We hope that this information on the characteristics and properties of potters raw materials, frits and oxides is useful to you, especially the way that they interact with each other to create helpful and not so helpful results.

This information is provided in good faith, however like all ceramic calculations it needs to be viewed in perspective. Always remember that glazes are made of materials that have chemical, mineralogical and physical properties and you cannot ignore any of these or the way that all materials are affected by others present and many other factors such as age of material, time left standing, contamination from dust, refractories, the weather and of course Murphy’s law.  

See the downloadable pdf in the Downloads Section also for MSDS and Spec sheets

ALUMINA     Al2O3 Calcined and Al2O3.3H2O or Al2(OH)6 Hydrated aluminas are fine granular white powders that have good flow properties increasing glaze viscosity, firing range and resistance to crystallisation. This material has a very high melting temperature. It is important that the powder is very fine. The hydrated version of alumina stays in suspension better in glaze slurries and has better adhesive qualities than calcined. Also using hydrated alumina in glazes and glass can promote the fining operation of coalescing and removing finely dispersed gas bubbles. Small additions of fine alumina hydrate added to a glaze can also enhance the colour of Cr-Al pinks. Larger additions (15% of glaze) of fine material can impart matt and opaque effects if the glaze is able to take it into solution (sourcing alumina from kaolin and frits may be more practical). Also used in Batt Wash.

ANTIMONY     Sb2O3. Antimony oxide is used as an opacifier in porcelain enamel (mainly leadless but it has been replaced to an extent by titania) and ceramic glazes. However it can give a yellowish colour (1-2%) if the glaze contains lead, due to the precipitation of yellow lead antimonate (known as Naples yellow). Antimony is also used as a yellow body stain in combination with rutile or titanium. It is slightly fluxing in higher temperature glazes.  

BALL CLAY Fine secondary or tertiary clay mined in Devon and Dorset. Usually grey or blue in colour. It is very plastic and fuses relatively easily with greater density than china clay. Called ball clays because they used to be rolled into balls for loading onto transport. Not normally used on its own due to its high shrinkage - up to 17% so it is usually mixed with china clay

BALL CLAY - Blue - Puraflo AK    32% Al2O3, 50% SiO2, 1.1% Fe2O3, 2.1% K/Na2O. A Devon ball clay, very plastic, clean burning ball clay. recommended for glazes needing ball clay as an ingredient. 

BALL CLAY - Hymod AT     29% Al2O3, 55% SiO2, 2.3% Fe2O3, 3.6% K/Na2O3. A Dorset ball clay with a high iron content, high strength, useful in the production of warm coloured textured bodies at stoneware temperatures. At earthenware temperatures the colour tends to be deeper. 

BALL CLAY - Hyplas 71     20% Al2O3, 70% SiO2, 0.8% Fe2O3, 2.3% K/Na2O3. A high siliceous Devon ball clay giving good plasticity with medium strength and usefully low iron content.  

BARIUM CARBONATE     BaCO3. Used in casting slips at low levels to prevent scumming  (up to 0.5%). Used in high temperature glazes as a flux and produces matt and semi-matt surfaces at earthenware temperatures. At higher temperatures it can make a distinctive turquoise with copper. In bodies it seems to impart better translucency but can lead to weakness and excessive shrinkage.

BENTONITE     Al2O 2H2O SiO2. A clay mineral with incredibly small particle sizes. Must be added in the dry state. This, in combination with the active chemistry on the surface of the particles (that makes them hold onto water), makes it the most plastic and impermeable common clay material used in ceramics. Anyone who uses this material should have their eyes open to its advantages and disadvantages. Binder: Bentonite binds particles together in ceramic bodies to make them stronger in the green or dry state (up to 5%). Its minute particles fill voids between others to produce a more dense mass with more points of contact. Adding bentonite to glazes also imparts better dry strength and a harder and more durable surface. 1 part bentonite can plasticize a body as much as 10 parts kaolin. Bentonitic bodies are stronger in the dry form but dry slower, crack more and fire darker with potential iron specks (get a super fine ground grade). 1-2% bentonite can greatly improve glaze suspension by gelling it. In addition it will harden the dry layer. Coarser varieties can impart some glaze speck. If a glaze already contains more than 15% clay (kaolin, ball clay) you should not need more than 1% bentonite.  Firing cracks, explosions: Bentonite slows down water penetration. Not only does a bentonite-containing clay body dry slower but it does not dry as completely. Although ware might look dry it is not, several percent water tightly-bound between bentonite particles remains. If ware is not temperature-dried before being fired there is a risk that water will not be able to escape fast enough during firing and ware will crack, fracture under steam pressure. At stoneware temperatures it fuses to produce a typical brown colour.

BISMUTH SUBNITRATE     Obtained from metallic bismuth and is soluble in acids but insoluble in water. It gives a pearly luster to glazes and glasses, especially in reduction (and raku) firing. It is an ingredient in luster colours. It is a very very strong flux.

BONE ASH     CA3(PO4)2. Bone ash has traditionally been added to porcelain to achieve a high degree of translucency (hence the name 'bone china'). The manufacture of bone china is difficult to master because the clays are non-plastic, ware is unstable in the kiln, and it is difficult to burn consistently to the body's narrow firing range. Up to 1-2% bone ash can be used in enamels for opacification or milky character (more will usually cause pinholes). In glazes, as with enamels, too much or too high a temperature will cause blistering. Bone ash or calcium phosphate are used to opacify opal glass (1-3%) because the P2O5 content forms colourless compounds with iron impurities. Not suitable for slip casting.

CALCIUM BORATE FRIT     1030-1180oC. SiO2 17.9%, B2O3 50.3%, MgO 0.1%, Al2O3 4.9%, K2O 0.3%, CaO 26.5%. Insoluble colemanite frit for use where recipes specify greater than 5% colemanite. Can be used as a base in lead free glazes.

