13. Glaze oxides

This list of glaze materials only includes materials that we can expect to obtain easily. It also lists mineral sources for the oxides, which may be helpful when consulting with geologists. The formula suggestions are only meant as a rough guide for those who work with formulas.

The following list can be used as a reference when developing or modifying glazes. If you have a problem with pinholes, then go through the list noting down all materials that are mentioned as having high viscosity or high surface tension or the opposite. Comparing your glaze recipe with your notes will give ideas of what materials to increase or decrease.

"MP" means "melting point".

ALUMINIUM OXIDE, ALUMINA, Al2O3 (Stabilizer), MP 2050°C

Source:

- Aluminum oxide, alumina, (Al₂O₃)
- Clay (Al₂O₃ · 2SiO₂ · 2H₂O)
- Feldspar(K20/N20/CaO · Al2O3 · 6SiO2)

Mineral sources:

Kaolin, ball clay, bentonite, corundum, bauxite, silimanite, kyanite, gibbsite (hydrargillite), websterite (aluminite), alunogen.

Effect:

- Increases melting point, hardness, viscosity, surface tension.
- Reduces tendency of crystal formation.
- Reduces thermal expansion.
- Small additions help other opacifiers.
- Large amounts produce matt glazes.

Clay addition normally 5-15%. Clay helps to keep glaze materials suspended in the bucket. Large additions cause problems of cracking of raw glaze layer and crawling, pinholing.

Formula:

-	Ratio in shiny glazes,
	Alumina: Silica = 1 :6-1 :10
-	Ratio in matt glazes,
	Alumina: Silica= 1:4-1:2

Color effect:
- Increase of alumina makes red and yellow-brown iron oxide color more brownish.
- MnO colors turn brown and less violet.
- CoO colors turn darker.

BARIUM OXIDE, BARIA, BaO (flux), MP 1923°C

Source:

- Barium carbonate (BaCO₃). Poisonous if it enters the blood.
- Barium sulfate (BaSO₄)
- Selenite (BaO, SeO₂)

Mineral sources:
Witherite, barytes, celsian, bromlite, barytocalcite.

Effect:

- BaO reduces boron's tendency to form opaque "clouds" and therefore helps to make boron glaze transparent.
- Reduces chemical resistance.
- High amounts (above 25%) produce matt glaze due to formation of crystals. BaO matt glazes are not stable.
- Lowers melting point.
- Slow in giving off CO₂. Sometimes sulfate problems in coal-or oil-fired kilns.
- Helps formation of crystalline glazes.
- Improves hardness.
- Small amounts improve gloss.

Formula:

- Generally, below 1100°C BaO should be less than 0.10 mole.
- Above 0.3 mole BaO raises melting point of glaze.

Color effect:

- CoO colors turn more violet, Cr₂O₃ below 1% turns more yellow.
- CuO colors turn from green to blue-green.
- Iron colors are subdued.
- NiO colors turn more brownish.

BORIC OXIDE, B₂O₃ (stabilizer or glass former), MP 741°C

Source:

- Borax (Na₂B₄O₇ ·10H₂O)
- Boric acid (B₂O₃ · 3H₂O)
- Both materials are soluble in water and they are normally introduced in a frit.
- Colemanite, gerstley borate (2CaO · 3B₂O₃ · 5H₂O). The only insoluble mineral form of borax, only mined in the USA.
- Calcium borate (CaO · B₂O₃ · 6H₂O₂), the chemical form of colemanite.

Boric oxide is sometimes classified as a stabilizer (USA) and sometimes as a glass former (UK).

Mineral sources:
Borax (tincal), kernite, ulexite, colemanite, boracite, sassolin.

Effect:

- Strongly lowers melting point. Mainly used below 1100°C.
- Improves formation of an intermediate layer between glaze and body.
- Boric oxide below 15% reduces tendency to craze, higher amounts increase crazing.
- Lowers viscosity and surface tension.
- Low thermal expansion rate.
- B₂O₃ less than 10% lowers surface tension.
- High content of boric oxide forms opaque clouds especially in combinations with CaO and SnO₂. This is reduced by addition of BaO or SrCO₃.
- Extends the firing range.
- Reduces tendency to crystallize.

Formula:

- Boric oxide ratio to silica is normally 1:10 and should not be less than 1:2. In frits a ratio below 1:2 will leave the frit water-soluble.

Color effect:

- MnO colors turn a violet hue.
- Iron colors become yellowish-reddish.
- CoO colors become brighter.
- CuO colors change from green to bluish green.

Sometimes a small percentage of raw borax is added to glaze or to engobe. When the glaze layer dries, the borax recrystallizes and this gives strength to the raw glaze layer which means it will not be damaged during handling.

