A potter’s A–Z of glaze and clay body materials. What each material is, what it does in a glaze or clay body, and how it behaves at different cones. Most entries link straight to the product so you can order it.
85 materials · 72 available to order online · all cone references are Orton cones
Jump to a material: Additive A · Albany Slip Substitute · Alumina Hydrate · Antimony Oxide · Ball Clay · Barium Carbonate · Bentonite · Bone Ash · Borax · Boric Acid · C & C Ball Clay · Calcined Alumina · Cedar Heights Bonding · Chrome Oxide · Cobalt Carbonate · Cobalt Oxide · Copper Carbonate · Copper Oxide · Cornwall Stone · Cryolite · Custer Potash Feldspar · Darvan #7 · Darvan #811 · Dolomite · Feldspar · Fire Clay · Flint · Frit · Gerstley Borate · Glaze Clay #1 · Glomax LL · Goldart · Grolleg · Hawthorn Bond · Helmer Kaolin · Ilmenite · Indiana Red 101 · Iron Chromate · Iron Oxide · Kaolin · Kaopaque 10S · Kyanite · Laguna Blackbird · Laguna Borate · Lithium Carbonate · Lizella Clay · Macaloid · Magnesium Carbonate · Manganese Carbonate · Manganese Dioxide · Mississippi M & D · Molochite · Mullite · Nepheline Syenite · Nickel Carbonate · Nickel Oxide · Pearl Ash · Petalite · Pioneer Kaolin · Potash Feldspar G-200 · Pyrax HS · Pyrax RG · Pyrophyllite · Pyrotrol W-200 · Redart · Rutile · Sheffield Clay · Silica · Silicon Carbide (Black) · Soda Ash · Tin Oxide · Titanium Dioxide · Umber (Burnt) · Vanadium Pentoxide · Vee Gum Cer · Vee Gum T · Velvacast · Volcanic Ash · Wax Resist · Whiting · Wollastonite · Yellow Ochre · Yellowbanks Buff 401 · Zinc Oxide · Zircopax
Safety: many raw materials, especially colorants, barium, lithium, antimony and silica dust, are hazardous to inhale or ingest. Wear a NIOSH-approved respirator when mixing dry materials and keep your studio clean. See our studio safety equipment.
A
Additive A
A clay conditioner that gives clay more plasticity, higher green strength and greatly improved workability throughout our range of pre-mixed clays. The most dramatic improvement is in porcelain, white stoneware and sculpture clays. It has been thoroughly tested and will not change the color or texture of fired clays, and it can be used in any type of clay mixing. It reduces mixer and pug mill power requirements by about 25%, saving cost and wear on your clay mixing machines.
Albany Slip Substitute
Sheffield Pottery developed this Albany slip substitute from a native glacial clay (see Sheffield Clay) blended with fluxing agents. It produces a rich brown, semi-gloss slip glaze at cone 8–10 that mimics Albany slip in both reduction and oxidation.
Alumina Hydrate
Al(OH)3 · m.p. 3722 °F / 2050 °C
Used primarily in glazes as a source of alumina. It is often favored over the oxide form (Calcined Alumina) because it promotes glaze adhesion and helps keep a glaze in suspension. Also used in salt glazing, bungs and kiln wash.
Shop: Alumina Hydrate →
Antimony Oxide
Sb2O3 · m.p. 1166 °F / 630 °C
Used in glazes as an opacifier and, more often, as a colorant (cone 06–1); cost limits its use. In the presence of lead or iron it produces yellow. It is slightly soluble and very toxic.
B
Ball Clay
So named because it was first sold in England in the shape of a ball. A fine-particle secondary clay whose chief mineral is kaolinite, with varying amounts of illite, montmorillonite, quartz and organic matter. It is low in iron and other impurities and fires light grey to buff. Used in clay bodies (rarely alone, because of high shrinkage) to promote plasticity and higher firing ranges (cone 5–13). It is also a source of alumina in high-temperature glazes (cone 8–13) and helps keep a glaze slip in suspension.
Shop: Kentucky OM-4 → · Tennessee #10 → · Old Hickory M-23 → · XX Sagger →
Barium Carbonate
BaCO3 · m.p. 2480 °F / 1360 °C
The usual source of barium in glazes. Its use is limited by its low reactivity and its toxicity. In low-fire glazes (cone 06–5) it promotes matte, sometimes dull, finishes; at high temperatures (cone 8–13) it is a powerful flux. In clay bodies it controls scumming by rendering soluble sulfates insoluble.
