Complex clay and complex ions: the chemistry of ceramics from Earth to fired finish.

Decorate your home, furnish your reception area, or brush up on the chemistry of materials and light emission--with ceramics.

The ceramic process first began with the earliest discovery that earth mixed with water can be moulded into shapes. All pottery contains some amount of clay in combination with other materials. Clay allows the material to be easily manipulated and to maintain its new form with strength. Flux is another material present in pottery. It melts in the high temperatures of a kiln and reacts with other materials, increasing the pottery's strength. The remainder of the composition is filler--non-reactive material providing rigidity. Once clay bodies are moulded into shape and built with the materials to provide us the characteristics we desire, they are fired and glazed. That is where chemistry reveals a full spectrum of glorious colours! While pottery is often admired for its physical beauty, the associated chemical processes should not be overlooked.

Clay is not a pure substance. It consists mostly of kaolin ([Al.sub.2][O.sub.3], 2Si[O.sub.2], 2[H.sub.2]O), and also quartz (Si[O.sub.2]) and mica (soda--[Na.sub.2]O x 3[Al.sub.2][O.sub.3] x 6Si[O.sub.2] x 6[H.sub.2]O, and potash--[K.sub.2]O x 3[Al.sub.2][O.sub.3] x 6Si[O.sub.2] x 6[H.sub.2]O). The components of clay are what distinguishes its important characteristics like particle size, plasticity, and strength. The superficial view that particles are spherical or cubical will just not do in the case of kaolin. Kaolin consists of a layer structure with planes of Si-O alternate with planes of Al-O or O-H. The unit cell dimension is in the order of nanometres. For potters, sizes are often quoted in an equivalent spherical diameter--kaolin at approximately 0.05 micrometres. Clays with lower kaolin content have smaller equivalent particle diameters. It's this fineness that allows clay bodies to have high packing density. Clay bodies must also have high plasticity--allowing it to undergo large strains, quickly, without fracturing so that it can maintain an imposed shape. It's the plate-like nature of the kaolin, the fine particle size, and its affinity with water that attributes to its plasticity. Clay bodies need to be strong while being formed, while being fired, and as a finished product. Non-plastic materials with the water removed, like flint and stone, and dry clay powder have very little strength. However, clay remains firm even after the water has been dried out. The water in the clay allows the particles to remain close together, contributing to attractive forces. Its fine particle size permits a greater number of points of contact per volume, reinforcing the clay's strength. Additional plasticity and strength can also be achieved with other material binders.

When alkalis are present in clay bodies, they contribute to the formation of alkali-aluminum-silicate glasses. This glassy matrix also enhances the strength of the clay bodies. Feldspar (soda--[Na.sub.2]O x [Al.sub.2][O.sub.3] x 6Si[O.sub.2] and potash--[K.sub.2]O x [Al.sub.2][O.sub.3] x 6Si[O.sub.2]) allows ceramics to melt at normal pottery firing temperatures and to have a high viscosity to protect against deformation.

For clay bodies that need special characteristics, flux can be a chief component. Talc (magnesium silicate hydroxide--[Mg.sub.3][Si.sub.4][O.sub.10][(OH).sub.2]) can form cordierite (magnesium aluminum silicate--[Mg.sub.2][Al.sub.4][Si.sub.5][O.sub.18]) that gives the clay bodies very low thermal expansion.

Filler is mainly added to clay bodies to fill the space between the clay and flux. The filler must remain relatively unchanged during the firing of the clay piece to help provide strength, rigidity, and maintain form. The most common filler is silica, which is inexpensive in large quantities.

After the potter has chosen the clay, flux, and filler, and moulded the piece using material that has enough strength to maintain its shape, he/she has to ensure that the chosen materials will not distort or shrink when drying. Once the piece is formed, water needs to be driven out to strengthen the unfired pieces. The mechanics of drying are essential because a steep rise in temperature can be too drastic. Ceramic pieces will warp, crack, or rupture if the pressure increases before the water escapes. This is likely with big shifts in temperature. Ceramic pieces can dry at room temperature, but it's a lengthy process. Kilns operate by increasing the heat slowly. The temperature ranges and numbers mentioned below are specific to a typical earthenware clay body. Clay bodies that have different quantities of the clay, flux, and filler will have different temperature ranges and consistencies.

