Ball clays are characterized by their plasticity. The small platelet or particle-size structure of ball clays imparts great plasticity to the clay in the moist stare. However, ball clays require large amounts of water to achieve plasticity. Plasticity and water can result in dry shrinkage, fired shrinkage, and warping. That’s one reason why ball clays are not the sole component in any clay body formula . Another reason is because they can contajn elevated amounts of carbonaceous matter, which aid in plasticity but can lead to burn-out problems in firing stages. Burn out is when carbonaceous matter is trapped in the clay body and begins causing carbon deposits in the clay body.
Ball clay is one component of a clay body formula char can contain ocher types of clays, feldspar, Rint, and other raw materials. The raw materials determine the clay’s even tual forming method, firing temperature, and fired color.
Relative Sizes of Clay Particles
Bentonites and ball cloys have the smallest platelet size, followed by stoneware cloys, kaolins, and fire clays. Earthenware clays can vary in platelet size depending on their individual location and the geologic forces used in their formulation. Generally, the platelet diameter of clays can range from 100 microns to micron.
Several common types of ball clay are used in clay body and glaze formulas. There are variations in particle-size distribution, organic content, and chemical makeup, along with other variables throughout this group. However, each clay has a data sheet that can offer information on its eventual use in clay bodies and glazes. Data sheets can be obtained through the clay mine or ceramics supplier. Ball clays vitrify at approximately 2000°F to 1200°F (I093°C to 1204°C).
Fine Ball Clays: A typical data sheet listing ball clays will indjcate which clays have finer or coarser particle size distributions. Particle size can be charted on a graph, giving a fingerprint (or unique formula) for each ball clay. The distribution can be complex because all clays have particles as large as 100 microns and as small as 0.1 microns (I micron= l/24,500 of an inch). The number of particles in each range determines the overall fineness or coarseness of the clay.
Fine ball clays have greater plasticity and increased strength when dry, which makes them suitable for plastic forming operations, such as throwing and hand building.
Finer clays can tighten a clay body structure, causing an exothermic reaction (releasing heat), preventing the oxida tion of organic matter when heated in the 572o to 932°F (300° to 500°C) temperature range.
Coarse Ball Clays·: Coarse ball clays are less plastic and better suited for casting slip clay formulas. Coarser ball clays allow water to “wick” through the liquid clay into the mold , building up an acceptable clay thickness in the cast piece. Coarse panicle-size ball clays also allow the clay body to “firm” quickly, or develop durability after draining out the excess slip from the mold. The leather-hard piece can be handled faster when it is taken out of the mold. Coarser ball clays will contribute to greater durabi lity and firmness in a cast piece when it is removed from the plaster mold.
Coarse ball clays also can be used in clay bodies that do not require the degree of plasticity needed in throwing bodies. Larger particle-size ball clay can be used in hand building, coil, Ram press, or dry press forming clay bodies.