Shop: Barium Carbonate →
Bentonite
Al2O3·5SiO2·7H2O (approx.)
A decomposition product of volcanic ash, resulting in a super-fine (colloidal) clay; montmorillonite is the major mineral. Its particle size and mineral make-up give it a unique ability to add plasticity to clay bodies (typically under 4%) and to improve glaze suspension (typically under 2%).
Shop: Volclay 200 Mesh → · Volclay 325 Mesh → · HPM-20 → · Hectalite 200 (white) →
Bone Ash
3CaO·P2O5 · m.p. 2462 °F / 1350 °C
Natural: prepared by calcining selected animal bones (especially cow bones; some calcium carbonate contamination) and grinding to a set particle size. Phosphate tends to make a glaze opaque; too much can cause crawling or blistering. Used as a flux in higher-fired glazes (cone 8–13) and in porcelain bodies, where it is known to enhance translucency.
Synthetic: produced from non-bone calcium phosphate sources such as the mineral apatite, Ca5(PO4)3(OH,F).
Shop: Bone Ash, Synthetic →
Borax
Na2O·2B2O3·10H2O · m.p. 1366 °F / 741 °C
Also known as sodium tetraborate. Granular borax (5 and 10 mol) is coarse (99.9% under 2.4 mm) and readily soluble in water, so its unfritted use is limited. It supplies both sodium and boron and is a vigorous flux in low-fire glazes (cone 06–5). In small amounts (under 10%) in high-temperature glazes (cone 8–13) it increases fluidity, helping the glaze heal over scratches, cracks or pitting at maturity. Too much can cause pinholing and blistering.
Powdered borax behaves the same in a glaze, but dissolves much faster (minutes rather than hours).
Shop: Borax, Granular →
Boric Acid
H3BO3
Soluble, so its use in glazes is limited now that insoluble (fritted) boron sources are available. When it is the only flux in an alkaline glaze it is less likely to cause crazing than sodium fluxes. It can introduce boron into a clay body, though there is usually little advantage in doing so.
Shop: Boric Acid, Granular →
C
C & C Ball Clay
Mined in West Tennessee and produced by the H.C. Spinks Clay Company. A fine ball clay (69% under 1 micron) used mainly in clay bodies to add plasticity and improve strength. In a glaze it is a source of alumina and improves suspension (see Ball Clay).
Shop: C&C Ball Clay →
Calcined Alumina
Al2O3 · m.p. 3700 °F / 2040 °C
Produced by calcining alumina hydrate at 1200–1300 °C to convert it to alpha-alumina, the most stable form of aluminum oxide. In porcelain bodies (325 mesh) it is typically substituted for molochite or silica grogs. It reduces shrinkage, increases thixotropy (shear-thinning), minimizes firing warpage and adds fired strength.
Shop: Alumina, Calcined →
Cedar Heights Bonding
A fire clay mined in Ohio by the Cedar Heights Clay Company. Particle size can vary from lot to lot, so test each new lot in your clay body before production use (see Fire Clay).
Shop: Cedar Heights 50M Bonding →
Chrome Oxide
Cr2O3 · m.p. 4418 °F / 2270 °C
Used in small amounts (under 2%; it is very refractory) as a colorant for greens. Its effect is chameleon-like, depending on temperature and flux (KNaO, CaO, BaO or MgO). With tin it produces pink, and with zinc oxide, brown. Bright reds are possible in low-fire lead glazes that are low in alumina.
Shop: Chrome Oxide →
Cobalt Carbonate
CoCO3
Used as a colorant to produce blues (see Cobalt Oxide). Often preferred over the oxide for its fine particle size, which avoids the need for ball milling. Cobalt oxide is stronger, so use about two-thirds as much oxide in place of carbonate. With manganese, iron chromate or nickel it produces strong blacks.
Shop: Cobalt Carbonate →
Cobalt Oxide
Co3O4
The major oxide for producing blues (see Cobalt Carbonate). It gives comparable blues across almost all firing ranges and methods. A very strong colorant (typically under 2%). Ball mill glazes that use it if you don't want mottling. To substitute cobalt carbonate, use about 1.5 times as much.
Shop: Cobalt Oxide →
Copper Carbonate
CuCO3
The most common source of copper oxide, used at 2–5% as a major colorant for greens, turquoise and copper reds. Preferred over the oxide for its finer particle size, which favors turquoise in oxidation and copper red in reduction in alkaline glazes (see Copper Oxide). Copper is volatile above cone 8 and may require a sagger to protect other ware and the kiln.
