source of soda, used as deflocculant, or to supply soda in vapor-glazing process. Source of flux in Egyptian paste recipes. Brush solution over hi-fire glaze to increase carbon trapping. Highly toxic in inhalation and ingestion. Skin irritant.
soda feldspar
Na2O×Al2O3×6SiO2—feldspars contributing sodium (and potassium), primarily as a high-temperature flux. Kona F-4 feldspar, NC-4 feldspar and nepheline syenite. Toxic in inhalation.
soda firing; soda glaze
Vapor-glazing process touted as modern-day nontoxic replacement for salt firing. Gives slightly less gloss and orange-peel. Instead of salt, soda ash (sodium carbonate) in water solution is sprayed into kiln at maturing temperature, and sodium vapor combines with silica in clay to form sodium-silicate glaze. Recent research indicates that effluent gases are highly corrosive and [...]
sodium aluminum fluoride; cryolite
Na3×AlF6—small amounts promote crackle effects, larger amounts become very volatile with silica and may cause blistering. Used for special effect crater glazes. Toxic in inhalation.
sodium silicate; waterglass
Na2SiO3—comes as a liquid—used as a deflocculant in slips, as an air-setting binder for low-temperature refractories. Many clay artists use it in “magic water” in place of slurry for joining wet or soft leather-hard ceramic forms. Add 1.5% sodium silicate and 1.5% soda ash by weight to a measure of water. Toxic in ingestion. Skin [...]
sodium tetraborate; borax
Na2O×2B2O3×10H2O—a major LT alkaline flux, available in granular or powdered form. Gives smooth finish, bright colors. Water soluble, so is often used in fritted form. In excessive amounts creates brittle glass and can cause blistering and pinholing. Available in granular and powdered form. Sometimes used with salt or soda in vapor glazing to lower firing [...]
softbrick
See insulating firebrick.
soft paste
Traditional European term for a dense, white claybody that fires at lower temperature and was the precursor to true porcelain.
soluble salts
Range of soluble metallic salts like bismuth subnitrate, silver nitrate, copper sulfate, cobalt sulfate, ferric chloride, and stannous chloride, used to create fumed luster effects on glazed wares and to create surface effects on unglazed saggar-fired wares. Most are water soluble and highly toxic.
solution
A liquid mix where the components are fully dissolved (as in a water/soda ash solution for vapor-glazing), where materials will not settle out via gravity.
solvent
An agent that acts to accelerate the dissolution of a more resistant material. In glaze maturation, the flux-glass-former combination acts as a solvent on alumina.
spalling
Defect in repeatedly fired clay materials where outer layers shrink and peel away—most often occurs in kiln hot-face when refractories are repeatedly fired beyond their rated temperature.
specific gravity
The weight or density of a liquid measured in proportion to that of water. A glaze with a specific gravity of 1.2 is 1.2 times as dense as water per unit of measure. See hydrometer.
split
A specialized hardbrick, half the thickness of a standard brick.
spodumene
Li2O×Al2O3×4SiO2—lithium feldspar—powerful high-temperature alkaline flux. Promotes copper blues—good for thermal-shock bodies and matching glaze. Toxic in inhalation.
spooze
A repair medium for bone-dry greenware, made of equal parts vinegar and corn syrup mixed with dry powdered claybody to the desired consistency.
spray booth
Open-front enclosure with an exhaust fan at the rear, designed to draw off all overspray and other toxic dust or fumes.
spraying
Application of liquid slip, engobe, glaze, or stain, using mechanized spray equipment. Gives smooth glaze coating, but with less glaze pooling in recesses.
sprigging
Surface decorating technique in which small coils or balls of clay are affixed to the damp or leather-hard surface, usually with a layer of slip.
sprung arch
Kiln arch representing less than 180 degrees of curvature, and requiring buttressing to support outward thrust of the arch.
spyhole
See peephole.
stain
Ceramic colorants that have been fritted in order to eliminate solubility problems and give greater stability in firing and truer color before firing. Most are stable up to cone 5, many to cone 10. Can be mixed with 25—50% Ferro 3134 frit for Maiolica overglaze decoration. Most stains are ground glass and are highly toxic [...]