CALCIUM CHLORIDE     CaCl2 6H2O. Calcium Chloride is used as a suspending agent in glazes 0.05%. It works well with bentonite. Small additions (0.1-0.3%) to glaze slurries also produce gelling qualities that make it easier to apply coatings of even thickness. Vinegar has a similar effect although calcium chloride is said to work better. This material is the key to being able to apply a glaze to non-porous porcelain bisque ware.

CHINA CLAY     Al2O3.2SiO2.2H2O.  General purpose Kaolin. Because kaolinite mineral has a much larger particle size than ball clay and bentonite materials, blending it with them in bodies can produce a good cross section of ultimate particle sizes imparting enhanced working and drying properties. Kaoilns speed up casting rates in slurry bodies and drying rates in all bodies. Many porcelains contain only kaolin as their clay component but is has a relatively low plasticity and so is usually used (less than 50%) in combination with ball clays, bentonites and other plasticizers.  It is also the most common glaze suspender (15-20%). GROLLEG is lower in iron and a little stronger, so is whiter and more suitable for casting. 

CHROMIUM OXIDE     A very versatile stain which disperses well through a glaze, normally producing a green colour but giving reds and yellows in lead glazes, and pink in the presence of tin.

Chrome oxide can be used as a body stain in amounts to 5% to give grey-green, up to 3% in glaze recipes. Drab chrome greens can be moved toward peacock green with the addition of cobalt oxide (1% each gives bright color). This works in boron and soda glazes. Chrome in zinc glazes tends to form brown zinc chromate.

Because chrome reacts with normally inert tin to produce chrome-tin pink colors whiting and alumina are usually used instead of tin to lighten and clarify chrome green glazes. Chrome-tin pinks are much more consistent if the combination is premelted (i.e. commercial stain) and if the glaze is high in calcium or strontium, and free of zinc. Strontium is most effective if a wide firing range is desired (0.1-0.5% chrome, 4-10% tin).

Chromium oxide is added to enamels for green where borax and zinc are used to increase the brilliance of the colour. However, chrome in ground coat enamels tends to react with the metal to cause blistering.

Zircon opacifier 1-2% is often added to chrome glazes to stabilize them and prevent brown edges.

Chrome – Purple - Chrome-tin pinks move toward purple in glazes with significant boron. One glaze with 3.3 SiO2, 0.27 Al2O3, 0.2 B2O3, 0.15 Li2O, 0.5 CaO, 0.1 MgO, 0.15 Na2O employed 5% tin oxide, 0.6% cobalt carbonate, 0.17% chrome oxide to produce a good purple at cone 6.

Chrome – Green - Chrome is a classic green coluorant for recipes in oxidation and reduction at all temperatures. However, the shades it produces can be opaque, dull, and uninteresting. In the presence of CaO, the color moves toward grass green.

Chrome – Green Peacock - Drab chrome greens can be moved toward peacock green with the addition of cobalt oxide (1% each gives bright color, some MgO needed also). This works in zinc free boron and soda glazes.

Chrome – Brown - Chrome in zinc glazes tends to form brown zinc chromate.

Chrome – Orange - Chrome in high lead glazes forms yellow lead chromate. Zinc and chrome tend to produce orange.

Chrome – Black - Chrome is a constituent in almost all black oxidation colours. It is used up to 40% in Cr-Co-Fe blacks and as high as 65% in Cu-Cr blacks.

Chrome – Pink / Maroon - Chrome and tin are a widely used combination to produce pinks in zinc free glazes with at least 10% CaO and low MgO (alkaline glazes work well). Many stains are based on this system and typically have around 20-30 times as much tin oxide as chrome oxide. Tin would typically be around 4-5%.

Chrome – Chinese Red - Below 950C in high lead, low alumina glazes, chrome will produce reds to ranges, often with a crystalline surface. The addition of soda will move the colour toward yellow.

Chrome – Yellow - Chrome in high lead glazes forms yellow lead chromate. Alkalies are recommended in the base glaze. Added zinc can extend the range to orange. In other types of glazes, less than 0.5% chrome oxide will give yellowish or yellow green tints.

COBALT CARBONATE     CoCO3. Cobalt Carbonate -A pinkish tan powder. It is a strong colourant and almost always produces blue in glazes. The carbonate form of cobalt is very fine grained and disperses better and gives more evenly distributed colour than cobalt oxide. However, as with any carbonate, it produces gases as it decomposes and these can cause pinholes or blisters in glazes. Also the carbonate form contains less cobalt per gram, therefore colours are less intense than the oxide form.

COBALT OXIDE     Co3O4. Cobalt Oxide is a metallic oxide that produces blue in glazes at all temperatures. Black Cobalt Oxide is the principle source of CoO used in glazes, glass, and enamels. Cobalt is the most powerful ceramic colourant and it is stable in most systems. It is also useful as a body and slip stain. Cobalt will often produce glaze specking if it is not thoroughly sieved or ball milled to distribute the particles. Cobalt carbonate tends to disperse better in glazes to give even blue coloration because it is not as powerful and produces some glaze blistering problems. A high cobalt stain is also an alternative.

Cobalt - Violet, Lilac - In magnesia glazes a colour range is from violet to lilac is possible.

Cobalt – Blue - The shade of blue can, however, be affected in many ways by the presence of different oxides. Cobalt is powerful and often less than 1% will give strong colour. If the colour needs to be toned down, additions of iron, titanium, rutile and nickel may work. Brighter blue in alkaline glazes.

Cobalt – Soft Blue - Often calcined with alumina and lime for soft underglaze colours. Stains often employ mixes of alumina, cobalt, and zinc for softer blue colours.

Cobalt – Yellow - Used in combination with manganese and selenium to mask excess yellow coloration (yellow plus blue gives green which is masked by the pink of selenium).

Cobalt - Blue Slate - Combinations with iron and manganese can give a slate blue.

Cobalt - Blue-green - With barium shades of blue-green are possible.