Shop: Copper Carbonate →
Copper Oxide
Black: CuO · Red: Cu2O
The two stable forms of copper oxide, both coarser than copper carbonate. Black copper oxide (2–5%) is the more common in glazes: a strong flux and colorant producing greens in oxidation and reds in reduction. Red copper oxide can be precipitated during reduction firing as part of a copper red finish. Other colors include turquoise in soda-based glazes, yellow-greens in potash glazes and blue-greens in barium glazes. To substitute copper carbonate for black copper oxide, use about 1.4 times as much.
Shop: Copper Oxide Black → · Copper Oxide Red →
Cornwall Stone
Also called Cornish stone, from Cornwall, England. A feldspathoid used frequently in clay bodies and glazes (feldspathoids are chemically similar to feldspars but contain only about two-thirds as much silica, so they are richer in alkalis). Its distinctive feature is a roughly equal molecular balance of CaO, K2O and Na2O. In a clay body it lowers the firing temperature; in a glaze it helps minimize defects such as crawling and crazing (see Feldspar).
Shop: Cornwall Stone (H&G) →
Cryolite
Na3AlF6 · m.p. 1828 °F / 998 °C
Found naturally in Greenland. Used in glazes as a source of sodium or aluminum, but its fluorine content can make a glaze boil as it melts, and the escaping fluorine gas can cause blisters and pinholes. If the glaze is fluid enough, a half-hour soak can help heal them. The same effect is useful for crater glazes and crackle effects in alkaline glazes.
Shop: Cryolite →
Custer Potash Feldspar
Mined in Custer, South Dakota, by the Pacer Corporation. Used in glazes and clay bodies as a source of K2O, with about 10% potash and 2–3% soda. Custer is no longer produced; G-200EU potash feldspar is a common replacement with modest recipe adjustments (see Feldspar).
Shop: G-200EU Potash Feldspar →
D
Darvan #7
A high-molecular-weight, long-chain sodium polyelectrolyte made by the R.T. Vanderbilt Company. A general-purpose deflocculant (dispersing agent) for casting slips and glaze slips.
Shop: Darvan 7 →
Darvan #811
Similar to Darvan #7 and recommended for red casting slips.
Shop: Darvan 811 →
Dolomite
CaMg(CO3)2
An inexpensive source of magnesia in glazes. A double carbonate of calcium and magnesium; true dolomite is typically 56% CaCO3 and 44% MgCO3 by weight, with common impurities of alumina, iron and silica. In clay bodies it promotes longer, lower firing ranges (below cone 9).
Shop: Dolomite →
F
Feldspar
Feldspars are the main source of alkali flux (potash, soda) in high-fire glazes and the flux that vitrifies stoneware and porcelain bodies. See Potash Feldspar, Custer, Cornwall Stone and Nepheline Syenite.
Shop: G-200EU Potash → · Minspar 200 (soda) →
Fire Clay
A refractory clay that withstands high temperatures, used to add tooth, refractoriness and character to stoneware, sculpture and raku bodies, and in kiln and oven building. See Cedar Heights Bonding, Goldart and Hawthorn Bond.
Shop: Hawthorn Bond 35M → · Cedar Heights 50M Bonding →
Flint
SiO2 · m.p. 3119 °F / 1715 °C
Strictly, flint is the dark, porous variety of chert containing organic matter (chert is a hard, dense, dull to semi-glossy sedimentary rock of microcrystalline quartz). True flint is very fine, hydrated silica (about 1% bound water) and is less dense than quartz sand because of its fine porous structure. The name is a carry-over from England and France, where true flints were calcined and ground. Today potters use “flint” to mean ground silica (see Silica).
Shop: Flint 200 Mesh → · Flint 325 Mesh → · Flint 400 Mesh →
Frit
Made by combining water-soluble fluxes (alkali carbonates, nitrates, borates, etc.) with silica, then melting them into a glass and grinding it to a set particle size, which makes the fluxes insoluble. Harmful compounds such as lead are also fritted to reduce their toxicity. Frits are used in glazes and clay bodies wherever soluble or toxic materials need to be introduced safely.
Shop: Frit 3110 → · Frit 3124 → · Frit 3134 → · Frit 3195 → · Frit 3269 →
G
Gerstley Borate
2CaO·3B2O3·5H2O (approx.) · m.p. 1225 °F / 660 °C
A hydrated calcium borate with some sodium (under 5%). It is only slightly soluble, making it one of the few quasi-insoluble boron sources besides frits. Widely used as a low-temperature flux, it produces leadless low-fire glazes that craze less than other alkaline types, and it tends to give glazes a milky blue opalescence. Colemanite can replace roughly half the Gerstley Borate in a glaze. Gillespie Borate is a modern substitute.
