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Ceramic chemistry

To our help in dating antique Chinese porcelain there are a number of features of the pieces that we by training just recognizes. By experience we ideally know what features are to be expected - or not - on any ware, from any period, of the Chinese ceramic history. To the features we can see with out naked eyes and feel with our hands, we can add features that could only be seen in strong magnification and discovered by chemical analysis. Everything we feel or see have a reason and those reasons could also be described in Chemical terms. To help with this, I am working with this page.

Alkali

In ceramics alkalies are strong fluxing agents - reacting strongly with silica (an acid-behaving substance) to effectively lower its melting temperature - and thereby its fusion point. Alkalies are strong bases who can convert acids to neutral salts. In concentrated form, they are caustic enough to be corrosive to organic tissues.

In nature alkalies were found in the ashes of burned sodium- and/or potassium-bearing plants (such as kelp of beaches and moss of marshes) from which the oxides of sodium and potassium could be leached (usually by boiling in water to a thick syrupy brine). Earlier still, alkalies were leached from desert earths rich in the alkaline elements. Clay and glaze recipes from antiquity to present times abound which call for exotic ingredients such as finely sifted beach sand, ash of seaweed, ash of bog moss, ash of wine leves, soda ash, potash, etc., all indicating sources for alkaline substances.

Kaolin

Principal mineral is kaolinite. The kaolinite mineral is a pure clay crystal of one part alumina and two parts silica, also called China Clay. Kaolin clays are whiter and cleaner than other clays because they are mined near the site where they were weathered and altered from the parent rock. Kaolin ore is mixed with other rocks and impurities which have to be separated using various wet and dry processing methods. Kaolin is the paradigm for a pure clay but still, in its natural, unrefined, form, it often has substantial portions of other minerals such as muscovite, quartz, feldspar, and anatase.

Kaolin clay particles are flat and comparatively the largest of all clay minerals. They have a surface chemistry that gives them an affinity for water. The attached water both 'glues' particles together and acts as a 'buffer' to lubricate particle-against-particle movement in the plastic matrix. Because of the large particle size of the kaolinite mineral they are permeable to the passage of water. Thus kaolins, especially the larger sized ones, speed up casting rates in slurry bodies. Kaolins have relatively low plasticity when compared to other raw clay types.

When used in ceramic bodies, pure kaolin fires clean and white. Kaolin is a very refractory aluminum silicate. Melts at around 1200C, depending on impurities, but when mixed with a feldspar their combined alumina content will add refractoriness subsequently holding off the melting point up to 1450. To raise the melting point even further a magnesium mineral as talc (soapstone), dolomite, etc. can be added. Because of its purity it fires to white and because of its high refractoriness it can serve as a pillar to hold up surrounding materials as they vitrify - two qualities which, historically, made possible the white translucent ware of China which became known as porcelain.

For glazes the chemistry of kaolin makes it the primary source of alumina oxide. In glaze recipes kaolins are employed to keep other particles from settling out to the surface.

Ideal form: Al2O3, 2SiO2, 2H2O with a typical empirical analysis of SiO2 45.11%, Al2O3 38.99%, Fe2O3 0.46%, TiO2 1.46%, CaO 0.12%, MgO 0.04%, K 2O 0.06%, Na2O 0.03%, traces 13.97%.

Al2O3, Aluminum Oxide, Alumina

The primary source for Aluminia in porcelain is kaolin, clays and feldspar.

Alumina has a very high melting temperature and alumina ceramics are thus employed in many refractory materials that must withstand high temperature.

Alumina controls the flow of the glaze melt, preventing it from running off the ware by helping to build strong chemical links between fluxes and silica. It is thus called an intermediate oxide.

Alumina is second in importance to silica and combines with silica and basic fluxing oxides to prevent crystallization and give body and chemical stability to a glaze.

Alumina is the prime source of durability in glazes. Alumina increases melting temperature, improves tensile strength, lowers expansion, and adds hardness and resistance to chemical attack.

