An Introduction to Ceramics The Science of Clay: An Introduction to Ceramics Tara Maikranz

The Chemical Make-up of Clay Al203 2Si02 2H20 alumina, silica, water Calcium carbonate, talc, grog, iron oxide Plastic- malleable, easy to form and shape The clay body is held together firmly, but not rigidly, by the collapsed molecules of alumina and silica which intermesh like a basket weave

Dehydration Leather hard- beginning of dehydration process; stiff but moist Bone Dry- green ware; next step in dehydration process; water has evaporated; unfired; hard but fragile -

Kiln- enclosed heat source in which clay is fired (heated). Dehydration process is essentially complete at about 500° C when the chemically combined water is driven out of the clay in the kiln.

Vitrification Bisque; early stages of vitrification; clay is still porous; ready to glaze. Vitreous- the pottery will no longer absorb water chemically and can never return to its original plastic state. Vitrification- at the earliest stage, the molecules of silica and alumina begin to collapse together, partially filling the voids left by the escaped water and creating a relatively firm bond which gives the fired clay body its hardness and strength. Vitrification continues with the melting of some of the impurities in the clay, chiefly beginning with iron oxides. These combine and melt to form a glassy substance which flows around the remaining alumina molecules, gluing them together in a rigid bond. At higher temperatures mullite, an alumina silicate characterized by long, needle-like crystals, begins to grow and becomes interwoven with the glassy substances in the clay body. Glassification and the growth of mullite combine to form an extremely rigid and hard crystalline structure in the clay body which gives stoneware and porcelain its characteristic clear ring when tapped.

Bisque fire- bone dry clay fired at low temperature to bisque; early stages of vitrification; clay is still porous; ready to glaze.

History of Pottery Pot making dates to about 10,000 BC in parts of Asia Pinch or coil technique Pit firing Utilitarian All of the earliest work was earthenware with no glaze

Burnishing One of first surface techniques Functional Aided in the retention of water, but did not vitrify completely Rub with smooth object at leather hard stage Time consuming – usually takes at least three burnishings

Glaze Silica Forms a glass coating Glaze fire- fire after glaze is applied to bisque ware. Glaze is non-porous after firing.

The earliest known glazes are found in the Nile valley about 5,000 bc High temperature ceramics; stoneware, porcelain, glazes, were developed by the Chinese about 2000 years before the rest of the world

Potters Wheels are known to have been used in the Indus valley (Pakistan and northern India) about 4,000 BC but possibly earlier True potters wheels appeared around 3000 BC in several areas of the middle east and China

Slip, Score, and Coil Technique used to attach pieces of clay together Scoring- scratching the surface of the clay Slipping- adding water or liquid clay to scored surface Put slipped and scored surfaces together Coiling- add small coil (tube) of clay around slipped and scored edges and smooth to reinforce connection

References http://www.depauw.edu/acad/art/Faculty/DHerroldWeb/pages/clay.html http://www.mii.org/Minerals/photoclay.html http://www.pottery.netfirms.com/assignments/assign/terms/glossary.htm#potparts