Clay is the singular essential element for all pottery, with its plasticity being particularly vital. This characteristic evokes a primal memory, a fundamental understanding that clay originates from the earth, thereby engaging the ancient struggle between man and the forces of nature.
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The Formation of Clay
Clay originates from the breakdown of rock, and from a geological standpoint, it is categorised into two principal types: primary (also known as residual) and secondary (or sedimentary).
Primary (or Residual) Clay
Clays classified as primary or residual are those that have developed at the exact location where they are discovered, meaning they have not undergone transportation by water or wind. This in-situ formation process grants them exceptional purity, as they contain minimal impurities or secondary minerals typically carried by natural forces. Consequently, primary clays exhibit a more consistent texture and less variation in their composition compared to secondary clays.
A notable characteristic of primary clays is their reduced plasticity, which stems from the absence of the impurities that enhance the malleability of secondary clays. While this low plasticity makes primary clays less suitable for manual shaping or pottery wheel techniques without supplementary components, it renders them particularly valuable for specific types of ceramic production that demand a more stable, robust material.
The inherent purity of primary clays establishes them as a fundamental material in the creation of high-quality ceramic products, such as fine porcelain. The most important primary clay is china clay, or kaolin, which remains free from colouring impurity. The second one is bentonite, which originates from the decomposition of volcanic ash.
The utilisation of primary clays is crucial in crafting ceramic items that require outstanding thermal resilience, including dinnerware, ornamental figures, and other articles that must retain their form and integrity after high-heat firing in a kiln. Thanks to their superior heat endurance and ability to maintain structural stability during the firing process, primary clays are the optimal selection for producing premium porcelain and upscale ceramic creations.
Secondary (or Sedimentary) Clay
Secondary clays, also known as sedimentary clays, are those that have been transported from their original location by natural erosive forces such as water, wind, or ice. During this journey, clay particles intermingle with a range of substances and minerals, including metals, carbonates, organic matter, and other elements. This results in a more intricate composition compared to primary clays.
The impurities integrated during their displacement, such as oxides and carbonates, impart a wide spectrum of hues to secondary clays, spanning from reddish and brownish shades to deeper tones. Natural processes mechanically refine these clays by eliminating gravel and sand. However, nature also introduces impurities chemically or at the ultimate particle level, incorporating substances like iron and fluxes.
A significant advantage of secondary clays is their enhanced plasticity. The inclusion of impurities such as carbonates and organic matter increases the clay’s capacity to absorb water, making it more pliable and easier to manipulate. This heightened plasticity makes them a favoured option for hand modelling and pottery wheel techniques, which demand more adaptable and manageable clays.
Nevertheless, this very attribute can render secondary clays more prone to modifications during the drying and firing stages, given that their elevated water content and other constituents can compromise the structural integrity of the finished articles.
Owing to the inherent impurities, secondary clays exhibit a more inconsistent firing temperature range when contrasted with primary clays. The required firing temperatures can vary based on the nature and quantity of these impurities, necessitating precise adjustments to the firing procedure to prevent warping or fracturing of the items.
Extensive reserves of secondary clays are present in numerous locations, and materials possessing a wide range of properties can be found in adjacent layers. Ball clays, earthenwares, and stonewares are all examples of secondary clays.
The Three Main Types of Secondary Clays
Refractory Clays Â
These heat-resistant clays are typically found in regions with coal, occasionally even between coal layers. They are frequently referred to as fireclays. Their primary application is in the manufacture of firebricks and other heat-resistant products. Their texture can range significantly, from exceedingly fine to quite coarse. Given their refractory nature and often dense consistency, refractory clays are commonly incorporated into stoneware compositions.
Vitrifiable Clays
Vitrifiable clays exhibit significant diversity in their inherent characteristics and hues. Ball clay, a specific variety of vitrifiable clay, is distinguished by its exceptional plasticity. It constitutes a crucial element in the formulation of both earthenware and stoneware ceramic mixtures.
Fusible Secondary Clays
Sometimes they are called Tertiary clays. Fusible secondary clays, which are surface clays, typically contain a significant amount of iron oxide. This iron oxide is accumulated as the clays are transported from their original geological formations. These clays are commonly used to manufacture building bricks and tiles and to produce coarse pottery items such as flower pots and bread crocks.
The Preparation of Clays for Pottery
To enhance the characteristics of pottery clay, additional minerals can be incorporated, and different types of clay can be blended. For instance, grog is frequently added to alter its textural properties. While it is possible to prepare your own clay by combining powdered ingredients with water, both beginners and many seasoned potters often find that commercially prepared clay offers superior consistency and requires less effort.
There are four principal categories of clay—earthenware, stoneware, porcelain, and raku—each containing numerous variations. Every type exhibits unique properties and requires a distinct firing temperature. Although the choice of clay is largely influenced by individual preference, the primary factor determining its use is the specific kiln you use, owing to the precise firing temperatures required.
Additions to Clay Bodies
Different types of clay can be combined to integrate their respective handling properties. However, it is essential to remember that this process will invariably alter the clay’s firing characteristics. Conversely, if the intention is to modify the firing attributes, the clay’s workability will likewise be influenced.