Cobalt - Blue-black - With chrome and manganese blue-black and black are common.

Cobalt- Blue-green - With chrome and copper, cobalt can yield tints from pure cobalt blue, to greenish-blue, to the green of chromium. These effects work best when silica is not too high and there is adequate alumina.

Cobalt – Purple - With manganese (i.e. 1-3% cobalt carb, 3-5% manganese carb), purples and violets can be made. Less cobalt will lighten the colour. This effect works well in magnesia glazes. In high magnesia glazes, 1-2% cobalt alone will give purple. Add tin to move the colour toward lavender.

Cobalt - Lavender, Purple, Violet, Pink - With adequate SiO2 and high MgO (0.4 molar), purple, violet, lavender, and pinks can be made using 1% or more CoO. Mimimizing boron, alumina, and KNaO will help prevent it from turning blue. Note that the high MgO will generally make the glaze matte and it could suffer some ill effects associated with excessive MgO.

Cobalt – Red - With MgO, SiO2, and B2O3, red, violet, lavender, and pinks can be made.

COLEMANITE     B2O3 43.9%, CaO 26%, SiO2 4.5%. A popular natural source of insoluble boron for many decades. Gerstley Borate contained significant amounts of colemanite. Pure colemanite, however, is much higher in B2O3 than Gerstley Borate.  Be sure to screen out any materials coarser than 200 mesh, or ball mill the glaze. Gum or other binders also help. Acting as a powerful flux it can intensify the effect of colouring oxides and can increase craze resistance in glazes however

COPPER OXIDE      Black Cupric CuO. Red Cuprous Cu2O. Reduction firing reduces normal CuO copper oxide to Cu2O to produce bright red coloration in the reaction: 2CuO + CO -> Cu2O + CO2
Copper is an active flux and may increase melt fluidity and may increase crazing because of its high thermal expansion. Bright red colors are usually achieved with very small amounts of copper (i.e. .5%). If larger amounts of copper are present, the reaction could precipitate very tiny copper metal particles (colloidal copper) in the glaze melt to yield a red colour (i.e. flambâeš or sang-de-boeuf). Copper lustre can be produced by oxidation firing at low temperature glaze (950C) with heavy reduction cooling to leave a metallic layer of copper on the surface. 2-8% copper is required and cooling should be done in 15 minute cycles of reduction, interspersed with intervals where the atmosphere is allowed to clear. This can be carried out in cooling electric kilns by creating reduction through the introduction of flammable materials.

Copper - Green - Under normal oxidizing conditions the CuO molecule remains unchanged and produces clear green colours in glazes. The shade of copper greens can vary with firing rate and soaking changes. The best colours are generally obtained with fast firing and little soaking. Copper in calcium/magnesium glazes gives a green very different from that produced with lead.

Copper - Blue-green - Fluoride, when used with copper, can produce blue green colours.

Copper – Red - Copper is well-known for its ability to produce blood-red and fire-red colours in steady reduction atmosphere firings where CuO is altered to Cu2O. Bright red colours are usually achieved with very small amounts of copper (i.e. 0.2-0.5%) in a low alumina base with at least .4 molar equivalents of CaO and plenty of the alkalis. Tin oxide will enhance colour. Use of silicon carbide in oxidation (2%) can produce red.

Copper – Purple - Purple copper reduction glazes are the result of a mixture of copper in its green oxidized and red reduced forms. This effect appears most frequently in high lime glazes or where early stages of firing are oxidizing or latter stages are light or neutral. The use of boron in a copper red reduction glaze will give a purple hue.
The following formula produces good purple at cone 10: BaO 0.1, CaO 0.5, MgO 0.1, KNaO 0.2, ZnO 0.1, B2O3 0.15, Al2O3 0.2, SiO2 3.0.

Copper – Turquoise - In copper red glazes, barium additions in a high feldspar base will produce turquoise to deep blue depending on how much copper is added. Lithium contributes to the colour also. Combinations of Black CuO with tin or zircon will give turquoise or blue-greens when the glaze is alkaline (KNaO) and low alumina. Look for a frit with this profile for best results. Glazes of this type often craze.

Copper - Green Yellowish - K2O can turn a copper glaze yellowish. If Na2O or PbO are present, K2O should not exceed 0.15 equivalent.

Copper – Blue - Copper in a barium/zinc/sodium glaze gives a blue. Colour can also be enhanced by lithia. Tin and copper can produce turquoise to robin's egg blue 

Copper – Metallic - Large amounts of copper (7%) in a glaze give metallic and even graphite effects.

COPPER CARBONATE ( CuCO3) is bulkier than the oxide form, thus it tends to disperse better to give more even results. It is also more chemically reactive than the oxide form and thus melts better. As such, it is ideal for use in brush work where minimal speck is required. However it produces gases as it decomposes and these can cause pinholes or blisters in glazes. Also the carbonate form contains less copper per gram, therefore colours are less intense than the oxide form. Supplies of green copper carbonate often vary in colour and density. Despite variations in the physical appearance of the material, the amount of contained copper metal remains essentially constant but the ability to stay in suspension can be different from one manufacturer to another and so the ceramic grade must always be used.

Copper normally produces green colours in amounts to 5% whereafter it moves toward black. In reduction firing, it turns to Cu2O and gives vibrant red hues. Above 1025C copper becomes increasingly volatile and its crystalline structure breaks down. At 1325C CuO melts. This can affect the colour of other glazed pieces in the kiln. Glazes containing copper can change significantly because of loss of copper. Some potters alternate between reduction and oxidation, and even put a dish filled with copper carbonate in the centre of the kiln to minimize this phenomenon. It can act as a strong flux. It is the most stable form of oxidized copper ( Black Cuprous oxide oxidizes to Red cupric oxide in normal firings). The oxide form of copper can give a speckled colour in glazes whereas the carbonate form will give a more uniform effect.