Shop: Gillespie Borate →
Glaze Clay #1
A ball clay mined in Kentucky by the Old Hickory Clay Company. A fine ball clay (about 55% under 1 micron) used in clay bodies to add plasticity and strength. In a glaze it is a source of alumina and improves suspension (see Ball Clay). Recommended by Ferro for fritted glazes.
Shop: Old Hickory #1 Glaze Clay →
Glomax LL
Al2O3·2SiO2
A non-plastic calcined kaolin (60% under 2 microns) from Dry Branch, Georgia. Used in clay bodies to control firing cracks and shrinkage, and in high-temperature glazes (cone 8–13) as a source of alumina (see Kaolin).
Shop: Burgess Iceberg Calcined Kaolin →
Goldart
A ceramic-grade stoneware clay (95% under 74 microns) mined in Ohio by the Cedar Heights Clay Company. Used in clay bodies to increase refractoriness. Fires cream at cone 2 and buff at cone 8 (see Fire Clay).
Shop: Goldart →
Grolleg
A water-washed primary kaolin from Cornwall, England. Formed directly from altered granite, it is higher in alkali (potassium) than the secondary kaolins of Georgia. A coarse-particle kaolin used mainly in porcelain bodies, where its alkali content enhances translucency and whiteness (see Kaolin).
Shop: English Grolleg Kaolin →
H
Hawthorn Bond
A fire clay used in clay bodies fired from cone 1 to 14. It fires from cream to brown depending on temperature (see Fire Clay).
Shop: 20 Mesh → · 35 Mesh → · 50 Mesh →
Helmer Kaolin
A white-burning fire clay resembling china clay, containing the clay minerals kaolinite and halloysite (m.p. 3100 °F / 1700 °C). Halloysite has the same formula as kaolinite, Al2O3·2SiO2·2H2O, but a different crystal structure; halloysite clays tend to be more plastic and fire denser than pure kaolinite clays. It enhances flashing (brown to red surface color on unglazed wood-fired ware, cone 8–14). It is dry-ground from raw ore, which contains isolated pockets of goethite (FeOOH), a dark iron mineral that is blended in rather than separated.
Shop: New Zealand Halloysite →
I
Ilmenite
FeTiO3 · m.p. 2489 °F / 1365 °C
Used in glazes as a colorant for tans and buffs, and in granular form to produce mottling. In clay bodies or engobes it promotes iron spotting under reduction glazes. Rutile can be substituted, but it contains less iron and colors much more weakly.
Indiana Red 101
A glacially derived, plastic, high-iron (about 7%) red-burning earthenware clay similar to Sheffield Clay. High in silica, it fires yellow-buff to brown at cone 05–2 and becomes vitreous and dense by about cone 02. Produced by Unimin Corporation.
Iron Chromate
FeO·Cr2O3
Used in glazes (typically under 2%) for a range of colors: with tin, pink to red; with zinc, brown; in alkaline glazes, grey; and in potash and spodumene glazes (cone 5–12), mottled greys. In engobes it also gives grey.
Shop: Iron Chromate →
Iron Oxide
m.p. 2818 °F / 1548 °C
Red (Fe2O3): the most common form of iron used as a colorant in glazes and clay bodies, usually for tans and browns. Above about 4% it acts as a flux and makes the glaze more fluid.
Black (FeO / Fe3O4): behaves much like red iron oxide, and is preferred by many potters for celadons fired in reduction.
Yellow #5060: a manufactured product that gives consistent results from year to year.
Shop: Spanish Red → · Black → · Yellow →
K
Kaolin
Al2O3·2SiO2·2H2O · m.p. 3150 °F / 1760 °C
Also called china clay. Found in both primary and secondary deposits, with kaolinite as the major mineral. Kaolins are relatively coarse and much less plastic than ball clays. In glazes, kaolin is a major source of alumina, and large amounts can produce a matte surface. It is the principal ingredient of porcelain and, being low in iron, makes white stoneware bodies over a wide range (cone 5–16).
Shop: Pioneer (EPK sub) → · Grolleg → · Standard Porcelain → · Treviscoe → · Remblend →
Kaopaque 10S
A water-washed, delaminated secondary kaolin from Dry Branch, Georgia. Its large platelets have an aspect ratio (diameter to thickness) of about 11:1, compared with 6:1 for natural kaolin, and a median particle size of 2 microns. Used in extrusion bodies where fired whiteness, brightness and translucency are wanted (see Kaolin).
