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Glossary
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Ultrox
Zircon opacifier. See zirconium silicate. Toxic in inhalation.
undercut
Common flaw in plaster or bisque molds, where the clay or casting catches and will not pull free without breaking or distorting.
underglaze; engobe
Colored slips formulated to have low drying shrinkage, allowing application to bone-dry or bisque-fired surface before glazing. Commercial underglazes are available in a wide palette of colors primarily for low-fire, but many will survive high-fire.
underglaze decoration
Process of applying any decoration to the bare, (usually bisque-fired) clay surface directly before glazing.
unity formula; seger formula
A formula generated by glaze calculation, which lists all glaze oxides present in separate columns for fluxes, refractories, and glass-formers.
vanadium pentoxide
V2O5—weak yellow colorant—toxic, expensive—usually fritted with tin to produce stronger yellow. Highly toxic in inhalation and ingestion.
vapor glazing
Any glaze process where the glaze results from vapor deposited within the kiln—includes salt glazing, soda glazing, fuming and wood firing (where fly ash is deposited on the ware and melted into a glaze).
Veegum Cer
Combination of Veegum T and carboxymethylcellulose gum, used as a suspension and adhesion agent in glazes. Product of H.T. Vanderbilt Company.
Veegum T
Suspension agent/plasticizer similar to bentonite and macaloid. Up to 2% of dry materials weight as plasticizer in high-kaolin claybodies, and 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. Product of H.T. Vanderbilt Company.
venturi burner
Natural-draft gas burner featuring streamlined restriction in burner tube, which increases entrainment of primary air and the efficiency of gas-air mixing.
vermiculite
A porous expanded mica product used as filler in clay bodies and in insulating refractory layer on kiln exterior.
viscosity; viscous
Reference to a material’s resistance to flowing. A viscous glaze flows less. As glass is heated it slowly becomes less viscous over a broad temperature range.
vitreous; vitrified; vitrification
Fired clay that has fused together completely, so that the pores between refractory particles are filled with glass and the body is impervious to water. Vitrification is sintering in the presence of a fully developed glassy-phase.
vitreous engobe
An engobe containing sufficient flux to form to a vitreous clay coating.
volatization; volatize
Change from solid or liquid to gas during firing, resulting in outgassing from the clay or glaze. See outgassing, blistering.
volcanic ash; pumice
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.
W.C.I.; water column inches
Standard low-pressure measure of gas pressure, based on pressure required to lift a column of water. Household natural gas pressure is usually 7″ or 7 W.C.I. Industrial natural gas pressure may be 12″ to 15″ or more. Propane household systems usually carry 11″ of pressure, but propane kiln systems may be equipped with adjustable regulators [...]
wadding; wads
Small balls or rolls of refractory clay mixture (40 alumina, 10 ball clay, 50 kaolin) placed under wares and posts, and between pots, lids, etc., in vapor-glazing and wood firing processes, to keep surfaces from sticking together.
warping
Distortion of clay forms caused by uneven stresses within clay due to forming method, uneven drying, uneven support in firing, or uneven or excessive heat in firing.
waster
European term referring to a fired piece discarded due to damage in firing.
water-smoking stage
The stage during bisque-firing or single-firing, below 1200°F, when carbonates, nitrates, sulfates, organic contaminants, and chemically combined water volatize and outgas. See carbon coring, outgassing.
water of plasticity
Amount of water required to bring a dry clay to its state of ideal plasticity. Common clay test used to determine this amount. The more water needed, the finer the particle size, the more plastic the clay, and the greater the drying shrinkage.
wax resist
Melted wax or wax emulsion used to prevent slip or glaze from adhering to a clay surface, either in decorating, or in preparing work for glazing.
wedge brick
Bricks with angled side faces, tapering along the length of the brick, which when laid together form a curved arch. Standard wedge bricks give 9″ thick arch.
wedging
Process of kneading the clay with the hands to remove air bubbles and ensure homogenous mass.
wheel-wedging
Working the clay up and down in a cone shape on the wheel to align the platelates in a spiral formation and thus increase control in centering and throwing.
white lead; lead carbonate
2(PbCO3)×Pb(OH)2—former source of lead for glazes. Highly toxic in absorption, inhalation, and ingestion. Remove from studio and dispose of in a responsible fashion.
whiteware
Low-temperature white claybody.
whiting; calcium carbonate; limestone; marble; chalk
CaCO3—alkaline earth, contributing calcium oxide to glaze—powerful all-temperature flux—major high-temperature flux for glazes—gives strong durable glass. Sometimes used in low-fire claybodies to extend firing range and give greater fired strength.
wollastonite; calcium silicate
CaSiO3—used in partial replacement of silica and whiting in high-temperature bodies, improves thermal shock resistance. In some cases, it is used in place of whiting to eliminate L.O.I. Toxic in inhalation.
XX-Sagger
A plastic refractory stoneware clay, often used in sculpture and raku bodies.
yellow ochre
High-iron yellow clay mineral, used as colorant in glazes and slips, converts to red iron oxide in oxidation or black iron oxide in reduction and/or high-fire.
zinc oxide
ZnO—High-temperature flux that promotes brilliant glossy surfaces. In some glazes can encourage opacity. With titanium in low-alumina glaze can encourage macrocrystalline growth (crystalline glazes). Volatizes in high-fire reduction. Toxic in inhalation.
zirconium oxide
Common opacifier, often used in place of the more expensive tin oxide. Sold as Opax, Superpax, Ultrox, etc., use 7% in a slip and 10 to 12% in a clear glaze to get pure opaque white. Zirconium particles are extremely refractory and remain as inert inclusions in the glaze-melt. They will not affect mattness or [...]
zirconium silicate
ZrSiO4—zircon opacifier—inert dispersoid in glaze melt—low-cost substitute for tin oxide—use double the recipe weight of tin. Includes Zircopax, Opax, Superpax, Ultrox. All are toxic in inhalation.
Zircopax
Zircon opacifier, no longer being manufactured. See zirconium silicate. Toxic in inhalation.
zone of crystallization
Period during cooling ramp when a particular material tends to crystallize out of glaze-melt.