Note: When added to low lead solubility glazes copper can cause the solubility of the lead to be greatly increased. Copper also can have similar effects in other types of glazes at other temperatures. If an overnight soak in vinegar or acid changes glaze appearance, be careful.

CORNISH STONE     K2O 3.8%, Na2O 4%, Al2O3 15.3%, SiO2 69.5%. A secondary flux in earthenware temperature glazes, and an alternative to feldspar at higher temperatures, giving greater fired strength. A decomposed granite however it is not a fusible as feldspar due to its high silica content. Low iron and high silica content promotes whiteness and transparency and so particularly useful in porcelain and similar white bodies.

DOLOMITE      CaO3 31.4%, MgO 20.8%. Dolomite as a ceramic material is a uniform calcium magnesium carbonate. In ceramic glazes it is used as a source of magnesia and calcia to act as a secondary flux. Other than talc, dolomite is the principle source of MgO in high temperature raw glazes but can be used as low as 1060oC. 'Dolomite matte' stoneware glazes, for example, are highly prized for their pleasant 'silky' surface texture. Above 5% it begins to opacify eventually making a matt glaze. Dolomite by itself is refractory, but when combined with the typical oxides in a glaze (especially boron) it readily enters the melt.

ERBIUM OXIDE     Er2O3. Erbium oxide is a light baby pink colour. It is an expensive, dense, and weak colourant, but one of the very few ways you will ever get a transparent pink. Erbium oxide's density means it is absolutely essential that you use CMC gum. Erbium oxide gives its best pink colour at concentrations of 8-10%, but it is difficult to get more than 8% to fully dissolve in the melt. It has given a more lavender colour in the presence of iron traces when in reduction.

FERRO FRIT 3110     1000 - 1060oC. Soft low alumina sodium borosilicate frit for glazes. Often used in crystal and crackle glazes. This frit can be very useful to reduce the feldspar content in glazes (since many high feldspar glazes have low clay content and therefore poor slurry suspension properties and dried hardness). The chemistry of this frit is similar to feldspar (but with low alumina and CaO in addition to the alkali fluxes). That means if you substitute this for at least part of the feldspar you can increase the kaolin (to supply the alumina) and thereby improve slurry properties. In addition you will be able to reduce the amount of troublesome calcium carbonate. Helps in the production of copper blues and manganese purples and raku glazes.

FLINT     SiO2. A major source of calcined silica for glazes and clay bodies (15%). It increases the fired temperature and craze resistance of glazes and its low expansion and contraction helps to stabilise the glaze. It is added to bodies to reduce shrinkage in drying and firing and to give a certain rigidity and eliminate crazing.

FRITS Glaze materials that have been melted together and then ground to an appropriate particle size - this reduces the handling of toxic materials such as lead by converting them into a silicate. Many materials, particularly carbonates are soluble in water and by fritting them first this prevents this problem. It also removes many volatiles which would have created firing problems. The tend to have a consistent formula and so give predicatbale and relieable results. See Calcium Borate, Standard Borax, Lead Bi-silicate and Sesquisilicate and HAF

HIGH ALKALINE FRIT     860 - 1060oC. Typically SiO2 52.5%, B2O3 3.4%, Al2O3 5.2%, Na2O3 18.6%, K2O 10.3%, CaO 2.9%. A high alkaline (Soda and Potash) version of a borax frit. High expansion rate making them suitable for crackle glazes. Helps create turquoise copper blues and purple/brown manganese in glazes.

ILMENITE     FeO TiO2. Like rutile, ilmenite is quite variable in nature. You can tell the difference between granular rutile and granular ilmenite by doing a smear test against an abrasive surface (i.e. an unglazed white tile). The rutile will be tan or brown, the ilmenite will be black or dark brown. Ilmenite can be used in small amounts (-1%) to produce dark brown specks in bodies and specialized glazes. It is also used in combination with rutile to develop characteristic rutile break glazes; it seeds crystals in titania glazes.

IRON OXIDE     Fe2O3. Red iron oxide is the most common colourant in ceramics and has the highest amount of iron. It is available commercially as a soft and very fine powder made by grinding ore material or heat processing ferrous/ferric sulphate or ferric hydroxide. During firing all irons normally decompose and produce similar colours in glazes and clay bodies (although they have differing amounts of Fe metal per gram of powder).

In oxidation firing iron is an important source for tan, red-brown, and brown colours in glazes and bodies. Iron red colours, for example, are dependent on the crystallization of iron in a fluid glaze matrix and require large amounts of iron being present (eg. 25%). The red colour of terracotta bodies comes from iron, typically around 5% or more, and depends on the body being porous. As these bodies are fired to higher temperatures the colour shifts to a deeper red and finally brown. The story is similar with medium fire bodies.

In reduction firing iron changes its personality to an incredible extent, it changes to a flux, a very active flux. Iron glazes that are stable at cone 6-10 in oxidation will run off the ware in reduction. The iron in reduction fired glazes is known for producing very attractive earthy brown tones. Greens, greys and reds can also be achieved depending on the chemistry of the glaze and the amount of iron. Ancient Chinese celadons, for example, contained around 2-3% iron. Particulate iron impurities in reduction clay bodies 'blossom' during firing, creating large specks that bleed right up through glazes.

The fine nature of red iron is a great asset in spreading it evenly throughout a glaze or body mix. It disperses better in glazes than does black iron. However, it is also a nuisance material for the same reason. In addition, larger amounts of iron oxide tend to gel glaze and body slurries, making them difficult to work with. Some grades of red iron do have coarser specks in them and this can result in unwanted specking in glaze and bodies.

High iron materials with alternate names: burnt sienna, crocus martis, Indian red, red ochre, red oxide, Spanish red.

The black iron oxides (FeO) have a higher iron iron content and gives darker shades usually dark browns and can produce speckles and crystals as an aventurine glaze.

