Pottery clay has been used for thousands of years to create a range of useful and beautiful objects. For clay to have its unique properties, it must include specific clay minerals, such as aluminium phyllosilicates, iron, and magnesium. Clay is harvested from the earth from deposits that formed from a combination of chemical decomposition and weathering processes. Pure pottery clay is found in many different colour hues, from dull grey to deep red, depending on the particular chemical composition of its components. When in a dry state, clay is powdery, but the addition of water allows the particles within clay to form a cohesive and malleable whole, which can be formed, spun on a pottery wheel, stretched, carved, or rolled flat. After pottery clay is formed, it is typically fired or air-dried to produce a hard, durable object.
Pottery clay is sold in art or ceramic supply shops and online at marketplaces like eBay. Understanding how different types of pottery clays should be treated is key to creating pottery that has the durability, translucence, and usability that a buyer desires. Before shopping, buyers should understand the properties of pottery clay and the different types of pottery clay available in order to be certain that they make the most educated pottery clay purchases.
Pottery Clay Properties
Since there are a number of different types of pottery clay available, buyers should do some research before shopping to be certain that they purchase the right clay to complete their pottery projects. Pottery clays generally differ in terms of plasticity, porosity, and vitrification, so familiarity with these terms helps buyers to correctly choose their pottery clay.
The plasticity of pottery clay is what separates it from ordinary dirt or silts. To form an object, moisturised pottery clay must be able to form a mound when compacted together. This compacted material, however, needs to be pliable as well, so artisans can shape the clay into different forms. The small particles in clay, called platelets, give clay its plasticity, but too much plasticity can be unmanageable. Bentonite, for example, is a clay that is used in industrial applications and contains too many platelets to be appropriate for pottery making.
Aging Pottery Clay to Improve Plasticity
To get the correct amount of plasticity from pottery clay, artisans typically age the clay before working with it. Aging clay involves adding water to dry pottery clay, mixing the clay, and allowing the clay to sit for a specific time before it is used. Once the pottery clay has rested, artisans knead the clay to make it smooth before proceeding to form it. Buyers who prefer to skip the aging process can purchase pre-mixed, boxed pottery clay that has water added to it by the pottery clay's makers. Since this type of clay has had time to cure, it can be used immediately once it is removed from its packaging.
One vulnerability associated with clay is a tendency for clay to crack while it dries. To avoid cracking, pottery clay must have porosity. Porosity refers to the coarseness within the pottery clay's particles that allow water to dry at the same rate, reducing the occurrence of cracks while the clay cures. A high level of porosity, however, impacts the amount of water that a pottery clay object can retain once it is dried. Buyers who are hoping to make pottery objects, like pitchers or water bowls, should avoid pottery clays, like earthenware clay, which are too porous, and opt instead for pottery clays with a higher absorption factor, like stoneware clay.
Vitrification occurs once the clay begins to harden and forms a glass-like surface as it is fired in a kiln. During this process, pottery clay typically shrinks as water is forced out of its chemical composition. Too much vitrification can cause a pottery object to melt, however, so buyers should always read manufacturer's recommendations for the correct amount of temperature and time to use when firing a pottery piece.
Types of Pottery Clays
Different types of pottery clays have various plasticity, porosity, and vitrification levels, so buyers should choose pottery clays based on the pottery projects they have in mind. Since specific types of clay often do not include all of the properties that a project requires, buyers should be prepared to mix certain clays to achieve their desired results.
Kaolin clay is also referred to as "china clay", and is used in a number of industrial applications including cosmetics manufacturing, agriculture, and rubber making. For pottery making, kaolin clay is essential when creating porcelain or china pottery. Because kaolin has a low level of plasticity, however, it must be combined with other clays to form pottery objects. The purity of kaolin clay also produces a clay that has one of the highest firing temperatures available.