Kyanite
Al2O3·SiO2 · m.p. 3300 °F / 1800 °C
A non-plastic aluminosilicate, rarely used in glazes. Added to clay bodies to offset shrinkage (it expands when fired) and improve strength. Its main use is in high-temperature refractories such as kiln bricks and furniture (see Mullite).
Shop: 35 Mesh → · 200 Mesh → · 325 Mesh →
L
Laguna Blackbird
A substitute for Blackbird (also known as Barnard) clay, a naturally occurring high-manganese, high-iron clay-like mineral. Made by Laguna Clay Company of California and used mainly as a source of iron in dark glazes.
Shop: Barnard/Blackbird Substitute →
Laguna Borate
Developed by Laguna Clay Company as a direct substitute for Gerstley Borate. Its main ingredient is calcium borate, blended with additional fluxes.
Lithium Carbonate
Li2CO3 · m.p. 1330 °F / 720 °C
The most common source of lithium. A major flux for mid- and high-temperature (cone 5–13) alkaline glazes. It is less soluble than other alkaline compounds and gives more durable glazes. As a secondary flux it can lower the maturing temperature of a stoneware glaze by 2–3 cones. Powdered and granular forms differ in particle size and reactivity; the coarser granular form suits higher temperatures, where its lower reactivity can be overcome.
Shop: Lithium Carbonate →
Lizella Clay
About 3.9% iron. A direct substitute for Ocmulgee clay, which is no longer available. This 16-mesh clay has better quality control than Ocmulgee, so it has fewer flaws. Maximum temperature is cone 9 with very little reduction. Also available screened to 30 mesh from Sheffield Pottery.
Shop: Lizella Clay, Screened →
M
Macaloid
The same as Vee Gum T.
Shop: Vee Gum T →
Magnesium Carbonate
MgCO3
A source of MgO in glazes. In cone 5–10 glazes it can develop the buttery, satin-matte surface favored for functional ware. At high temperatures (cone 9–16) it behaves somewhat like alumina, raising glaze viscosity and reducing flow, which is useful in glazes such as crystallines where alumina is not wanted.
Shop: Magnesium Carbonate →
Manganese Carbonate
MnCO3
See Manganese Dioxide. Per unit weight, the carbonate is a weaker colorant than the dioxide; use about 1.3 times as much carbonate in place of dioxide.
Shop: Manganese Carbonate →
Manganese Dioxide
MnO2
A source of MnO in glazes and clay bodies, used to develop purples, browns and blacks. It is a vigorous flux at cone 5–10 in glazes and bodies (keep it under 5% in a body), and a strong oxidizing agent in glazes. The powdered and granular forms differ in particle size and reactivity; the granular form produces dark speckles in clay bodies and glazes.
Shop: Powdered → · Granular 60 Mesh →
Mississippi M & D
A super-fine ball clay (85% under 1 micron) mined in Sledge, Mississippi, by the Kentucky-Tennessee (KT) Clay Company. Used mainly in clay bodies (under 10%) to improve strength and plasticity; in excess it can cause high shrinkage and warping (see Ball Clay).
Molochite
3Al2O3·2SiO2 · m.p. 3150 °F / 1760 °C
High-purity kaolin calcined at high temperature to convert as much of the kaolinite as possible to mullite. Available in sizes from 8 to 325 mesh. Typically used as a clean, low-impurity grog in porcelain and white clay bodies to reduce drying shrinkage and improve thermal shock resistance.
Shop: 30 Mesh → · 50–80 Mesh → · 120 Mesh → · 200 Mesh →
Mullite
3Al2O3·2SiO2 · m.p. 3300 °F / 1800 °C
Commercial mullite is made by calcining kyanite; natural mullite is rare and is named for one of its few known deposits, on the Isle of Mull off the west coast of Scotland. Like kyanite it is a non-plastic aluminosilicate and rarely used in glazes. Added to clay bodies to stabilize shrinkage, improve strength and modify thermal expansion.
Shop: 35 Mesh → · 200 Mesh →
N
Nepheline Syenite
K2O·3Na2O·4Al2O3·9SiO2 · m.p. 2200 °F / 1200 °C
A feldspathoid used frequently in clay bodies and glazes. Relative to its silica content, it carries more K2O and Na2O than a typical soda or potash feldspar. In a clay body it lowers the maturing temperature; in a glaze it can reduce defects such as crazing and extend the working range of glazes from cone 011 to 5 (see Feldspar).
