CLAY PRODUCTS FORMING TECHNIQUES

Clay products forming techniques are of three basic types Surfacing, Casting and Moulding.

Surfacing are mainly in the form of thin coatings. These are applied by Daubing -rubbing in a thin paste of clay, Brushing or Coating -applying or spraying very thin layer, Dipping -immersing a product in clay slurry, Painting -creating very thin lined patterns or area fill-ins.

Castings are made with forms for shaping. For centrifugal process single outer form is required, but for other methods a set of inner and outer moulds may be necessary. Removal of inner mould from enclosing shape is tricky, and may require multi unit moulds. Bricks are called table cast because it requires a ring or edge frame over a flat surface. Casting can be done by extrusion through a shaped orifice.

Moulding is a process of shaping. Shaping the liquidised or pliable raw material with or without a shaping form called a mould. Other techniques include traditional hand-building methods such as rolling soft or hard slabs, coil formation and pinching. Small hand tools are used for shaping edges, cutting and engraving. A potter’s wheel is most popular shaping device.

The forming process, is determined by the clay ingredients available and the type of item required. Manufacturing processes consist of one or more of the following three methods.

1. Dry to semi dry or semi plastic process

2. Wet or plastic process

3. Liquidized process.

Dry or semi plastic processes are adopted in several conditions such as: where water supply is scarce, the condition of soil is very plastic and where high density mass is required.

In dry arid areas and sea coast locations availability of water for construction is a major issue. Quantity of water required to achieve homogeneous mass can be partly reduced by greater compaction. However, high compaction of the soil mass requires energy or an industrial set up. Such dry or semi-dry compaction is used for formation of high end ceramic products, such as the electric insulators and flooring tiles. For dry or semi dry high pressure castings, the ingredients have to be in pure state, thoroughly mixed, and homogenized with low amounts of water.

Dry or semi plastic processes are required for highly plastic soil which swell in volume due to addition of water. To avoid cracking on drying, the proportion of water is reduced with higher compaction. Outdoor clay surfaces such as the Cricket pitches, tennis courts, cinder tracks, rural raw roads are all created with low proportion of water but greater compaction. 

Wet or plastic processes allow sufficient time to a crafts-person to mould and finish an article. Moisture removal from such articles needs to be very slow and at a controlled rate. The item should have even wall thickness and transition from one thickness to another, if any, must be very gradual. Some wet or plastic clays require addition of fibrous materials to prevent cracking on drying. Sun-dried bricks or adobe and clay sods used to insulate roofs in a hot arid climate, Kutchh style wall decorations (Gujarat, India) of clay are examples of this type. Most of the household pottery products are manufactured through this process. 

Liquidized processes are used where through mixing of ingredients by wet grinding and solution with water is necessary. Liquidized process allows greater quality control. Industrial processes where standard size and shape items of very fine wall thickness and tolerance are necessary, porosity of the product is not a problem, very costly or rare ingredients are to be conservatively used, and where an additional quantity water for chemical reaction (such as for Plaster of Paris) is required. Removal of any additional water and moisture is more acute here than in wet or plastic processes. Most of the cow dung coating or daubing or mud plasters are of this type.

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