If a glaze contains too much alumina, then it may not melt enough but will likely be more hard and durable if firing temperature is increased. If a glaze has inadequate alumina, then it is likely that it will lack hardness and strength at any temperature.

Increasing Al2O3 stiffens the melt and gives it stability over a wider range of temperatures (although excessive amounts may tend to cause crawling, pinholes, rough surfaces). The addition of alumina prevents devitrification (crystallization) of glazes during cooling because the stiffer melt resists free movement of molecules to form crystalline structures. Thus crystalline glazes tend to have less than .1 molar equivalents of Al2O3. The addition of small amounts of CaO will help reduce the viscosity of a melt and make it flow more freely.

Calcined alumina does not work well in glazes or enamels as a source of Al2O3 ; however, the hydrated form can be effective to matte a glaze if it has a very fine particle size. If possible, kaolin or feldspar (and nepheline syenite) are the best sources. Kaolin especially is ideal because it is so important to other physical slurry properties (i.e. suspension, adhesion, and shrinkage control). If glaze batches are being calculated from a source formula, it is normal to supply all possible alumina from feldspar and kaolin until the alkali targets are met, then furnish any additional alumina requirements with Bayer process alumina hydrate. Sometimes Bayer alumina is added where exceptional freedom from iron is needed.

In most cases, the addition of alumina raises the melting temperature of a glaze. However, in some soda lime formulations, a small alumina addition can decrease melting temperature.

Alumina (preferably in the calcined form) can be used in clay bodies as an aggregate and filler in place of flint. This can increase the firing range, decrease quartz inversion firing problems, and increase hardness and whiteness in the fired body. However, alumina is much more expensive than flint.

Alumina hydrate promotes opacity in enamels and glazes by generating gas bubbles in the glaze melt.

In glazes Alumina is used in combination with chrome, manganese, and cobalt to achieve pink colors.

Cobalt depends on the presence of alumina or it will fire pinkish. Chrome reds like alumina also.

Alumina is a Surface Modifier - Crystalization - Since Alumina stiffens the glaze melt, it will prevent the growth of crystals during cooling because it is more difficult for the specific oxides needed to form the crystal, to travel to the site of formation. Thus most highly crystalline glazes have very little alumina. Alumina is also a Surface Modifier - Matte - The ratio of alumina to silica is mainly responsible for the degree of matteness in glazes. In the absence of boron, ratios of less than 5:1 are generally quite matte; ratios of greater than 8:1 are usually glossy in the absence of high titania, zinc, magnesia, or calcia (which cause volatile melting or crystallization during freezing). Ratios of 1:18 are possible, but certainly not typical. If a glaze remains matte when fired higher, it is a true alumina matte.

SiO2 (Silicon Dioxide, Silica)

Sources: Flint, Feldspar, Kaolin, etc.

Silicon Dioxide is the principle, and often only glass forming oxide in glaze. Normally comprises more than 60% of most glazes and 70% of clays.

The proportion of Silicon Dioxide in relation to fluxes, regulates melting temperature and gloss.

More Silicon Dioxide and less flux makes glaze harder, more durable and brilliant, raises the melting temperature, increases acid resistance, lowers expansion, increases hardness and gloss, and increases devitrification.

Less Silicon Dioxide and more flux increases the melt fluidity.

It is normal to use as much Silicon Dioxide as possible in any glaze to keep expansion low, to prevent crazing, and enhance body/glaze fired strength.

With boron and alumina, it has the lowest expansion of all oxides.

In clay bodies, flint mineral particles act as a filler and behave as an aggregate, while Silicon Dioxide in feldspar, kaolin, ball clay, etc., participates directly in the chemical reactions taking place to build silicate glasses. Thus the particle size of the parent material is often important in determining whether contributed silica affects the chemistry and/or mineralogy or acts as an aggregate.

