Week 6 Flashcards
Ceramic
Hard, brittle, nonmetallic material made from clay and other Earth materials and hardened by firing at high temperature.
Contains minute silicate crystals suspended in a glassy cement. Unlike a glass, a ceramic has a crystalline structure.
Cohesion
When slightly wetted, clay is also very cohesive (i.e. sticks together very well).
This again is due to the high surface area of the mineral grains.
Small amounts of water cling to the particles. Surface tension produced by the water film allows particles to stick together.
Primary Clays
Found close to their original site of formation due to little or no transport of clay particles.
Consist mostly of the clay mineral kaolinite
Secondary Clays
Found far away from their original site of weathering
Grog
crushed quartz or pre-baked clay fired under high temp. It adds rustic appearance (“tooth”) to ceramic items.
Why add grog?
Addition of grog to clay body has beneficial effect on shrinkage because grog does not retain water as fresh clay does.
Also allows gases to escape during drying process.
more grog = less shrinkage, less grog = more shrinkage
Earthenware
Made from secondary clays. Well suited for the manufacture of thick-walled articles.
Cheapest to make (secondary clays more common than primary clays)
Stoneware
Made mostly from primary clays, suited for the manufacture of vessels that are stronger than earthenware.
Also used to make non-porous products such as floor tiles and drainage pipes.
Allows for a wider range of glaze effects
Porcelain
Made from primary clays, suited for manufacture of very thin objects that are very strong
Methods of shaping
1) Hand building
2) Throwing
3) Slipcasting
Handbuilding
“Freeform” method of forming an item without the use of a potter’s wheel.
Throwing
Clay item is shaped on a spinning potters wheel
Slipcasting
Item is produced as a cast inside a mould. Slurry of water and ceramic ingredients (called “slip”) is poured into mould.
Air Drying
A ceramic piece is air-dried to “greenware” state before it is fired.
When clay dries, water evaporates from it.
As this happens, clay particles are drawn closer together, resulting in shrinkage.
If drying (and therefore shrinkage) is uneven, stresses are produced in the clay, forming cracks or warped areas.
This is why it is important to ensure that the piece is of fairly uniform thickness.
Firing: Klin Drying
Complete drying doesn’t take place until the piece is in the kiln.
This happens when the boiling point of water has been reached (100°C).
Vitrification
Finally, the mineral components of the ceramic fuse together due to intergrowth of mullite crystals and silica is molten.
When cooled, the molten silica filling spaces between interlocking mullite crystals becomes glass, binding the whole structure together.
The ceramic, now made of fused mineral components is hard and durable.
Differences in firing temperatures
Primary clays vitrify at higher temperatures because the clay body (almost pure kaolinite) has a very high melting point.
Secondary clays vitrify at lower temperatures (as low as 1,000°C) because iron oxides serve as a flux to lower the average melting point of the clay body b.
Bisqueware
A ceramic that has been fired in a kiln but has not yet been glazed. In cases where porosity/permeability is desired
(e.g. in flowerpots), this is the final stage of production.
Glaze consists of
1) Glass-forming minerals (e.g. silica, feldspar)
2) Stiffeners (such as clay)
3) Fluxes which lower the melting point of the other glaze components
