The earliest method of shewing was a simple squeezing of a thick paste of clay and water by hand into desired shape.
Invention of the potter's wheel sometime around 5000 RC.
in a town.named Worka in ancient Mesopotamia (modern Iraq) turned pottery into an industry. It was the first step in mechanization of the making of pottery. A potter's wheel is a rotating disk on which the potter throws the thick clay paste and shapes it using both his hands. The rapidly moving lump of clay needs only a gentle pressure from hands to be moulded into the desired shape. It is a fascinating operation to watch, but this seemingly easy job requires a high level of.
skill and training. As you might have seen, it is still a very-
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Jiggering (a) and jolleying (b) processes for making pottery
common method of making pottery in our villages. Even in the more advanced countries of the West, it is used for making individual art pieces of pottery.
For commercial production of a number of articles of the same shape and size like cups, saucers and vases processes known as "jiggering" and "jolleying" are used. These proces- ses use the potter's 'wheel in conjunction with plastic moulds.
Thus,'instead of throwing the clay on the wheel and shaping it by hand, clay is pressed with the help of a metal outline onto a convex (bulging outwards) plaster mould which repli- cates exactly the inner surface of the plate. This process is
Slip casting process for making pottery
known as jiggering. When the plaster mould is concave as in the making of a cup, the process is known as jolleying.
If a thin slurry of clay in water is made, it can easily flow and can take the shape of the container. This is the basis of a shaping technique called" slip casting". The free flowing clay and water mix, called" slip", is poured into a mould made of Plaster of Paris. In due course, the mould absorbs water leaving a thin layer of clay on the inside of the mould. This layer can be removed after drying to get an object that has the same shape as the inside portion of the mould. A number of pottery articles are made this way. Apart from these conven- tional methods, a number of modern methods of shaping exist. These are usually applied when dry powders are the starting materials, as in the case oJ advanced ceramics.
Broadly, pottery may be divided into three categories:
earthenware, stoneware and porcelain. The first is also known as porous pottery and the latter two as dense pottery.
The appearance and characteristics of the different forms depend upon the raw materials used and the method of firing, specially the temperature of firing.
Earthenware, is the oldest and simplest variety of pottery, which is still being widely produced in many countries in-
cluding India. In its most common varieties, the only raw material used is naturally occurring clay. After shaping the product by one of the methods described earlier, the article is fired at 900- 1000°C. Clay does not melt below 1600°C.
Hence, heating at 900- 1000°C only removes water. It sinters the pottery but no vitrification or melting occurs. This leads to a more porous and coarser product than that obtained when fluxes are present and vitrification occurs. The colour of the product is usually buff to dark red or grey to black.
This is due to the presence of iron in the commonly occurring clay, which on heating gives the dark brown coloured com- pound ferric oxide. Pots and jars, are the common articles made this way.
Some of the earliest potters applied primitive type of varnishes on the surface of the products to give them a smoother look and overcome disadvantages due to porosity.
But the resulting thin layer could give way on prolonged use.
Some ingenious souls found a much better and permanent solution. They covered the fired, coarse vessel with glass powder, feldspar or common salt and fired the vessel again.
During this second firing, the covering layer of glass powder, feldspar or common salt melted and the melt fllled the pores
Three varieties of pottery: (a) earthenware, (b) stoneware and (c) porcelain
in the clay body giving it a smooth finish. This process, known as glazing, has been widely practised since the ancient times for decorating as well as reducing the porosity of earthenware.
Glazing Brings A Shine
A new innovation was made when a tin salt was used for glazing- it gave an opaque white covering to the product.
Tin glazed earthenware became quite popular in different countries acquiring varied names. Thus Majolica, Faience and Delft are essentially the same material, the names vary- ing according to the place of origin. In fact, the art of tin glazing is quite old. Probably Assyrians (ancient inhabitants of the basins of river Tigris in West Asia) discovered this art as shown by the discovery of white enameled decorated brickwork of that period. Tin glazing was practised in Mesopotamia also from where it spread to Spain and then to Italy via the island of Majolica. Hence the popular name Majolica pottery came into being.
If a low melting substance, called flux, is added to the naturally occurring clay used for making pottery, the prod ucts obtained are hard and opaque. These are called stoneware. Fluxes used are usually feldspars. As the pottery object made from a mix of clay and flux is heated to around 1200°C, the clay particles remain unaffected and hold the object in shape, but the low melting flux becomes liquid or vitrifies. This liquid fills the pores in the otherwise porous body of the object leading to a non-porous or opaque body.
Usually, its colour is red, brown, grey or black. However, white stoneware can also be obtained if instead of the natural- ly occurring clay, a specially chosen clay which does not contain iron salts is used.
Much of the modern crockery is porcelain. Porcelain is translucent pottery obtained by using some specially selected white burning clay. This variety was first discovered in China
during the Tang dynasty (618 - 907 A.D.). Its widespread prod uction, however, started during the Yuan dynasty (1279- 1368 A.D.).
Kaolin, a white burning clay, and petuntse, a feldspar, were heated to 1450°Cwhen the feldspar vitrified to produce a transh:lcent product. The name kaolin itself is derived from Chinese "kao-liang" meaning high ridge, indicating that this white burning clay was first used for pottery making in China. The Chinese porcelain products were of great attrac- tion throughout the world. For long, European potters tried to unlock the closely guarded Chinese secret of making por- celain. Ultimately success was achieved in Germany in the 17th century by an alchemist named Johann Friederich
BOTTGER in the town of Dresden. This marked the birth of famous Dresden pottery.
In England, during the 18th century, a new variety - bone china - was discovered. This soft variety of porcelain was obtained by adding bone ash alongwith ground glass as fluxes to kaolin clay (also called China clay). These added substances vitrified to give a soft translucent look to the products.
In fact, one can have many variants of porcelain by varying the proportions of the basic raw materials-China clay, feldspar and silica. If clay is 50 per cent and silica and feldspar are 25 per cent each of the total raw materials taken, a hard porcelain is obtained. If the amount of clay is decreased by 10-20 per cent, a soft porcelain is obtained. Typical English bone china contains still less of clay, only 25 per cent. Of the remaining 75 per cent, two-thirds is bone ash and one-third cornish stone, a variety of feldspar. All these varieties are translucent but vary in colour from pure white to off-white to bluish white.