Shop: 270 Mesh (Minex 3) → · 400 Mesh (Minex 4) →
Nickel Carbonate
NiCO3
See Nickel Oxide.
Shop: Nickel Carbonate →
Nickel Oxide
Green (NiO): used in glazes (typically under 3%), most often in porcelain enamels, to produce blues, greys, browns and yellows depending on the balance of alkalis and alkaline earths. It can also soften cobalt and copper colors.
Black (Ni2O3): reduces to green nickel oxide at about 600 °C in a glaze and gives similar results, though more may be needed for the same effect.
Shop: Nickel Oxide Green → · Nickel Oxide Black →
P
Pearl Ash
K2CO3 · m.p. 1000 °F / 540 °C
Potassium carbonate. It absorbs moisture from the air and is extremely soluble. Where it is used in a glaze, its main effect as a low-temperature flux (cone 011–3) is to soften other colorants such as copper and cobalt.
Shop: Pearl Ash →
Petalite
Li2O·Al2O3·8SiO2 · m.p. 2550 °F / 1400 °C
A lithium source used mainly in clay bodies, particularly flameproof and refractory ware, to lower thermal expansion and increase thermal shock resistance.
Shop: Petalite →
Pioneer Kaolin
An air-floated, plastic secondary kaolin (60% under 2 microns) from Dry Branch, Georgia, noted for high green strength and white firing. Used in slip casting, ram pressing and extrusion bodies, and as a source of alumina in glazes. A good substitute for EPK.
Shop: Pioneer Georgia Kaolin →
Potash Feldspar G-200
Mined in Georgia by The Feldspar Corporation. Used in glazes and clay bodies as a source of K2O, with about 10% potash and 2–3% soda, and interchangeable with Custer feldspar with modest adjustments. The original G-200 has been replaced by G-200EU, which we stock (see Feldspar).
Shop: G-200EU Potash Feldspar →
Pyrax HS
m.p. 2790 °F / 1530 °C
A high-sericite pyrophyllite blend from the R.T. Vanderbilt Company (sericite is a fine-grained muscovite mica). It is about 35% foliated (platy) pyrophyllite, 35% quartz, 25% sericite and 5% kaolinite. Rarely used in glazes; mainly used in wall tile bodies to reduce abrasive wear on molds and dies. It also lowers maturing temperature and moisture expansion (improving craze resistance) and adds strength in vitreous bodies.
Pyrax RG
m.p. 2970 °F / 1635 °C
A refractory-grade, high-pyrophyllite blend from the R.T. Vanderbilt Company: about 45% foliated pyrophyllite, 45% quartz and 5% kaolinite. Rarely used in glazes; mainly used in insulating firebrick bodies. It can replace feldspar and part of the quartz to reduce thermal expansion in clay bodies (cone 5–14), and it helps stabilize shrinkage because it expands during firing. The expansion depends on particle size.
Pyrophyllite
Al2O3·4SiO2·H2O · m.p. over 3300 °F / 1800 °C
Used mainly in wall and floor tile bodies. It can also be used in ovenware bodies and to control shrinkage and firing cracks, but keep it under 15% of the body because it is non-plastic. Rarely used in glazes, though it can replace kaolin to reduce thermal expansion and crazing.
Shop: Pyrophyllite (Pyrax B) →
Pyrotrol W-200
m.p. 2970 °F / 1635 °C
A 200-mesh, refractory-grade andalusite (Al2SiO5) and pyrophyllite blend from the Resco Company: about 25% massive (blocky) pyrophyllite, 25% andalusite and 50% quartz. Rarely used in glazes; mainly used to control shrinkage in stoneware bodies (cone 5–14), since it expands during firing. The expansion depends on particle size. It is not a direct substitute for Pyrax RG because of their different minerals and particle shapes.
Alkatrols: made by adding sericite (a fine-grained muscovite) to andalusite–pyrophyllite blends to broaden the firing range. Typically added to whiteware bodies to improve whiteness, reduce warping and fired shrinkage, and improve thermal shock resistance. Variations differ in their andalusite : pyrophyllite : sericite ratios. Produced by Piedmont Minerals and the Resco Company.
Shop: Pyrotrol 2253 →
R
Redart
A fine-particle earthenware clay mined in Ohio by the Cedar Heights Clay Company, firing at cone 06–1. Used in earthenware bodies for the intense red it develops when fired.
Shop: Redart →
Rutile
TiO2 · m.p. 3452 °F / 1900 °C
An economical source of TiO2 where pure white is not needed. Used in clay bodies and glazes (typically 1–3%) for ivory through yellow to dark tan in oxidation, and purples and blues in reduction. Its most frequent use is to modify other colorants and create effects such as mottling and streaking, especially striking in Gerstley Borate and colemanite glazes.