CALCIUM OXIDE, CALCIA, CaO (flux), MP 2570°C

Source:

- Calcium carbonate (CaCO3), limestone, whiting, marble.
- Wollastonite (CaO SiO2).
- Dolomite (CaCO3 · MgCO3).
- Anorthite, lime feldspar (CaO · Al2O3 · 2SiO2).
- Calcium sulfate (CaSO4), plaster of parts.
- Calcium borate (2CaO · 3B2O3 · 5H2O).
- Calcium fluoride (CaF2).
- Calcium phosphate (Ca3(PO4)2) (bone ash).

Mineral sources:

Glauberite, fluorspar, apatite, lime, calcite, chalk, limestone, marble, gypsum, alabaster, seashells, coral, portland cement.

Effect:

- Combines readily with silica in glaze and, if CaO is present in body, it reacts with SiO₂ in glaze to form a strong interface, reducing crazing.
- Increases hardness, especially with boron glazes.
- Reduces tendency to craze.
- Primary flux for temperatures above 1100°C.
- Below 1100°C small additions act as secondary flux.
- High CaO produces opacity in boron glazes, and white matt wax-like glazes can be produced.
- Too high CaO gives dull, matt finish.
- CaCO₃ gives off CO₂ at 825° C.
-In zircon white glaze CaO increases pinholes and a dull surface.
-Decreases lead solubility.

Formula:

- At cone 03 CaO not above 0.25-0.28 mole
- At cone 01 CaO not above 0.30-0.35 mole

Color effect:

- CaO turns Cr₂O₃ colors yellow.
- MnO browns and violets are improved with CaO.
- CaO is important for production of iron-red, chrome-green and blue color pigments.

LEAD OXIDE, PbO (flux), MP 888°C

Source:

- Litharge (PLO)
- Red lead (Pb3O4)
- White lead, lead carbonate (2PbCO3Pb(OH)2)

Lead is a very good flux but it is very poisonous and expensive. It should never be used in ware that will contain food, but still is used frequently for decorative ware. If you use lead, it should always be in frit form.

Mineral sources:

Cerussite (white lead), galena (blue lead), minium (red lead), anglesite, pyromorphite.

Effect:

- Smooth, shiny low-temperature glazes.
- Strong flux.
- Good for transparent glazes.
- Reduces viscosity and surface tension.
- Reduces hardness and chemical resistance.
- Evaporates easily during firing.
- Combined with boric oxide, it is a common flux for earthenware glazes.
- It is more dangerous with copper oxide, which increases lead release 10 times.
- Small amounts in high temperature increase smoothness.

Formula:
Simple lead-alumina-silicate combinations make glazes in the following ratios3:

PbO · 0.10 Al2O3 · 1.0 SiO2	900°C
PbO · 0.11 Al2O3 · 1.1 SiO2	920°C
PbO · 0.12 Al2O3 · 1.2 SiO2	940°C
PbO · 0.13 Al2O3 · 1.3 SiO2	960°C
PbO · 0.14 Al2O3 · 1.4 SiO2	980°C
PbO · 0.15 Al2O3 · 1.5 SiO2	1000°C
....
PbO · 0.25 Al2O3 · 2.5 SiO2	1200°C

Color effect:

- Good with almost all colorants.
- Lead transparent glazes produce pleasant colors for engobe decorations.
- With iron, rich tans, browns, reds.
- With copper, rich greens (caution: lead release is increased 10 times).
- With antimony oxide, yellow.

LITHIUM OXIDE, Li2O (flux), MP above 618°C

Source:

- Lepidolite (lithium mica) 1.5%-6% lithium oxide.
- Petalite (Li₂O · Al₂O₃ · SiO₂), 2%-4% lithium oxide.
- Spodumene (Li₂O · Al₂O₃ · 4SiO₂), about 8% lithium oxide.
- Lithium carbonate (Li₂CO₃).
- A number of artificial lithium chemicals exist.
- High price.

Effect:

- A strong flux
- Lowers viscosity.
- Improves hardness.
- Improves gloss.
- High Li₂O content furthers formation of crystals in the melted glaze.
- Already 1% additions of Li₂CO₃ improve gloss and smoothness of glaze.

Color effect:

- CuO turns to blue colors.
- In lithium glaze 1% SnO₂ + 0.5% CuO produces Chinese reds in reduction firings.

MAGNESIUM OXIDE, MAGNESIA, MgO (flux), MP 2800°C

Source:

- Talc (3MgO · 4SiO₂ · H₂O).
- Magnesite (magnesium carbonate)(MgCO₃)
- Dolomite (CaCO₃ · MgCO₃)

Mineral sources:
Soapstone or steatite, serpentine, meerschaum, vermiculite, periclase magnesia, magnesite, brucite.