Like kaolin clay, ball clays are usually combined with other clays to form pottery. The biggest drawback of using ball clay alone is ball clay's excessive level of shrinkage when it is fired. Shrinkage levels for pure ball clay objects can exceed 20 per cent of an object's original form, making it an inappropriate option for most pottery applications. Ball clay has a high level of plasticity, however, which can help increase the malleability of other types of clay. Buyers using ball clay to strengthen kaolin clay for porcelain or china pottery objects should limit the amount of ball clay they include in their clay compound to less than 15 per cent if they want their finished product to have a high level of translucence.
Fire clays are pottery clays classified by the amount of heat resistance, also called refraction, that they contain. To have a high level of refraction, fire clays contain little-to-no iron. Able to withstand intense heat, fire clay is instrumental in creating the interiors of many industrial-grade furnaces. To take advantage of fire clay's properties in relation to pottery making, buyers can blend fire clay with stoneware clay to give a finished object a rough appearance. Fire clay is also popular when creating terra cotta pottery with a porous, coarse look.
Earthenware clay is sometimes called "common clay" because it is the most frequently found type of clay in nature. In its natural state, earthenware clay can include colours ranging from neutral tones like grey or brown to colourful hues like red or green. Earthenware clay retains some of its original colour once it is fired, so buyers interested in creating primitive pottery projects may prefer to experiment with natural finishes for earthenware clay. If buyers aim to create water-retaining pottery objects, they must combine earthenware clay with another, non-porous clay, like stoneware clay, to offset the porous nature of earthenware clay.
Stoneware clay is a good option for artisans who want to create pottery that is both durable and water-resistant. Generally, stoneware clay has the plasticity and porosity that can match most pottery projects without combining the clay with another type. Additionally, stoneware clays are highly adaptable with a variety of finishing techniques, such as the application of most glazes.
Drying Pottery Clay
When shopping for pottery clay, buyers find that there are often one of two types of finishing techniques listed on a clay package: air-dry or kiln-fired finishing. Both types of finishes have their advantages and disadvantages, which buyers should be aware of before purchasing their clay.
Pottery Clay Type
Can be dried by allowing an object to sit uncovered in open air, usually over the course of a number of days; good option for buyers who do not have access to a kiln
Fired in a kiln at high temperatures and can be dried within a few hours
Air-dried clays do not have the strength or water resistance of kiln-dried clays and lack transparency when they are finished, so they are often not applicable to many pottery projects. Buyers who want to experience some level of pottery making without the expense of firing pieces in a kiln, however, can still use air-dried clays to complete many decorative pottery pieces.
Buying Pottery Clay on eBay
eBay is a great resource for buyers who are interested in purchasing pottery clay to meet the challenges of any pottery projects. When shopping on eBay, buyers can source a variety of clays as well as related products, such as colouring oxides, moulds, clay tools, and brushes. Using a keyword search is the fastest way to find pottery clay items and accessories on eBay, so buyers can add relevant keywords, like "electric potter's wheel", to the search field on any eBay page to locate items quickly. Since pottery clay is sold in packages by weight, buyers should make sure they purchase enough clay to finish a project before they begin. A single kilogramme of clay is usually sufficient for creating a soup bowl, while a large soup tureen with a lid may require approximately 2.5 kg or more when waste is taken into account. To find the exact weight of a package of pottery clay sold on eBay, buyers should read the product listing that accompanies the clay, which lists the weight specifications of each package.
Pottery clay has been used since ancient times to create objects that are desirable for both their form and function. The specific elements that make up the composition of pottery clay range across pottery clay types, so buyers interested in purchasing pottery clay should understand what to look for before buying clay supplies for their pottery projects. Pottery clays vary based on plasticity, porosity, and vitrification, which, in turn, determines how pottery clay can be used. Most pottery clay types need to be combined with other types to achieve a workable compound. Because of this, buyers should determine the blend of clays to purchase based on the type of pottery object they are creating. Additionally, pottery clays can be air-dried or kiln-fired to set the clay, which impacts the durability of a finished pottery piece. Luckily for buyers, eBay carries a wide selection of pottery clays and related products that can match the challenges of any project a buyer has in mind.