Dark Milled: between ilmenite and rutile, with more iron. Light Ceramic: a fine true rutile with some iron. Granular: a coarse true rutile with some iron; it is less reactive than the Light Ceramic grade, so it is used to promote mottling at higher temperatures (cone 8–13).
Shop: Dark Milled → · Granular →
S
Sheffield Clay
Mined and processed by Sheffield Pottery in Sheffield, Massachusetts. A glacially derived, high-iron (about 8.5%), narrow-particle-size red-burning earthenware clay containing biotite, hornblende, chlorite and quartz. Use it in clay bodies (dense at cone 5) and in glazes; on its own it becomes a stoneware slip glaze at cone 11–12.
Shop: Sheffield Slip Clay → · Sheffield Raw Clay, Hammer-milled →
Silica
SiO2 · m.p. 3119 °F / 1715 °C
Second only to clay in importance as a ceramic material, and the most abundant mineral in the earth’s crust. The main siliceous materials are crystalline quartz, quartzite, sandstone, silica sand, and organic and amorphous silica such as flint and diatomaceous earth. Silica is often called “potter’s flint” and is often simply ground sandstone; the terms silica and quartz are used interchangeably.
In clay bodies, silica modifies shrinkage, porosity and strength, depending on particle size and crystallinity. Quartz undergoes a rapid phase change at 1063 °F / 573 °C (quartz inversion), with a volume change that can cause firing cracks in bodies high in free quartz. In glazes, silica is the most common glass former and controls fusibility and viscosity.
Common sizes are 200 mesh (under 74 microns), 325 mesh (under 45 microns) and 400 mesh (under about 38 microns). The coarser 200 mesh helps control shrinkage in bodies; the finer 325 and 400 mesh are more reactive and melt into glazes more readily.
Shop: 200 Mesh → · 325 Mesh → · 400 Mesh → · Silica Sand F-65 →
Silicon Carbide (Black)
SiC · m.p. 5100 °F / 2800 °C
Contains up to 10% free silica and is highly refractory. Used mainly in high-temperature kiln furniture (cone 5–16). Small amounts (0.5–1%) in alkaline glazes have been used to produce local-reduction copper reds in oxidation.
Shop: Silicon Carbide 500 →
Soda Ash
Na2CO3 · m.p. 1564 °F / 851 °C
Also called soda ash light; anhydrous sodium carbonate. A very active flux for low-temperature glazes (below cone 5). Because it is water soluble, it is usually used in fritted form to prevent it recrystallizing in the glaze. Glazes using raw soda ash should be well ground and used immediately, or dry-mixed and only the needed amount mixed with water. Small amounts are also used as a deflocculant in clay slips (see Darvan #7) to reduce the water needed for fluidity. Sodium bicarbonate can substitute as a deflocculant at about 1.6 times the amount.
Shop: Soda Ash →
T
Tin Oxide
SnO2 · m.p. 2066 °F / 1130 °C
Found in nature as the mineral cassiterite. The most effective opacifier at every temperature from terra cotta to porcelain. 4–5% is typical, though anywhere from 2% to 8% is used. Above 8%, calcine it first to prevent crawling. Zircon opacifiers can substitute, but more is needed (up to about 10%) for a similar result.
Shop: Tin Oxide →
Titanium Dioxide
TiO2 · m.p. 3360 °F / 1850 °C
Produced by digesting ilmenite in sulfuric acid and purifying the result. Used in glazes as an opacifier; it tends toward cream, where tin oxide gives white. It improves acid resistance and adds texture, such as mottling similar to rutile but without the iron. Light Ceramic rutile can substitute where iron is not a concern.
Shop: Titanium Dioxide →
U
Umber (Burnt)
An iron-bearing earth consisting of limonite and manganese dioxide. Used in engobes and clay bodies for red-brown color (see Iron Oxide).
Shop: Burnt Umber →
V
Vanadium Pentoxide
V2O5 · m.p. 1274 °F / 690 °C
Used in glazes as a colorant for yellow, greenish yellow and reddish brown; 5–10% gives yellow. Combined with titanium or tin it gives a range of yellows. Toxic; handle with care.
Vee Gum Cer
A Vanderbilt Company blend of Vee Gum T and a medium-viscosity sodium carboxymethylcellulose (CMC). Used in glazes to harden the unfired glaze surface, stabilize viscosity and improve suspension.