Actual Yellow Ochre iron oxides (Fe2O3) are around 85% Fe2O3 and about 12% LOI with some impurities (e.g. SiO2, CaO). In ceramics, yellow irons are used where its raw colour or other raw properties are important to the manufacturing process or colour of the unfired product. It can produce yellowish honey colours (3%) but generally gives shades of brown (up to 8%) especially in lead glazes. It has excellent hiding power, absorbs ultraviolet light, is compatible with a broad range of vehicles, disperses well in aqueous and solvent systems, does not contain heavy metals

LEAD BI-SILICATE FRIT     900 - 1100oC. SiO2 29%, Al2O3 12%, PbO 59%. A good clear, general purpose lead frit for developing rich, bright surfaces and has a good reaction to colours . The safe way to introduce lead. Particularly suitable for red clays.

LEAD SESQUISILICATE FRIT     860 - 1080oC. As above but specially coated to reduce solubility.

LITHIUM CARBONATE     Li2CO3. Lithium Carbonate is the best source of lithium oxide for glazes. It is slightly soluble. It is unusual to see more than 5% lithium carbonate in glaze because it promotes devitrifaction. Because of the low expansion of Li2O, high lithium glazes tend to shiver. There are certain basic properties of lithium which are of interest in ceramics. Since lithium has a very small ionic radius in comparison to the other alkali metals, it has a higher field strength. Low expansion coefficients are generally imparted to ceramic compositions containing lithia. Lithium carbonate is a very strong flux (also true of lithium fluoride). It is the lightest, smallest, and most reactive flux. In addition to being soluble, lithium carbonate produces gases as it decomposes and these can cause pinholes or blisters in glazes. In frits and glazes, lithia is used to reduce the viscosity and thereby increase the fluidity of the coatings. This reduces maturing times and lowers firing temperatures. 1% additions can increase glaze gloss to a marked degree and slightly greater amounts (3%) can reduce melting temperature by several cones and affect surface tension of the melt

Lithium – Blue - Lithia can produce blue effects with copper.

Lithium – Pink - Lithia can produce pinks and warm blues with cobalt.

Lithium – Variegation - Lithia contributes to mottled and flow effects when used in small amounts (-1%).

MAGNESIUM CARBONATE     MgCO3. In high temperature glazes it acts as a flux (beginning action about 1170C) producing viscous melts of high surface tension and opaque and matte glazes. Like CaO, its melting action drastically accelerates at high temperatures. The surface tension of MgO-containing melts is less of a problem in reduction. Zircon and Magnesia melt at 2800C making them the highest melting oxides. Remarkably, MgO readily forms eutectics with other oxides to melt at surprisingly low temperatures.

It is valuable for its lower expansion and crazing resistance. When introduced into a glaze it should preferentially replace calcia, baria, and zinc before the alkalis to maintain surface character. Adding too much will generally move the surface texture toward matte or dry. MgO is a light oxide and generally is a poor choice for glazes to host bright colours. However, it does work well in earthtone and pastel glazes, especially in high temperature reduction firing. Likewise, it may be harmful to some under-glaze colours. Does not volatilize.

Magnesia is well known for the pleasant vellum 'fatty matte' and 'hares fur' tactile and visual effects that it produces around 1200C, especially in reduction firing (dolomite matte). The mechanism is phase separation of the suddenly melting MgO, but MgO can also produce matte effects at lower temperatures as a refractory melt-stiffening additive.

MANGANESE DIOXIDE     MnCO3. Above 1080C, half of the oxygen disassociates to produce MnO, a flux that immediately reacts with silica to produce violet colours in the absence of alumina, browns in its presence. Thus if it is being used in glazes fired below 1080C it should be considered as MnO2, if above it should be taken as 81.5 MnO and 18.5 LOI.
Smaller amounts are easily dissolved in most glaze melts, however, around the 5% threshold, the manganese will precipitate and crystallize. In large amounts in a glaze (i.e. 20%), metallic surfaces are likely. In glazes below 1080C, it can give coffee colour browns when used with tin. In glazes it will behave in a refractory manner, stiffening the melt. Because of the expulsion of oxygen at 1080, glazes using manganese should avoid this temperature range to reduce the chance of blistering and ruining of the glaze surface.

Manganese dioxide is the key to Rockingham brown wares which are made by employing about 3% iron oxide and 7% manganese in a transparent lead glaze of a recipe such as: Feldspar 28, Kaolin 14, Flint 4, Lead bisilicate 40, Whiting 4.

Manganese – Black - Manganese and cobalt mixture produce black. Iron can also be used. For example, a mix of 8 iron, 4 manganese dioxide and 0.5 cobalt make a raw black stain.

Manganese - Purple, Violet - Purple colours can be produced in glazes of high alkali (KNaO) and low alumina, especially in combinations with cobalt (look for a frit with this profile for best results).

 Manganese – Black - When added to terra cotta bodies in amounts around 5% manganese dioxide will produce dark gray to black firing bodies.

Manganese – Metallic - Large amounts of manganese can produce metallic effects in a glaze. However, these glazes must not be used on food surfaces.

MOLOCHITE     Al2O3 37%, SiO2 48%. A calcined china clay or aluminium silicate used as a fine powder to increase the firing temperature of glazes as it introduces alumina and silica; it reduces the tendency to crawl in glazes with high clay contents. Large sizes are used as grog for white bodies and to open the texture of bodies; it induces mechanical stability and resistance to thermal shock through the development of mullite crystals.

NEPHALINE SYENITE     K2O 9.1%, Na2O 7%, Al2O3 24.9%, SiO2 56%. A beneficiated mineral similar to but more fusible than feldspar, it has lower silica and higher soda and potassium levels and so may be used when a lower melting temperature is required. It has a fairly narrow vitrification band typically used in vitreous bodies.

NICKEL OXIDE     NiO3. Gives brownish greens through to grey colours (1-3%). In reducing conditions a yellow or blue may be obtained in high zinc stoneware glazes – 0.15 nickel with 0.15 zinc oxide gives a brown, 0.25 zinc a reddish purple, and 0.35 zinc a dark blue.