stannous chloride; tin chloride
Soluble metallic salt. Fuming agent to create mother-of-pearl luster on glazed surfaces. Introduced in cooling ramp at dull red heat, which will affect everything in kiln. Highly toxic in inhalation and ingestion. Fumes are especially toxic.
stilts
Term often applied to all kiln posts, but more correctly referring to specialized refractory furniture pieces equipped with ceramic or metallic points designed to support fully glazed wares during firing.
stoneware
High-fired vitreous ware, literally as hard and durable as stone. Matures from 2200-2400° F. (cone 5–11). Also naturally occurring refractory clays with adequate fluxes to fire in stoneware temperature range.
strontium carbonate
SrCO3 Alkaline earth, high-temperature flux, similar to barium, slightly more powerful. Gives semimatt surfaces. Nontoxic in balanced glaze. Substitute 75 parts strontium to one part barium.
super-cooled liquid
A material that, in its solid form, maintains the amorphous physical structure of a liquid rather than the ordered structure of a crystalline material. A crystalline material softens or solidifies at a specific freezing point, whereas a super-cooled liquid like glass remains amorphous, softening or solidifying gradually over a broad temperature range.
Superpax
Zircon opacifier. See zirconium silicate. Toxic in inhalation.
suspension
A liquid mix where insoluble particles are distributed throughout without dissolving and may settle out from gravity, as in a glaze or slip.
talc; magnesium silicate; steatite; soapstone
3MgO×4SiO2×H2O High-temperature 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.
temper; tempering materials; fillers
Gritty materials like sand, grog, volcanic ash, crushed seashells, etc., added to clay to open upthe body, give physical structure in wet-working and increase thermal shock resistance.
Tenmoku
Classic East Asian high-iron gloss glaze giving black where thick, breaking to brown or red-brown where thin.
terracotta
Low temperature, porous earthenware claybody, fires red-brown due to high iron content, which also fluxes clay, making it the most durable low-fired clay after firing.
terra sigillata
Ultrarefined clay slip that can give a soft sheen when applied to bone-dry wares and if polished or burnished while still damp may give a high gloss. All ancient Greek red-black pottery and Roman red wares were finished with this technique, without the use of glaze.
tessha
A Japanese glaze similar to kaki, in which re-oxidation and crystallization of iron in the glaze results in significant areas of purplish-red, often with some black or brown surface showing.
thermal expansion
The physical expansion and contraction that accompanies the heating and cooling of most materials. See coefficient of expansion.
thermal shock
Effect of sudden temperature changes during firing or during subsequent heating and cooling in daily use.
thermocouple
Temperature probe that produces minute variable electrical current dependent on degree of heat exposure—used in pyrometers and Baso valves.
thermoplastic
See Pyroplastic.
throwing stick
Wooden tool traditionally used by Japanese potters as an extension of the hand for raising the inside of pots, especially tall narrow forms.
Tile-6 kaolin
Al2O3×2SiO2×2H2O—air-floated secondary kaolin—broader particle distribution than primary kaolins, greater plasticity, green strength.
tin chloride; stannous chloride
Soluble metallic salt—fuming agent to create mother-of-pearl luster on glazed surfaces. Introduced in cooling ramp at dull red heat, it will affect everything in kiln. Highly toxic in inhalation and ingestion. Fumes are especially toxic.
tin oxide
SnO2—most powerful opacifier, but expensive—inert dispersoid in glaze melt—5–7% will produce opaque white in a clear glaze. Toxic in inhalation and ingestion.
titanium dioxide
TiO2—matting/opacifying agent. Promotes crystal growth, visual texture in glazes.
tombo
A t-shaped Japanese throwing gauge, used to measure the depth and rim diameter of a vessel, usually when throwing off the hump.
triaxial blend
Method for testing three-way combinations of glaze materials, where proportional amounts vary through a series of samples between three limits. May involve change in glaze materials, or addition of colorants or modifiers. See line blend.
trimming
At the leather-hard stage, removal of excess clay from a piece, using any of a variety of sharp cutting tools.
tube burner
A simple atmospheric burner with no venturi effect present.
tube kiln
Sequential-firing Korean kiln design evolving from Chinese bank kiln, with long sloping tubular firing chamber. See anagama.
