PROCESSING OF CERAMICS

A) Methods of Making Clay

In order to process ceramics, mud, or clay is obtained and cleared of all plant material, stones, paper, metal scraps, wooden pieces and other impurities. Afterwards, it is mixed with water and thoroughly pounded until a smooth, even mixture is obtained.

Next comes the shaping of this mixture into the desired object. This art of using earthen material to make various objects of earthenware, stoneware, porcelain and others is known as pottery. A very useful tool in pottery is the potter’s wheel shown in Fig 2.16. This device has a flat, horizontal disc, which can be rotated. When in use, earthen material is placed on the disc and this disc is given a gentle spin. A potter then places his hands against the material to mould the desired object as the disc spins.

After shaping or moulding, the next process is baking or firing. Baking is done in a kiln or a fire. The objective of this process is to harden the moulded articles by heating them. Afterwards, the objects are allowed to cool gradually. They are now ready for use although a further process of decoration may be carried out on them before use. Some techniques of decorating pottery are impressing and stamping, incising, carving, piercing, slip decorating, burnishing and polishing, decorative glazing and painting.

The same basic processes are involved in the manufacture of high temperature bricks (otherwise called refractory bricks), red bricks, facing bricks and others. Although, in this case use is made of several machines since these are usually done on a fairly large scale.

 

B) Processing Glass

There are two types of glasses namely:

• Inorganic or mineral glass.
• Organic or safety glass.

Inorganic glass is classified as a ceramic while organic glass is usually classified as a plastic. Most glasses in use are of the inorganic type.

Basically, inorganic glass is produced by melting together various proportions of silica (SiO2), soda ash (sodium carbonate, Na2CO3), limestone (chalk, CaCO3) and dolomite by firing them in a furnace. All these become a molten mixture which afterwards is very slowly cooled using a process called annealing. Glass results from this cooling process.

Various shapes of glass are made by blowing air into the mixture while in its molten state. All these stages are undergone through the use of appropriate tools and equipment.

 

C) Processing Asbestos

Asbestos is a mineral fibre obtained from certain types of rocks known as asbestos rocks. A material is described as a fibre if its structure is made up of barely visible threads. Asbestos fibre usually has a whitish colour although some varieties may have shades of blue, green or yellow.

Asbestos rocks are mined or quarried. A quarry is a place on the ground surface, where rock is obtained. Afterwards this rock is crushed and the fibre separated usually by blowing air through the crushed material. The material is then made into various shapes.

 

D) Processing Bricks and Tiles

A brick is a small building unit in form of a rectangular block, formed from clay or shale, or their mixtures and baked in a kiln in order to become strong, hard and resistant to heat. Shale is a kind of soft rock that splits easily into layers when crushed. Bricks have various standard sizes.

A tile is a plate of baked clay for covering walls (such as in the bathroom), roofs, floors etc. A tile may be plain or decorated with hollow spaces. Bricks and tiles may be used with the natural reddish brown colour, painted or glazed (i.e. covered with a glassy surface).

 

E) Processing Cement

Cement is the binding material used in building and other civil engineering works. The commonest type of cement is Portland cement.

To manufacture Portland cement, substances containing calcium carbonate CaCO3 (e.g. limestone, chalk) and substances containing alumina or silica are obtained and cleared of undesirable foreign materials like vegetable material and gravel. These substances are then mixed and ground. The ground mixture is heated in a kiln. Chemical reactions cause the mixture to turn into dark coloured hard balls called clinker. This clinker is ground into powder, thus forming cement. Cement sets and hardens when mixed with water.

 

F) Processing Porcelain

Porcelain is made in various types referred to as soft, hard, and others. However, the most common porcelain is called bone china. In making bone china, bones of cattle are roasted producing bone ash. This bone ash, rich in calcium sulphate (CaSO4) is ground and mixed with ground feldspar rock. The product is porcelain.