PETALITE     K2O 0.2%, Na2O 1.6%, Li2O 4%, Al2O3 15.7%, SiO2 76.1%. A secondary Lithia, alumina, silica bearing flux for use in high temperature bodies such as porcelain and high temperature glazes. It can be used to alter colour response and to reduce thermal expansion lowering maturing temperatures without shortening of firing range, especially when used as a replacement for feldspar.

POTASH FELDSPAR     K2O 11.3%, Na2O 3.2%, Al2O3 18.5%, SiO2 65.8%. One of the most important materials for medium and high temperature ceramic glazes. Potash feldspars are not usually as pure and white as soda spars. Glazes high in feldspar (35% or more) are likely to produce crazing problems. 'Flux saturated' glazes with more than 50% feldspar may be unbalanced and lack adequate glass former or alumina. SODA FELDSPAR (K2O 2.8%, Na2O 8.5%, Al2O3 18.5%, SiO2 69.5%) is more suitable at lower temperatures. If recipe only states Feldspar it usually means Potash Feldspar. FFF Finnish Floatation Feldspar (K2O 7.5%, Na2O 3.2%, Al2O3 18.5%, SiO2 67.5%)  is a high quality product half way between Soda and Potash Feldspar.  (The name Felspar is sometimes used but Feldspar is traditionally the correct name and is used throughout Europe).

PRASEODYMIUM OXIDE     Pr6O11. Praseodymium Oxide gives a small range of vibrant lime green colours in oxidation and reduction at concentrations of 5-8%. In small amounts (0.65%) in reduction with a trace of iron, gives a bright spring green colour.

QUARTZ     SiO2. Quartz sand is often used in bodies as grog for texture and to increase thermal expansion and craze resistance. Powdered quartz is used in glazes and bodies also. Quartz of very fine particle size (-400 mesh) will typically enter the feldspathic melt or convert to cristobalite during firing if fluxes are lacking, coarse powdered grades help to 'squeeze' glazes into fit. Intermediate sizes (200-300 mesh) seem to be best however, since their greater surface area exerts more compressive squeeze per unit.

RUTILE     TiO2 +. Rutile is the mineral name for natural crystals of titanium dioxide. In nature rutile is always contaminated by up to 15% other minerals (especially iron but also things like tantalum, niobium, chromium and tin). The term 'rutile' is thus generally understood to refer to the brown powder into which these minerals are ground and industry accepts up to 15% contaminants and yet still calls it rutile (below 85% titanium is called ilmenite). Rutile is considered an impure form of titanium whereas ilmenite is considered as FeTiO3.

In ceramic glazes rutile is more often considered a variegator than a colorant. As little as 2% can impart significant effects in stoneware glazes. It is normally used in combination with a wide range of metal oxide and stain colorants to produce surfaces that are much more visually interesting. In glazes with high melt fluidity (e.g. having high boron), large amounts of rutile (e.g. 8%) can be quite stunning. The rutile encourages the development of micro-crystals and rivulets. Since rutile contains significant iron its use in combination with other colourants will often muddy the colour that they would otherwise have or alter it if they are sensitive to the presense of iron. Even though rutile generally makes up less than 5% of stoneware glazes that employ it, they are often called 'rutile glazes' in recognition of its dramatic contribution.

Excessive rutile in a glaze can produce surface imperfections. In addition, when rutile is employed in higher percentages (e.g. 5%+) a given percentage might work well whereas a slightly higher amount can look drastically different. Such situations are vulnerable to chemistry changes in the supply of rutile. Thus people will often do a line blend trying a range of percentages to determine an optimal amount.

In glazes rutile can be quite sensitive to the presence of opacifiers. While an unopacified glaze glaze might appear quite stunning, the addition of a zircon opacifier will usually drastically alter its appearance and interest because the variegation imparted is dependent on the glaze having depth and transparency or translucency. Strangely rutile and tin, another opacifier, can produce some very interesting reactions and it is quite common to see tin in amounts of up to 4% in rutile glazes. In these cases the tin appears to react in the crystal formation rather than opacify the glaze.

Rutile powder, although its colour makes it appear to be a very crude ground mineral, normally contains 90%+ titanium dioxide. However this does not mean that you can use a 90% titanium:10% iron mix and get the same result in a ceramic glaze (obviously line blending would be needed to match the amount of iron). The mineralogy and significant other impurities in rutile are a major factor in the way it acts in glazes and are not easily duplicated using a blend of other things. Sometimes the special effects that rutile produces in glazes are also partly a product of a coarser grade (larger particle size). These likewise cannot be easily duplicated by more refined materials.  

SCMC GLAZE BINDER     Glazes being brush applied generally incorporate up to 5% of a glaze binder. Also useful in 0.5 to 2% addition to prevent colour transference.  

SILICA SAND     A medium silica sand used for either placing or as a grog, helping glaze fit and making the body more refractory.

SILVER CARBONATE & NITRATE     Silver nitrate is highly soluble and forms Ag+ ions when dissolved in water. This ion transfers to ware as adhered material and when fired and reduced the Ag metal gives a gold sheen. It is possible to dry blend the nitrate and carbonate forms and putting these into a glaze to get a better gold sheen. The Nitrate is hazardous the Carbonate is not. 100 grams of Carbonate is equivalent to 123 grams Nitrate. Primarily used in Raku firings to give yellow or gold effects.

SODIUM CARBONATE     Na2CO3. In ceramics, a common use of soda ash is as a soluble deflocculant in ceramic slips and glazes. It works well in combination with sodium silicate to produce slips that do not gel too quickly and whose rheology can be adjusted for changes in the hardness of the water. Higher soda ash in proportion to sodium silicate will produce a slip that gives a softer cast (stays wet longer). The total soda ash and sodium silicate amount should be tuned to create a slip that will eventually gel if left to stand. This thixotropic behavior will prevent it from settling.