Its fusion temperature is 1,720 C

Silicon Dioxide is a Surface Modifier - Matte - in such a way that a Low silica / high alumina glaze will produce matte effects. The silica to alumina molar ratio is considered a good indicator of this type of matteness. A ratio of 5 to 1 is matte while 10 to 1 is glossy.

CaO (Calcium Oxide, Calcia)

Calcium Oxide is a Flux. Tpical sources are Feldspar, Dolomite and Quicklime.

Together with SrO, BaO, and MgO it is considered one the Alkaline Earth group of oxides. It has a cubic crystal structure.

Quicklime is pure calcia, but it reacts with water to produce calcium hydroxide or slaked lime. Calcium oxide, on the other hand, is an extremely stable compound.

Calcium oxide is the principle flux in medium and high temperature glazes, beginning its action around 1100C. It must be used with care in high-fire bodies because its active fluxing action can produce a body that is too volatile (melting if slightly overfired).

Calcia usually hardens a glaze and makes it more scratch and acid resistant. This is especially so in alkaline and lead glazes. Its expansion is intermediate.

Calcia and silica alone resist melting even at high pottery temperatures, but when soda and potash are added, calcia becomes very active in both oxidation and reduction. CaO contributed by wollastonite is more readily fusible than that contributed by whiting (calcium carbonate). This synergy between CaO and other fluxes and differences in the mechanism of its fluxing action generates some disagreement among experts regarding the nature of CaO since it does not appear to the a 'stand-alone' flux compared to others.

Hardness, stability, and expansion properties of silicates of soda and potash are almost always improved with the addition of CaO.

CaO is not effective below cone 4 as a flux in glazes but in small amounts (less than 10%) it can dissolve in earthenware glaze melts especially with lead, soda, potash) to add hardness and resistance to leaching. In non-lead mixes it can also help reduce crazing. In larger amounts, it encourages the growth of crystals which can give decorative effects to glossy glazes and produce matteness (i.e. 30%).

High CaO glazes tend to devitrify (crystallize). This occurs either because of the high melt fluidity imparted by CaO at higher temperatures or because of the readiness with which CaO forms crystals. Fastfire glazes can contain more CaO because the quick cooling does not give the crystallization a chance to occur.

Calcia is a moderate flux in the cone 5-6 range, but a very active one at cone 10.

High calcia glazes tend to have good (although sometimes unexpected) color responses. For example, in oxidation iron glazes calcia likes to form yellow crystalline compounds with the Fe2O3 producing a 'lime matte'. Without the calcia, glossy brown glazes are the norm.

CaO is not found pure in nature but rather is contained in various abundant minerals (i.e. calcite, aragonite, limestone, marble) but vary greatly in their purity (impurities usually include magnesia, iron, alumina, silica, sulfur). Of these iron and sulfur are most troublesome (i.e. where clarity is important in glass). Lime minerals vary in the degree of crystallization and cohesion of the crystalline mass and the homogeneity of the matrix. The term 'lime' encompasses several different minerals.

Calcined limestone is burned lime.

Dolomite (magnesium carbonate) is a mineral which supplies some magnesia in addition to its CaO complement. It is preferred in many situations because it more readily fluxes and the magnesia imparts desirable properties. Properties Glaze Color - Yellow - In oxidation, iron glazes calcia likes to form yellow crystalline compounds with the Fe2O3 producing a 'lime matte' even in an otherwise fluid glaze. Glaze Color - Glossy Brown, Black - Calcia likes to form yellow crystalline compounds with Fe2O3 producing a 'lime matte', especially in fluid glazes. Thus iron glossy brown or black glazes should be low in CaO. Surface Modifier - Variegation - CaO can mottle glaze surfaces at high temperatures if significant amounts are present. Surface Modifier - Matte - High molar amounts of calcia combined with adequate silica and preferably lower alumina will form a calcium silicate crystal matte (lime matte). The presence of zinc will increase the size of crystals.

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