Definition; Fine grained, earthy material, plastic when wet and hardens when heated, consisting hydrated silicates of aluminum.
Constituents; -silicon, aluminum, magnesium, potassium, and ion. Terracotta; – Unglazed, reddish brown hard baked clay, often used to make pottery items. It’s fired clay in a permanent irreversible state.
=Clay is a recyclable material, but when fired it becomes permanent and irreversible.
Properties of clay
- Shrinkage under firing.
- Color change after firing.
Uses of clay
-Making bricks, tiles, pots, and sculpture.
Possibilities of clay;
-Flexible material, easily manipulated with hands.
-Common material in most swamps in Uganda.
-Recyclable if not fired.
-Permanent material when fired.
Limitations of clay
-Long preparation process (especially for pottery).
-Irreversible if fired.
This involves mixing of materials, removal of large stones, roots, crushing of large particles est.
- The plastic method;
-Is common to the brick industry & pottery.
-Moisture available in ciay delivered is utilized. If the clay is too dry, water is added. It’s a Simple and cheap process, requiring no equipment.
-Mixed clay is used directly for forming products by hand throwing, coiling, extrusion est.
-Time consuming, and results in unevenly mixed clay.
-Less suited for clay types that do not slake easily i.e. hard and dry lamps need much time to soak up water, dissolve and become plastic.
- The wet method;
-Clay is dissolved in water to form slurry or slip. It is most suitable for casting, but slip can also be used to join parts of clay together.
- The dry method
-Clay is dried, pounded and crushed into powder only used for dry pressing in making tiles.
- 4. The semi dry method
-Combines dry and plastic methods.
-Dry and crush clay to powder, then add grog and sieve after mixing to eliminate large particles, add
Water and mix till plasticity is attained.
Merit; Produces a homogeneous mixture.
Demerit; Working in a dusty environment.
Steps for clay preparation
*Mine clay from the swamp and dump it in a clay bin.
*Remove impurities like large particles and roots while clay is moist.
*Spread out the clay to dry.
*Crash the dry clay to powder.
*Sieve the powder to eliminate large particles and impurities.
*Soak powder clay in water for some time.
*Add grog to the clay. (Grog is crushed fired clay).
*Mix the clay, knead and wedge it to plastic stage.
watch this video for more information about clay preparation
Testing plasticity; Make a coil and wrap it around your finger, if it doesn’t develop cracks, then it Is plastic.
Store the clay. Cover it with plastic sheeting to keep the moisture inside.
Terms in clay
Grog; Also known as fire sand, is defined as crushed fired pottery of any type that is added as a temper to unfired clay.
Kneading; Process of folding, pressing and stretching a soft substance such as clay and working it into a smooth uniform mass.
Plasticity; Flexibility or ability for a given material such as clay to hold shape.
Slab; Is a thick flat broad piece of material, especially when cut or trimmed.
Slake; the process of dissolving dried, unfired clay in water to produce a slip or slurry.
Slip; A mixture of clay and water used as a decorative layer on pottery or for casting in moulds to form an actual piece.
Slurry; A liquid mixture of water and an insoluble solid material such as cement or clay.
Terra-cotta: Are Italian words for baked earth. Unglazed reddish-brown hard baked clay often used to make pottery.
Temper; Are non-plastic additions to clay, used to control shrinkage prior to the firing process.
Wedging; Process used to prepare clay for working by giving it an even texture.
Vase; is an open container, often used for storage purposes (made from ceramics) & glass and often decorated.
Kiln; An oven used for baking pottery/ Furnace for firing pottery.