In low solubility glazes it will weaken their resistance to acidic attacks.

Sodium carbonate is the preferred deflocculant for thinning glaze slurries. Its solubility makes it an ideal flux for Egyptian paste glazes.

SODIUM SILICATE     Na2 SiO2. The most popular deflocculant used in casting slips for many years. It is nearly always used with soda ash (when employed alone it can make a slip 'stringy' and thixotropic). The material is effective, reliable and inexpensive. However, it attacks the plaster in molds much more than more modern deflocculants and it is easier to over-deflocculate a slip with sodium silicate.  The 75TW with Bone China and Terracotta, the 140TW with everything else.

STANDARD BORAX FRIT     900 - 1140oC.  Often used in the production of earthenware glazes when a lead-free glaze is required. Slight milkiness, especially at low temperatures, may be evident over red clays, and the colour response with oxides etc is not as vivid as with lead frits.

STRONTIUM CARBONATE     SrCO3. A flux typically used in glazes at stoneware temperatures (above 1090oC) with similar effects to whiting and zinc oxide but with less pinholing and as an alternative to lead oxide.  It has been used in glazes at low temperatures as low as cone 01 giving a gloss mirror finish with good heat shock resistance. Excessive additions will precipitate a crystalline matt surface.

TALC     Typically MgO 32/33%, SiO2 46%. No talcs have the theoretical chemistry, the most common impurities are CaO (up to 8%) and Al2O3 (up to 9%). Along with dolomite, and to a less extent magnesium carbonate, it is an important source of MgO flux for bodies and glazes. Dolomite and magnesium carbonate have high loss on ignitions which can produce glaze bubbles, blisters and pinholes, talc is much less of a problem in this respect.

High temperature magnesia matte glazes employ MgO from talc and magnesium carbonate to form magnesium silicate crystals on cooling to give both opacity and a matte silky surface. It reduces plasticity and so is more suitable for castware. Talc is also used to produce thermal shock resistant / flameproof stoneware bodies where it acts as a low expansion flux that reduces body expansion by converting available quartz mineral, mainly in kaolin, to silicates of magnesia. Talc is a curious material in that, by itself it is a refractory powder; yet in amounts of 1-5% in middle temperature stoneware bodies it can drastically improve the maturity and melting. In ceramic slips, where 50% is often used, it produces a body that melts suddenly by cone 4. In glazes at middle temperature talc does not participate much in the melt and its presence tends to create an opaque silky matte surface, at cone 10 it is a powerful flux.

TIN OXIDE     SnO2. Tin oxide has long been used to opacify glazes (make transparents opaque) at all temperatures. Hand decorated tin glazed earthenware of the 1700/1800s is the most famous use of tin in glazes (delftware-England, faience-France, maiolica-Italy). While many potters are keeping this tradition alive today most now use zircon based opacifiers instead.

Thus any discussion about the use of tin oxide as an opacifier ends up comparing it with zircon products: Twice as much zircon is required to produce the same level of opacity. Like zircon, tin melts at very high temperatures and thus does not go into solution in typical glaze melts. Zircon will stiffen the glaze melt more than tin. Zircon is likely produce a harder glaze surface. Zircon will reduce the thermal expansion of the glaze more than tin. The quality of the white colour is different (tin tends to be more of a blue white, zircon a yellowish white). Tin is very expensive, this is likely to be the main reason for its much more limited use as an opacifier today. Zircon tends to have less of an effect on the development of metal oxide colors (e.g. tin reacts with chrome to make pink). Tin can react with titanium and rutile to variegate the glaze.
If gloss is an issue, silica might have to be reduced to compensate for the silica introduced by a zirconium silicate opacifier being substituted for tin. While there are other products that produce varying degrees of opacity, none are as neutral and non-reactive as tin and zircon.

Other opacifiers also tend to variegate the glaze. Copper red glazes require tin, with iron in oxidation tin makes a warmer shade of brown than zirconium does.

Tin – Pink - Chrome and tin are the most well known way to produce pink. For example, 7.5% tin and 0.5 chrome oxide will produce pink. Many Cr-Sn stains are available to make many shades of pink. However this mechanism requires that the glaze chemistry be right (e.g. no zinc, boron not excessive).

Tin – White - The quality of colour tends to be a 'soft-bluish white'. As little as 4-7% can produce brilliant white, although it is more typical to use 8-10% for full opacity. However, be aware that even tiny amounts of chrome in the kiln will volatilize and combine with the tin to produce pink shades.

Tin - Variegation - Tin/Iron Effects - Tin reacts with iron in fluid glazes to produce variegated surfaces. A good example is the Albany Slip 85, Tin 4, Lithium 11 glaze for cone 6.

TITANIUM DIOXIDE     TiO2. Although titanium is the strongest white pigment known for many uses, in ceramics the whiteness (and opacity) it imparts to glazes is due to its tendency to crystallize during cooling. Although titanium dioxide is used in glazes as an opacifier, it is not as effective and easy-to-use as tin oxide or zircon. It can be used as an additive to enliven (variegate, crystallize) the colour and texture of glazes (rutile works in a similar manner). In moderate amounts it encourages strong melts, durable surfaces and rich visual textures

In amounts below 1% titania can dissolve completely in a glaze melt. In slightly greater amounts it can give a bluish-white flush to transparent glazes (depending on their amount of alumina).

Above 2% it begins to significantly alter the glaze surface and light reflectance properties through the creation of minute crystals. This crystal mechanism gives soft colours and pleasant opacity, and breaks up and mottles the surface. In the 2-6% range, it increasingly variegates the glaze surface. Many potters add titania to their glazes or paint on overglaze titania washes for this purpose.

Large amounts (10-15%) will tend to produce an opaque and matte surface if the glaze is not overfired. They will also subdue colour and can add sparkle to the surface. As much as 25% can be absorbed by some lead glazes. Up to 0.8 molar can be used to effect crystal melts in glossy glazes.  Minute amounts (i.e. 0.1%) can be used to intensify and stabilize colours (i.e. iron can be altered to produce yellow and orange). It can alter and intensify existing colour and opacity in a glaze.