[ILLUSTRATION OMITTED]

The word "ceramics" is derived from the Greek word "keramos," meaning burnt material. The heating of clay bodies to temperatures greater than 1000 degrees Celsius is an irreversible process. The permanent changes to the ceramic pieces are due to the loss of water. Clay undergoes tremendous weight loss with a rise in temperature above 150 degrees. From 400 to 600 degrees, a large percentage of weight and even more water is lost. Small amounts of combustible matter are burned off at temperatures upwards of 1,000 degrees. Measuring thermal expansion versus temperature shows us that there is a gradual small increase in volume until the 400 to 600 degree range (again!) when the clay rapidly contracts. The endothermic reaction occurring in that region is the breakdown of the kaolin molecules releasing water, which is then driven off by the high heat ([Al.sub.2][O.sub.3] x 2Si[O.sub.2] x 2[H.sub.2]O [right arrow] [Al.sub.2][O.sub.3] x 2Si[O.sub.2] + 2[H.sub.2]O).

The kaolin has lost its plasticity and strength at this point. The strength can be regained through other reactions at higher temperatures. The heat brings about a decrease in volume and absorption of water and also allows for an increase in the glass character of the clay body. The ideal firing range is determined by the degree of porosity. Vitrification occurs between 1,000 and 1,100 degrees as the fluxes melt and produce the glassy matrix. Porosity will decrease to approximately zero at temperatures upwards to 1,220 degrees. Beyond that temperature, the piece becomes overtired as the few remaining pores expand, leading to warping and other problems.

Once a finished, fired, and dried clay piece emerges from the kiln, decoration is the next step. Not all ceramics need glazing. But a 100-micrometre-thick layer of a smooth, glossy glaze can become a very important part of the overall pottery process. The glaze does not only enhance the appearance of a ceramic piece. It becomes an impermeable layer with either glossy, matte, or satin effects, making the piece easier to clean and providing additional mechanical strength.

Glazes can be applied to dry, un fired bodies, to partially fired bodies, or in the most common case, to fired ware. Glazes are usually in an aqueous form and can be applied either by dunking, spraying, or painting. Glazes must have a sufficient viscosity to have good flow without flowing down surfaces and ruining desired artistic effects. Along with viscosity and adhesion properties, thermal contraction must be watched. Glazes contain silica and are very similar to glass. Silica itself melts at 1700 degrees. Adding oxides to silica can create a eutectic where the melting point is lower than that of either component. The oxides break the silicon-oxygen bonds, and the mixture now melts at 1,000 degrees. Lead oxide is a popular choice since it has low thermal expansion qualities, good flow, and a high degree of refraction for a shiny glossy layer. Other alkali, earth alkali, and transition metal oxides are also used, each possessing their own pros and cons.

Glazes' vibrant colours are produced during firing when high temperatures excite colourants' electrons into higher energy levels. Colourants are added to the glaze in very small amounts (approximately five to ten percent) and generally, the most common colourants are the transition metals and their oxides. The transition metals can readily form complexes in solution. The surrounding ligands can distort the energy levels of the ion, changing the energy differences to bring about a new colour. For example, cobalt chloride (Co[Cl.sub.2]) in solution can form Co[([H.sub.2]O).sub.6], which has a characteristic pink colour. Cobalt chloride with HC1 will give Co[Cl.sub.4] with a resulting blue colour. Ligand field theory is important in the underlying chemistry of glazes; however, the oxides used with the silica in the base glaze can have an effect on the colourant and some colouring ions such as Fe or Co have been known to take the place of Si in the silica and oxide network. Along with resulting colour, the choice of colourant is dependent on the temperature range desired.

The chemistry of pottery and glazes is further complicated when you take into account all the different types of pottery bodies available to today's potters. It's a dynamic art form providing further opportunity to appreciate the beauty of chemistry.

Anne Campbell, MCIC

References

Allan Dinsdale, Pottery Science--Materials, processes and products (West Sussex: Ellis Horwood Limited, 1986).

Anne Campbell, MCIC, has an MA in chemistry from Brown University in Providence, RI. She is the CIC career services