Effect:

- Raises melting point.
- High surface tension.
- Reduces crazing due to its low thermal expansion.
- Small amounts increase gloss.
- Larger amounts make matt glaze (best above 1100°C).
- With double glazing, good for special-effect crawling glaze.

Formula:

- Below 1100°C, less than 0.1 mole MgO increases gloss and 0.2 -0.4 mole MgO produces matt glazes.

Color effect:

- CoO blue turns violet with MgO.
- MgO glaze on red iron rich body turns the red color to a dirty yellow-brown color. Therefore transparent glaze should contain no MgO.
- Cr₂O₃ green only accepts small amounts of MgO. Large amounts bleach the green color.

PHOSPHORUS OXIDE, P2O5 (glass former), MP 569°C

Source:

- Bone ash, calcium phosphate (Ca₃(PO₄)₂)
- Apatite,3Ca₃(PO₄)₂Ca(Cl F)₂.

Mineral sources:
Bone ash (made from calcining animal bones), apatite, wavellite, vivianite.

Effect:

- P₂O₅ can replace some of the SiO₂ in the glaze.
- Strong flux, especially with MgO, BaO and alkalis.
- Additions above 5% form opaque glaze, especially in combination with ZnO and in lead-free glazes.
- Additions of up to 4% may increase melting and reduce pinholes. However, bone ash often increases pinholes due to high release of gas (instead add the bone ash to the frit).
- High additions (above 10%) produce matt glaze.
- Additions above 25%-30% make the glaze too soluble (less acid-or weather-resistant).

Color effect:

- CoO blue turns more violet.
- In B₂O₃ glazes iron colors turn yellowish.
- In alkaline glazes iron colors turn white with high amount of P₂O₅.
- CuO greens turn bluish and with high P₂O₅ spotted.
- MnO colors turn more violet.
- Cr₂O₃ colors are improved to lighter shades.
- Interesting special surface effects with high P₂O₅.

POTASSIUM OXIDE, POTASH, K2O (flux), MP 896°C

Source:

- Potassium carbonate, potash (pearl ash) (K₂CO₃), water-soluble.
- Potassium nitrate, saltpeter (KNO3), water-soluble -also used as fertilizer.
- Potash feldspar (K₂O · Al₂O₃ ·6SiO₂), exists as minerals named orthoclase and microcline, melting at 1200°C.
- Nepheline syenite (3Na₂O · K₂O · 4Al₂O · 8SiO₂).

Mineral sources:
Saltpeter, potassium bichromate, leucite

Effect:

- Potash's effect is very similar to soda, but it is a slightly less powerful flux.
- Potash increases crazing, but a little less than soda does.

SILICON OXIDE, SILICA, SiO2 (glass former), MP 1710°C

Source:

- Quartz, SiO₂
- Clay, Al₂O₃ · 2SiO₂ · 2H₂O
- Feldspar, Na₂O/K₂O/CaO · Al₂O₃ · 6SiO₂
- Talc, 3MgO · 4SiO₂ · H₂O
- Zirconium silicate, ZrSiO₄
- Wollastonite, CaO · SiO₂

Mineral sources:

Flint, chalcedony, chert, sand, quartzite, diatomite, granite, part of all rocks.

Effect:

- A glass former, a part of all glazes.
- Generally raises melting temperature.
- Low thermal expansion, addition reduces crazing.
- Addition to body also reduces crazing (see glaze faults).
- Increases viscosity of glaze melt.
- Increases acid and weather resistance.
- Increases hardness of glaze.
- High amounts make the glaze shiver.

Formula:

- Addition of 0.1 mole SiO₂ increases melting point by 20°C.
- Amount of SiO₂ depends on other glass-forming oxides. In general, earthenware 1-2.5 mole SiO₂ and stoneware 1-4 mole SiO₂.

Color effects:

- SiO2 has little influence on effect of coloring oxides.

SODIUM OXIDE, SODA, Na2O (flux), MP about 800°C

Source:

- Sodium carbonate (Na2CO3) as crystal soda or calcined soda -also named soda ash, soluble in water, absorbs moisture from the air.
- Sodium nitrate (NaNO3). Sodium saltpeter (Chile saltpeter), soluble in water.
- Sodium chloride (NaCl). Table salt, water-soluble, used in salt glazing, used in frit for reducing discoloration of frit by iron compounds.
- Soda feldspar or albite (Na₂O · Al₂O · 6SiO₂), a white mineral melting at 1170°C.
- Nepheline syenite, (K₂O · 3Na₂O · 4Al₂O₃ · 8SiO₂), mineral melting at 1100°-1200°C.

Mineral sources:

Natron, halite, hauynite, plagioclase,oligo-clase, sodalite, glauberite, cryolite, glauber salt.

