VARIOUS KINDS OF CLAY

The principal raw material is of course clay, some people say even paste !
 

1. Its chemical and physical characteristics

Clay is a permanent production of the Earth. It is produced by the erosion of the surface of the earth's crust, in particular by the action of the water (precipitations, rivers and glaciers) which disintegrates by crushing and chemical action the rocks by splitting them in increasingly small particles.

Its chemical composition is very similar to the average decomposition of the surface of the Earth, as a whole, which is made up for 75 % of silica and alumina. The other elements are various oxides and of course water. Clay is primarily produced by the most common mineral on the Earth : the feldspar. One defines clay in general, in its absolute chemical formula - kaolinite - as an aluminosilicate hydrated (Al₂O₃ 2SiO₂ 2h₂O).

Humidified clay tends to keep the form that one gives him, this property is called plasticity.
 

2.  Various kinds of clay

• Kaolin : it is a primary clay formed by the decomposition of feldspar, very refractory (its melting point exceeds 1800°), especially used in the composition of glazes.

• Very plastic clays or "ball clays" : it is almost impossible to only use them in pottery because of their great plasticity. They enter also often in the composition of glazes.

• Fireclays : They do not melt nor do not become deformed up to 1500°approximately. They are used in the manufacture of refractory bricks or radiants for the furnaces, boilers and crucibles. The coarse texture of some of these clays make of these a good element for sculptures of great dimension.

• Clay for saggars or "gazettes" : they are the clay boxes in which one cooks objects safe from the flame and direct radiation of the furnace. It is a rather plastic clay which must resist once cooked the thermal shocks and many cookings.

• Porcelains : These are clays rather not very plastic, rather refractory, whose vitrification of surface ranges between 1280°and 1400°. They are remarkable by their whiteness and a certain transparency after cooking.

• Stoneware : These are plastic clays whose maturation or vitrification is carried out from 1200°to 1300°approximately. With these temperatures one says that the shard " is closed " : clay is not porous any more.

• Earthenware : it is the majority of the usable clays, one can thus call them common clays. They are cooked from 950° to 1100° approximately. They are contained in majority of the potteries in the world, in the manufacture of bricks, tiles or pipes.

• The adobe : it is the clay used for the manufacture of bricks dried with the sun. It contains sand and stones.

• Terra-cotta : clay cooking at low temperature, with grog, it is used for manufacture of large sculptures.

• Bentonite : volcanic clay of origin which is used to give plasticity to the pastes and to deflocculate the glaze.

Of these various kinds of clay, the most used in our studios of pottery  is without any doubt earthenware. It is indeed a clay which offers large varieties of aspects,  as in color as in texture. Here after is what you can find in this " family ":

• Colors : Clay white, red, leather, yellow, brown, black, pink, mixed, even green.

• Textures : You have the choice between fine or grogged earthenware. The grog is added in the original clay (except in the case of the naturally grogged brick clay) ; it is either made of already cooked clay crushed in more or less large particles, or of pozzolana (volcanic rock). These small hard grains are classified according to various sizes (0 to 2 mm) and are contained in clay according to various percentages. It can be even smoothed perfectly and polished very easily. Grogged earthenware allows the development of thicker parts, it supports the differences in thickness better and decreases the risks of cracks in drying. Its aspect with the touch can be less pleasant and its not so easy to polish it.