Clay Composition

How do you distinguish different types of clay? What are the unique attributes to each? How does their chemical composition play into each of these?

Clay Body Composition & Distinctions

Clays are categorized by their firing temperature and origins. See below the three types of clay, and what makes them unique!

Earthenware

A rough, brown or red clay body known for its high concentration of iron oxides and other metallic cations. It is composed of 50% soft, kaolin rich clay, 15% feldspar, and 35% flit which gives it the distinctive red-brown coloration and graininess. Earthenware fires at a lower temperature (1742°-2102°F) and shrinks about half as much compared to porcelain, owing it its higher silica content from the feldspar. Earthenware is porous, making it useful for water drainage. Because of this, earthenware is often used to make planters (think terra cotta) and bricks for building primitive housing.

Stoneware

A non-porous intermediate between natural clays like earthenware, and highly pure clays like porcelain. Coloration is most commonly gray, however many variations from orange to beige exist after bisque firing. Stoneware clays are composed of 40% china clay (high kaolin content), 40% highly plastic 'ball' clay, 15% feldspar and 5% quartz. Stoneware is considered a great beginners clay, being relatively plastic and easy to shape, while retaining rigidity both before and after firing.

Porcelain

An incredibly soft, pure white clay known for its unique interaction with glazes. Porcelain is categorized by its kaolin-rich composition, with little metallic contaminants like iron oxide to replace aluminum and silicon in its molecular structure. The composition of this clay consists of about 55% kaolin, 5% bentonite, 27% feldspar, and 17% quartz.

Atomic Structure

Atoms bind to each other based on charge differences. One of the most common causes of this charge difference is a lack or abundance of electrons in an atom's 'valence' shell, or outermost shell of electrons available for bonding. Atoms prefer to have exactly the right amount of valence electrons, so they will share with each other through bonding to fill or empty their shells. These bonds can repeat across many molecules, forming sheets or crystals in 3D.

Clay is composed of sheets of silica (SiO₄) and alumina (2Al₂O₃) stacked on top of one another. Silica is a tetrahedral molecule, with one positively charged silicon atom at the center of four negatively charged oxygen atoms, creating a pyramid around the silicon.

Alumina is an octahedral molecule, with six oxygen atoms surrounding the positively charged aluminum molecule at the center. The oxygens at the corners of these molecules can contribute to the structures of surrounding silica and alumina molecules, allowing these structures to bind together into sheets.

Crystal Structure

When grouped together, silica and alumina form thin sheets of repeating units. These repeating units are stacked on top of one another through hydrogen and ionic bonding of the oxygens. Depending on the order and types of bonds holding the sheets together, different types of clay structures are created as a result. When combining these clay structures further down the line, different proportions of these structures will constitute distinct of clay bodies with varying properties.

From Crystal to Clay - Subunit Structures

Kaolin (Al₂Si₂O₅(OH)₄) consists of repeating single sheets of alumina and silica, bound together by hydroxyl groups. The hydroxyl groups come from water, providing the extra hydrogens which the negative oxygens on both the silicate and aluminate are attracted to. This type of attraction between oxygen, hydrogen, and water molecules is referred to as hydrogen bonding in the field of chemistry. Hydrogen bonding is relatively weak in this context, where heavy sheets of silica and alumina must be bonded together. This gives kaolin an incredibly high plasticity, being easily molded into shapes, however it lacks rigidity, it being difficult to hold these sheets of clay together against external forces.

Illite (2n(Al₂Si₂O₅(OH)₄) + n(cation)) is a set of two silica layers inverted outwards to form a

sandwich around the alumina layer. Similar to kaolinite, these layers are formed through bonding of the oxygen atoms between the silicon and aluminum ions, sharing their electrons to hold the alternating sheets. These sets of three sheets are then held together by metallic cations, which are attracted to the exposed oxygens in the silica sheets. These cations include magnesium, potassium, and iron oxides. These reactions between charged molecules are referred to as ionic bonding. Ionic bonds like this are generally stronger than the hydrogen bonds in kaolin, giving the structure more rigidity, but less plasticity. Firing temperatures with illite rich clays are generally lower, with iron oxides acting a flux. Fluxes are materials which decrease melting temperatures of other materials when mixed, as it allows for alternative bonding patterns to emerge as it starts to heat up. The spacing between sheets is greater than that of kaolinite, owing to the presence of potassium and other ions being the cross linking molecule. The room left between the sheets allows them to run across each other more easily, meaning clays rich in this subunit have high plasticity.

Implications on Pottery

Clays like porcelain that contain high kaolin to illite ratios in their composition have less structural integrity than earthenwares with other constituent metals like iron and magnesium. The lack of these molecules which ‘gunk up’ the crystal structure of the clay causes it to feel buttery and have a higher plasticity, making them easy to manipulate. Shinkage is a major factor to consider when selecting a clay body, with the higher ratio of silica to alumina in earthenware clays causing a lower rate of evaporation from the clay bodies. As the greenware is dried to bone-dry and fired, losing nearly all water in the clay composition, the sheets of silica and alumina squeeze closer together and chemically fuse during the firing process. If a clay body shrinks too much or too little during firing, or if the shrinkage occurs too fast, it will crack or warp during the glaze firing.