Effect:

- Strong fluxing agent.
- Improves gloss.
- Very high thermal expansion induces crazing.
- Lowers elasticity of glaze, which becomes brittle with high amount of Na2O.
- Low viscosity, causes glaze to run. Short melting range.
- Evaporates easily above 1100°C (salt glazing).

Color effect:

- High amount of Na₂O or K₂O produces "alkaline colors", noted for their brightness and interesting shades.
- Copper oxide turns blue instead of green.
- Manganese oxide turns violet.
- Cobalt gives a light blue.
- Iron oxide produces red in connection with boron.

Formula:

- In alkaline frits I mole alkali with at least 2.5 mole SiO₂, otherwise the alkalis Na₂O and K₂O will remain water-soluble.

TIN OXIDE, SnO₂ (glass former group), MP 1930°C

Source:

- Tin oxide, SnO₂ (artificial)

Mineral sources:

Cassiterite (tinstone), stannite, tin pyrites.

Effect:

- Opacifier with 5-10% addition, less efficient in alkali-rich glazes.
- Opacifying effect increases with CaO, TiO₂ and ZrO₂. Fine grinding improves opacifying effect.
- Increases viscosity and melting point.
- Increases hardness and acid resistance.
- Increases elasticity of glaze (reduces crazing)

Color effect:

- In leadless glaze turns CuO bluish.
- Produces pink in combination with Cr₂O₃ and CaO.
- Iron brown colors turn redder.
- Manganese brown turns more violet.
- Used for stabilizing colors in pigment production.

TITANIUM DIOXIDE, TiO₂ (glass former group), MP 1855°C

Source:

- Titanium dioxide, titania, TiO₂ (artificial)
- Rutile, TiO2 (85-98% TiO₂)
- Perovskite, CaO · TiO₂
- Titanite, CaO · TiO₂ SiO₂
- Ilmenite, FeO · TiO₂

Mineral sources:

Rutile, anatase, brookite, titanite (sphere), ilmenite.

Effect:

- Opacifier but not so reliable. Opacity improves with addition of ZnO and CaO.
- Above 10% TiO₂, glaze turns matt due to forming of small crystals if cooling is slow. Mattness depends very much on firing conditions.
- Reduces crazing.
- Increases acid resistance.
- Reduces lead solubility when introduced in small amounts.
- Used for crystal glazes in combination with ZnO.

Color effect:

- Pure TiO₂ produces white colors in alkali-rich, lead-free glazes.
- In lead glazes and high boron glazes with small amounts of iron oxide a slight yellow color is obtained.
- Rutile contains some iron. The pure TiO₂ will work as rutile with an addition of about 5% iron oxide.
- On iron-rich bodies (red firing) TiO₂ combines with the iron of the body to form yellow-brown colors.
- TiO₂ addition turns CoO blue to gray-blue and with high CoO to green.
- Low CuO turns yellowish, high CuO bluish.
- Cr₂O₃ becomes dirty greyish.
- MnO₂ turns greyish
- NiO red and blue colors changed to green.

ZINC OXIDE, ZnO (flux), MP 1975°C

Source:

- Zinc oxide, zinc white, ZnO
- Zinc borate, ZnO · B₂O₃
- Zinc chloride, ZnCl₂
- Zinc phosphate, 3ZnO · P₂O₅

Mineral sources:

Sphalerite or blende (zinc sulfide), the original zinc ore, smithsonite, hydrozincite, willemite.

Effect:

- Above 1100°C a strong flux.
- In small amounts increases brilliance.
- High amounts produce matt glazes.
- Reduces viscosity, increases surface tension.
- Increases boron clouds and helps opacity in combination with other opacifiers.
- Reduces crazing due to its low thermal expansion and high elasticity.
- Its high drying shrinkage may cause crawling if added without prior calcination.
- In high amounts best agent for forming crystals.
- Produces special surface and color effect in high boron glazes.

Color effect:

- Generally increases brightness of colors.
- Chrome-green turns gray.
- Cobalt blue becomes lighter with less of a violet hue.
- Manganese violet turns brown.

ZIRCONIUM OXIDE, ZrO₂ (glass former group), MP 2700°C

Source:

- Zircon, zirconium silicate, ZrSiO₄
- Zirconium oxide, zirconia, ZrO₂
- Commercial opacifiers
- Commercial zircon frits.

Mineral sources:

Zircon is found in beach sands, baddeleyite (ZrO₂).

Effect:

- Zircon additions of 10-20% produce opaque white glaze (due to its high price zirconium oxide is seldom used).
- Used in combination with ZnO, MgO, BaO, SnO₂ opacity is increased.
- Opacity is furthered by fine grinding and by adding zircon to the frit instead of the batch.
- Increases melting point.
- Increases hardness, viscosity and surface tension.
- Increases tendency to form pinholes.
- Reduces crazing.