Materials other than clay can be added to clay bodies to modify their colour, surface texture, melting characteristics, or heat resistance. Here are the main ones:
Flint
Flint, produced by calcining and grinding to an extremely fine state, is virtually pure silica. Its addition can provide increased whiteness, greater density, and enhanced refractoriness. It possesses no inherent plasticity. Its application can also improve rigidity and help prevent warping. Nevertheless, an overabundance of flint can make a body prone to cracking.
Silver Sand
Silver sand, also known as silica sand, is considered the cleanest variety of sand and can be added to smooth clays to provide texture and improved grip. Since sand is composed almost entirely of silica, its inclusion generally enhances the refractoriness of ceramic bodies.
Significant prudence should be exercised with sand of unknown provenance, including that sourced from builders’ merchants or found naturally, and it must always be thoroughly tested. This is because sand might contain troublesome iron pyrite, and some types could even tend to fuse at fairly common working temperatures.
Quartz
Quartz, which is essentially pure silica in its crystalline state, can be used as a substitute for flint. It imparts robustness, minimises contraction, and helps prevent fissures. However, its crystalline composition undergoes transformation at elevated temperatures, necessitating specific precautions during kiln firing.
When incorporated into clay bodies, quartz offers structural reinforcement, substantially diminishes clay contraction during the drying phase, and enhances overall resilience. In glazes, because it is composed of pure silica, quartz serves as the fundamental glass-forming matrix. This contributes to the development of the hard, vitreous, and occasionally matte finish that bestows a practical lustre upon completed ceramic pieces.
Grog
Grog is created with grinding fired clay. It can be bought in various levels of fineness with pre-mixed grog, or you can also make it by grinding up broken bisque ware or dried, unfired clay scraps, then mixing them with stains to produce vivid colours. Its purpose in clay bodies is to provide texture and lessen shrinkage. Although grog is typically made from refractory, light-firing materials, it can certainly be produced from darker-firing clays to impart a coloured texture to a body.
Feldspar
After clay, feldspar stands as the most significant component in ceramics. It serves as a natural flux in both clay bodies and glazes, melting at high temperatures to create a durable glass that fuses surrounding materials. In clay bodies, feldspar promotes vitrification, contributing to their hardness and density. When fired appropriately with china clay, it yields translucent porcelain. The main varieties of feldspar are potash feldspar and soda feldspar.
Bentonite
Bentonite is an exceptionally fine, highly plastic volcanic ball clay. Potters use it to enhance glaze suspension and boost clay body plasticity. Adding up to 5% can significantly increase a body’s workability without substantially altering its other properties. A mere 1–3% inclusion prevents glaze settling (hard-panning) and binds unfired clay, though it requires sparing use to avoid excessive shrinkage.
Bone Ash
Bone ash, or calcinated animated bones, is a classic and highly versatile component in pottery. It is produced by intensely heating animal or human bones to eliminate organic material. The resulting calcium phosphate gives traditional bone china with its characteristic translucency and strength, while also acting as a potent flux and opacifying agent in ceramic glazes.
Traditional bone china recipes generally incorporate around 50% bone ash, 25% kaolin (also known as china clay), and 25% china stone. This particular blend enables the pieces to be cast exceptionally thin while still possessing remarkable strength and translucency. In glazes, bone ash contributes calcium oxide, a potent flux, and phosphorus pentoxide. It aids in fostering opalescence, facilitates distinct crystal formations, and can intensify specific glaze colours, such as iron reds or celadons.
Numerous modern commercial ceramists now use synthetic hydroxyapatite compounds or dicalcium phosphate. This allows them to attain the same chemical properties without processing actual bones.
The Wonderful Nature of Plasticity
The defining characteristic separating clay from standard dirt is its plasticity, a property allowing it to be easily moulded and shaped under pressure without cracking or breaking, and to permanently retain that new form once the pressure is removed. Highly plastic clays (e.g., ball clays) stretch easily on a pottery wheel but can be sticky and prone to shrinking or warping. Less plastic clays (e.g., porcelain) are stiffer, more elastic, and harder to shape, yet they hold their form beautifully.
Plasticity is a magical property as it transforms raw clay into a workable medium, allowing us to produce complex hand-building or wheel-throwing pieces. Its importance goes through several stages of pottery creation.
Plasticity dictates how readily the clay can be worked. Highly plastic clays (which typically contain finer particles and organic substances) enable potters to build delicate walls on a wheel, construct sizable coils, or curve forms without tearing.
Plasticity also provides what ceramic artists term “wet strength.” This enables damp, heavy, or broad pieces to be repositioned and reshaped without collapsing, drooping, or distorting under their own mass.
And it helps to reduce defects as appropriate plasticity ensures consistent shaping, thereby cutting down on material waste and superficial imperfections.
Conclusion
So, to wrap things up, understanding how different types of clay and clay bodies additions work can really make or break your pottery projects. And plasticity is a huge deal when you’re getting creative with clay.
Don’t just take our word for it – go ahead and experiment with these tips to find your perfect mix! And hey, we’d love to hear your own experiences and favourite clay body experiences by sharing your valuable experiences and questions in the comments section.
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