Titania can be reduced to produce colours in keeping with the elements present. If highly reduced it can yield a red, with iron the colour could be yellow, brown or green. Other combinations can yield blues, greens, yellows. Titania is oxygen-hungry and will quickly oxidize from its reduced state if given the chance.

Titanium - Crystal Matte - Titanium can be used in glazes to produce a matte surface with increasing amounts of crystallization in amounts up to 25%. The effect works in most stoneware glazes and is better when the glaze is slow cooled.

Titanium – White - Titanium is a crystalline mineral and encourages crystal development during cooling and freezing of the glaze melt. This generally produces opacity. However, titanium opacified glazes have a much different character than zircon or tin types. The latter produces a much more even and bright white coloration. When used as an opacifier the batch amount can range to 10% or more of the recipe. Lead greatly enhances the yellow at low temperatures.

Titanium – Variegation - Smaller amounts of titanium dioxide (i.e. 5%) added to coloured or opacified recipes can variegate the surface and make it more interesting (e.g. it alters the shape of crystals, shade of colours). The more you use the greater the effect (up to 10%).

Titanium – Red - In high fire matte glazes, iron oxide and titanium can produce red colours.

VANADIUM PENTOXIDE     V2O5 A colouring agent that produces yellows in amounts up to 10%, especially when in combination with tin oxide. Its colour in generally weak, but can be strengthened when fritted with tin and zirconia. Although yellows can be prepared with antimony, vanadium is more stable at higher temperatures. The most vibrant colour is obtained in leaded glazes.

WHITING     CaCO3. A fine calcium carbonate (limestone) used as the principle source of lime in glazes and also a flux at high temperatures. Also, contributes hardness and durability. Under reducing conditions it assists in development of the celadon colours; and can enhance salt glazes by enabling a thicker glaze. Too much whiting in recipes will lead to dull and rough surfaces.

WOLLASTONITE     CaO47% SiO2%. The fibrous form of wollastonite can be very beneficial in bodies. In low fired ceramics wollastonite reduces drying and firing shrinkage and drying and firing warpage. It also promotes lower moisture and thermal expansion in the fired product. It fires with no LOI and its fibres help vent out gassing. These factors have made it a valuable component in tile bodies, especially for fast fire. Vitreous and semi vitreous bodies can also show reduced shrinkage with small additions (2-5%), however wollastonite becomes a stronger flux as temperatures go above 1100C.

At higher temperatures the powder form is valuable as a source of CaO flux in glazes (and bodies). The other main raw source of CaO is whiting but it releases a high volume of gases of decomposition which produce suspended micro-bubbles that demand slow firing to clear. Wollastonite is useful in recipes where pinholing is a problem. Also, since wollastonite sources silica as well, glaze recipes employing it do not need as much raw silica powder. Further the SiO2 and CaO react more readily to form silicates. Thus wollastonite is used as a major flux in high temperature sanitaryware and electrical insulators. Wollastonite is also used in stain and frit formulations to supply CaO in a more easily melted form.

ZINC OXIDE     ZnO. Zinc oxide is a fluffy white to yellow/white powder with a very fine particle size coupled with high surface area. It can be an active flux in smaller amounts. It generally promotes crystalline effects and matteness/softness in greater amounts. If too much is used (10%) the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Also at high levels other surface defects like pitting, pinholing, blistering and crawling can also occur (because its fine particle size contributes to glaze shrinkage during drying and it pulls the glaze together during fusion). The use of zinc in glazes is limited by its price, its hostility to the development of certain colours and its tendency to make glazes more leachable in acids. Zinc oxide will produce opacity or whiteness, especially at low temperatures, if the calcium content is low. It does not opacify as well in boron glazes. It works well in combination with tin.

Zinc has a complicated colour response. It can have harmful and helpful effects on blues, browns, greens, pinks and is not recommended with copper, iron, or chrome.

Zinc – Enabler - Almost all crystalline glazes are high in ZnO, its presence coupled with low alumina and adequate SiO2 is the secret. The very fluid melt created is perfect for growing a wide range of metallic zinc-silicate crystals.

Zinc - White/Off-White - In larger amounts ZnO can produce opacity or whiteness in glazes. It exhibits refractory properties and can contribute to the development of a crystal mesh surface.

ZIRCONIUM SILICATE     ZrSiO4. Zirconium silicate is extremely stable and will survive to very high temperatures in a glaze melt without dissolving (although small amounts do dissolve). 4-7% produces semi opaque, 8-10% usually produces full opacity but some transparent glazes require even higher amounts; amounts beyond 20% reach saturation where crystallization begins to occur. The exact amount needed varies between different glaze types. It is thus most effective at low temperatures. Tin oxide can be a more effective opacifier than zircon (it has various advantages and disadvantages).

High amounts of zircon opacifier can cause cutlery marking (because of abrading angular micro-particles projecting from the glaze surface).

Zircon has a low expansion, so it will tend to reduce crazing. In addition, it will increase melt viscosity, which means that crawling and pinholing can occur in glazes having a lot of zircon. These problems can normally be solved by cooling slower and taking measures to get a better bond between raw glaze and body during application.

Because of its high thermal stability zircon is also employed in making various hi-tech porcelain bodies and materials. It is a major source for the production of zirconium oxide ZrO2.

Exeter Branch - Telephone  01395 233 077  Fax 01395 233 905

Doncaster Branch - Telephone 01709 770 801 Fax 01709 770 803

CTM Potters Supplies is a trading name for Simba Materials Limited www.simbamaterials.co.uk - Co Reg No 5659119; VAT No GB873606309

Send mail to admin@ctmpotterssupplies.co.uk with questions or comments about this web site.
Last modified: July 09, 2009