Vee Gum T
A super-fine (colloidal) purified bentonite (hydrated magnesium aluminosilicate) from the Vanderbilt Company. Like bentonite, it adds plasticity (in white bodies, since it is iron-free) and keeps glazes suspended (typically under 2%; see Bentonite).
Shop: Vee Gum T →
Velvacast
A water-washed, coarse-particle secondary kaolin (39% under 2 microns) from Dry Branch, Georgia. Used mainly for fast-casting slips, where it controls the rate of water removal to reduce warping, and for easy mold release (see Kaolin).
Shop: optiKasT (Velvacast substitute) →
Volcanic Ash
Pulverized pumicite (volcanic dust: minute fragments of volcanic glass). In a glaze it behaves like a silica-rich potash feldspar (roughly 70% feldspar / 30% flint). Its high iron content limits it to buff or dark stoneware bodies, where it produces vitreous bodies with reduced shrinkage and less warping over a long firing range (cone 5–9).
Shop: Volcanic Ash (Pumice) →
W
Wax Resist
A wax emulsion, oil- or water-based, applied to keep glaze off areas of a pot. Oil-based wax thickens as it ages; thin it by adding water very gradually (no more than ½ cup, since it is easy to add too much), and clean brushes with boiling water, steam or kerosene. Water-based emulsions are more stable and much easier to use and clean up.
Shop: Aftosa Wax Resist → · Wax Resist Potter’s Pen →
Whiting
CaCO3
Finely ground, essentially pure calcium carbonate from limestone, chalk or marble, and the most common source of calcium in glazes and clay bodies. In stoneware glazes (cone 7–12) it promotes a matte finish and makes the glaze harder and more chemically durable. In stoneware bodies (cone 7–12) it is a flux (typically over 3%) that increases shrinkage and strength by lowering the vitrification point and decreasing porosity. Snocal 40, from Hammill & Gillespie, comes from the famous Yorkshire, England chalk deposit. Vicron 2511, from Specialty Minerals, is ground from North American limestone for a narrow particle size distribution.
Shop: Snocal 40 (glazes) → · Vicron 2511 (clay & slip) →
Wollastonite
CaSiO3 · m.p. 2804 °F / 1540 °C
A naturally occurring calcium silicate, mined mainly in Willsboro, New York. It can replace silica and whiting in clay bodies and glazes. In stoneware bodies (cone 5–13) it promotes low moisture expansion and improves shrinkage, strength and thermal shock resistance. In a glaze it acts like a frit of CaCO3 and SiO2, giving smoother, brighter surfaces than whiting.
Shop: W-10 (200 Mesh) → · W-20 (325 Mesh) → · W-30 (400 Mesh) →
Y
Yellow Ochre
Fe2O3·H2O (limonite) · m.p. 2415 °F / 1325 °C
Common in earthenware clays, and one of the main yellow or buff colorants in raw clay; rarely used as a glaze colorant. When fired, its bound water is driven off, leaving red iron oxide, so it is a major source of red color in earthenware bodies. Used in engobes for ochre yellow, tans and browns. Natural yellow ochre is less expensive than manufactured yellow iron oxide.
Shop: Yellow Iron Oxide →
Yellowbanks Buff 401
A plastic, high-alumina stoneware clay with ball-clay-like plasticity, firing white to yellow-buff at cone 8–12. Produced by Unimin Corporation.
Z
Zinc Oxide
ZnO · m.p. 3587 °F / 1975 °C
Derived from zinc sulfide ores. An important part of many glazes (cone 5–13) as a flux, opacifier and color modifier. It lowers glaze expansion (second only to magnesia), improves craze resistance, gloss and whiteness, and widens the maturing range. In low-alumina crystalline glazes it promotes crystal growth.
Shop: Zinc Oxide →
Zircopax
ZrSiO4 · m.p. over 3600 °F / 2000 °C
A zircon opacifier for glazes and slips. Compared with finer grades such as Superpax, it is coarser and less reactive, so more is needed for the same opacity, though it costs less. Zircopax Plus is a higher-purity grade with a brighter white. Ultrox is a comparable zirconium opacifier that we stock (see also Tin Oxide).
Shop: Ultrox Zirconium Opacifier →
Looking for something not listed? Browse every raw material and ceramic chemical we carry, or call us. We’re happy to help with substitutions.
*The definitions in our dictionary were developed exclusively for Sheffield Pottery, Inc. by CeraSci Consultants. No duplication or reproduction is allowed without the consent of Sheffield Pottery, Inc. All cone references are to Orton cones.