Histories of UK potters and pottery manufacturers
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Last updated: 1st August 2011
GLOSSARY - CERAMIC MATERIALS
Antimony
A metallic oxide used as a yellow pigment.
Ball clay (Pipe clay)
Ball clay is a secondary clay, laid down as a sedimentary deposit. Its composition is 20 - 80% kaolinite; 10-25% mica, and 6-65% quartz, plus other minerals and varying amounts of carbonaceous material laid down in the same deposit. It is thus highly variable in composition and properties.
The name ball clay is derived from the original method of mining where blocks of clay roughly 25 cm on each side were dug by hand. The blocks were transported by horse and cart or barge and became rounded as the corners were removed in handling. The alternative name ‘Pipe clay’ arose as the clay was used to make the white clay pipes used in the 18th and 19th century for smoking tobacco.
Ball clays are highly plastic and are used in conjunction with less plastic clays (kaolinite) to improve plasticity and workability. Used alone, ball clay is susceptible to excessive shrinkage when dried or fired.
Bone ash
Bone ash is calcined (heated, cooled and milled) animal bone. Bone ash is hydroxyapatite (Ca5(OH)(PO4)3) and when used in the manufacture of bone china, can be added at up to 50% of the paste (the other 50% being kaolinite). The bone ash replaces feldspar in the paste and during firing, the hydroxyapatite is converted to anorthite (a calcium-rich feldspar) and tricalcium phosphate. In addition to adding translucency to bone china, the bone ash imparts a high resistance to heat transfer. Although strong and translucent, bodies containing bone ash are difficult to work due to low plasticity and are subject to warping in the kiln, it has been largely replaced by other materials.
China clay
Alternative names: kaolin, kaolinite
China clay is an non-plastic, hydrated alumino-silicate mineral with the chemical composition Al2O3.2SiO2 (Al2Si2O5(OH)4). It is the major component of most ceramic bodies. See kaolinite.
China stone (Cornish stone, Cornwall stone)
China stone is a feldspar-rich mineral derived from partially decomposed granite. China stone includes feldspar, mica, fluorospar, quartz and other granite-derived minerals such as kaolinite, but lacks iron-bearing minerals. China Stone is used worldwide as a flux in the manufacture of ceramic bodies and glazes. In the United Kingdom, the material is mined in one small area of Cornwall.
Clay
‘Clay’ is a general term for a wide range of naturally occurring minerals that have the common properties of being plastic at certain water contents and which will harden with dried or fired.
Clays are composed primarily of hydrated alumino-silicates formed into tetrahedral or octahedral plates. The particle size of a clay mineral is generally less than 0.002 mm (2 micron), much smaller than the silt and sand components of a soil.
Clays are formed by the weathering of igneous, silicate containing rocks (granites) and may occur ‘in situ’ as a primary clay, or as a secondary clay deposited after erosion, and water transportation from the source. Secondary clays often contain organic matter and other minerals deposited at the same time.
There are about 30 different clay minerals recognised by their differing chemical composition and consequent properties. All are hydrous alumino-silicates, but different members contain variable amounts of iron, magnesium, sodium, potassium, calcium and other positively charged cations. A clay’s properties are determined by its chemical composition, the spatial arrangement of the atoms in the clay plates, electrical charge on the plates and the presence of inter-layer cations that balance the electrical charge.
All clays have in common a very high surface area, the ability to absorb and bind water, and the properties of being coherent and plastic when wet and hard when dry. On firing at high temperature, certain clays form a very hard, vitreous mass. See china clay, ball clay.
Feldspar
Feldspar is a general term for a group of alumino-silicate minerals containing significant quantities (10-15%) of alkaline oxides (Na2O, K2O, CaO etc). In nature it is a common (50% of the planet) pale-coloured crystalline mineral found in granite and other metamorphic rocks.
