PROPERTIES OF CLAY

What distinguishes clay a potter uses from plain ol’ dirt?

Three Essential Properties

• Plasticity

• Porosity

• Vitrification

PLASTICITY

• The ability to hold its form while at the same time be pliable enough to be moved by the potter’s hands

• Determined by the size and shape of very fine grains or particles of clay called platelets

Platelets

Platelets slip & slide & align themselves like bricks in a wall.

Clay must be wedged thoroughly to mixthe platelets up, which helps minimize cracking during the drying phase.

PLASTICITY (cont’d.)

• Small platelets are easier to form; however,…

• If the platelets are too small, the clay will be too plastic = too sticky = hard to form.

• Bentonite is a type of clay whose platelets are extremely small not good for potters

PLASTICITY (cont’d.)

• Water is very important to plasticity.• The amount of H2O must be just right for the

size of the clay particles in the particular clay body.

• Too little H2O clay too dry clay will crack when drying

• Too much H2O clay too wet won’t hold shape

POROSITY

• Clay body must be porous enough for H2O to escape evenly from all parts of the form.

• Clay is composed of chemically bonded H2O and H2O that is added to aid plasticity.

• Added H2O must be allowed to evaporate evenly or cracks will appear in the form.

• The smaller the particles, the less porous the clay.

• This causes H2O to be trapped, which in turn causes

• uneven drying• cracking• breakage

VITRIFICATION(vi-tri-fi-ca-tion)

• The process of becoming glasslike• Any substance will melt at some temperature.• Most materials tend to become soft and deform before

they melt, but…• Clay can hold its shape and not sag or slump in the

primary melting stages of the firing process. • Different clay bodies are fired at particular temperatures.

If fired at a too high temp, the clay will melt into a blob!• Earthenware clay is fired at lower temperatures.• Stoneware clay is fired at higher temperatures.

Phases of Clay(shaping forms phase)

•Plastic–workable stage –molding stage–can recycle by adding water –Can easily join to other pieces

Phases of Clay (drying phases)

• Leather-hard–stiff and will hold its shape; – join to other pieces; –carve into; –can recycle by adding water

• Greenware–bone dry; –can be carved into; –very fragile; –can recycle by adding water

Phases of Clay(firing phases)

• Bisqueware–fired once in kiln; –cannot be recycled; –underglaze can be applied

• Glazeware–second firing; –cannot be recycled; –Gloss glaze is turned to glass

Storing & Drying Clay Work

• The added water in the clay body will evaporate if left out & exposed to the air.

• Humid air will cause the piece to take longer to dry.

• Dry air will cause the piece to dry faster.

Storing & Drying Clay

• All work must be stored in an air-tight plastic bag each day to prevent drying.

• It is best to dry your work slowly and evenly.

• A solid ball of clay (such as the ‘body’ of an animal you might make) will never dry because the added water on the inside can never escape.

Firing Clay

• Low-fire clay is heated to about 1200O -1500O

• Water boils at 212O

• Any added water that has not evaporated, and all chemically bonded water will boil. Boiling creates steam. Steam needs to escape. If it’s trapped inside the walls of your piece and cannot get out, it will become violent and explode your piece all over the kiln, which is not a good thing and could ruin some or all of the other pieces in the kiln.

• If clay is fired to a temperature beyond what is recommended for that particular clay body, it will melt into a blob of hardened goo.