Feldspar, or feldspar-rich materials are added to ceramic bodies to act as a flux to assist the melting and fusion of the other components. Sodium-rich feldspar (albite, Na2OSi2O3 6SiO2) is used in glaze formulations whilst potassium-rich feldspar (sanidine) is used as a flux in ceramic bodies to promote vitrification. A base formulation for a hard-paste porcelain body could be one quarter each of kaolinite, ball clay, silica and feldspar.
Fireclay
A clay with a mineral composition capable of withstanding high temperatures (c. 1500 degrees C) without fusion.
Flux
A material that, when heated with other materials lowers the melting point of the clay and other materials making up the ceramic body and helps to form a glassy, vitreous structure. Feldspar is the most common material used as a flux because it delivers the oxides of sodium, potassium, calcium and magnesium which are the actual fluxing agents.
Frit
A ‘glassy’ material made from a mixture of silica (sand), flux and other materials. The ingredients are fused at high temperature to make the glass-like compound which is then cooled and ground to a fine powder – the frit.
Frits are used in the ceramics industry as a component of pastes, glazes and fluxes because of their uniformity of composition, greater industrial safety, the absence of volatile contaminants and because they can increase the solubility of some highly insoluble compounds.
Glaze
The smooth, impervious, glassy coating applied to ceramics primarily to make the body hygienic and non-porous. A glaze may be transparent, translucent or opaque, white or coloured, and highly glossy to matt in surface texture.
Gloss
Gloss describes the shinyiness and light-reflectivity of a glaze. Glazes high in glassy materials (SiO2, B2O3) are glossy. Those high in Al2O3 tend to be matte and the SiO2:Al2O3 ratio is taken as a general indicator of glaze glossiness.
Grog
Grog is a filler material added to ceramic bodies to impart a slightly rough texture. Grog is manufactured by firing high-temperature fire clays, grinding the fired bricks, and then sieving to produce uniform particle size. An alternative name is ‘firesand’.
Iron red
Colour pigment extracted from iron oxide.
Kaolinite (Kaolin, China Clay)
Kaolinite is the most important clay used in the manufacture of ceramics, however, its main industrial use is in the paper industry. It is an alumino-silicate with the chemical composition Al2Si2O5(OH)4. Alternative names are China clay and kaolin, a name of Chinese origin. Kaolinite is formed from the weathering of other alumino-silicate minerals such as feldspar and is generally found as a primary clay mineral in soft white deposits formed in-situ.
Compared to many other clay minerals kaolinite has relatively low plasticity and for ceramics manufacture requires the addition of other materials.
Lead glaze
A glaze made from silica, salt, potash and fused with lead oxide. Lead glaze is transparent and glossy and was used from circa 1500 onward. Because of their toxicity, lead glazes were phased out for use on tablewares from the early 1800s.
Manganese
Metallic element used as a colour pigment to produce a colour palette ranging from from light purple, through puce and purple to browns and blacks.
Mazarine blue
A deep blue colour, derived from cobalt, widely used as a ground or banded colour.
Paste
A generic term for the clay mixture or formula used to make the ceramic item. Manufacturers would develop and use different ingredients in different proportions to make their own ‘paste’ and the formula would be a closely guarded secret. Thus, even within the category of body known as earthenware there would be a multitude of pastes.
Petuntse (Pottery stone)
Petuntse is a historical Chinese term for rocks with a high content of mica and feldspathic minerals that have been used as one of the ingredients in the manufacture of Chinese porcelain. It is generally similar in composition and use to China stone. Petuntse, with china clay, forms the base ingredients for hard-paste porcelains similar to those produced in China in historic times. See China stone.
Pipe clay
A clay mineral used for the manufacture of white clay smokers pipes and sometimes included in other ceramic pastes.
Slag
A stony material recovered as a waste in iron smelting and metal refining.
Slip
Clay diluted to a creamy consistency with water.
Smalt
Cobalt pigment mixed with molten glass, ground to a fine powder and used as an enamel.
Tin glaze
A lead-based glaze to which tin oxide has been added producing, when fired, an opaque, brilliant white glassy surface. Wares were dipped in the glaze and allowed to dry, leaving a uniform white powder coating that fused in the kiln forming the characteristic tin glaze.
