‑‑ property of a material which permits change of shape when an
external force is applied and retention of that shape when the force is removed.
of the Material --
slip ‑‑ a clay in liquid suspension
plastic ‑‑ clay that can be manipulated without cracking
leather hard ‑‑ most moisture has left the clay, and it cannot be
bent without cracking
bone dry or greenware‑‑ physical water has left the clay
bisque ‑‑ preliminary firing of ware to harden it for glazing
vitrification ‑‑ hard, glassy and non‑absorbent quality of a
clay body or glaze
maturity ‑‑ the temperature or time at which a clay or clay body
characteristics of maximum non porosity and hardness; or the point at
the glaze ingredients enter in complete fusion, developing a strong bond
the body, a stable structure, maximum resistance to abrasion, and a pleasant
of the clay in either drying or firing; in drying, shrinkage
between plastic and leather hard (7‑15% for stoneware clay bodies. 20
for porcelain clay bodies.) During this transitional period, work should dry
under plastic or in the damp room.
Defects ‑‑ warping and/or cracking caused by:
forming imperfections ‑‑improper melding of seams or stressing of
creates points of weakness
differential water ‑ all forming operations‑ even though starting
water distribution ‑ result in uneven distribution of water (i.e. water
bottom of thrown piece, outside edges of extruded form, outer surface of
non‑uniform drying ‑ such as top drying first, or side exposed to
moisture gradient ‑ result of drying process, where water leaves surface
replenished from interior
drying too quickly.
Defects ‑‑ warping and/or
cracking caused by:
improper support for cantilevered parts
uneven temperatures in kiln
discrepancy between clay and glaze rates of expansion
cracking due to ware being prematurely removed from the kiln
AND CLAY BODIES
formed from the disintegration of granite and other feldspathic rocks. The clays
used by potters consist mainly of the mineral kaolinite. Kaolinite particles are
shaped like very thin plates less than 2 microns in size, fine grains of sand
being huge in comparison. These particles are flat and cling together like a
deck of wet playing cards, giving clay its property of plasticity.
clays are mixed together to create clay bodies. Each type of clay contributes
its particular properties to the clay body ‑ color, texture, plasticity,
dry strength, etc.
Clays: have remained more or less at the site of the decomposed rock from which
they are formed. They are less plastic than sedimentary clays, and because they
have been subject to fewer erosive forces, their particle size is much larger.
Clays: have been transported far from the site of the parent rock, by the action
of wind c‑ running water. This action had considerable effect on the
mixture and breakdown of minerals, and therefore the particles are very fine and
the clay more plastic.
a very pure form of clay that is white in color and vitrifies only at very high
temperatures. An important ingredient in all high‑fire whiteware and
porcelain bodies. May be of a sedimentary or residual type, and more or less
Clay: extremely fine‑grained, plastic, sedimentary clay, often including
much organic matter. Added to clay bodies to increase plasticity.
plastic, and fire in the middle range of temperatures, from cone 5 to cone 10.
Contain more impurities than kaolin, such as calcium, feldspar and iron, which
lower the maturing temperature and impart color to the clay.
high‑firing clay with varying characteristics of color, composition and
clays that contain a high percentage of iron oxide .which serves to lower the
maturing temperature of the clay. Rather fragile and quite porous when fired.
firing clay that is white in color. Also known as China.
BODIES sold at STATE
Types of Clay Bodies:
Earthenware ‑ (not sold at State. If student brings earthenware to class,
student must see instructor for special firing instructions) white or red body
designed to fire at a low‑temperature (cone 04-1) and to remain porous.
The bisque firing is generally done one to two cones higher than the glaze
Stoneware* ‑ (primary clay used at State) high‑fire ware (cone 8)
with slight or no absorbency, and tan‑gray to red, yellow and brown in
color. More plastic than a porcelain clay body. Most bodies contain grog or sand
to make the clay stronger and more versatile in its plastic state.
Porcelain" ‑‑ hard, non‑absorbent clay body that is white
and translucent. The bisque is low‑fired and the glaze is high‑fired
(cone 8). Smooth texture allows fine detail.
Raku body ‑‑ generally a stoneware body with at least 20% grog to
counteract thermal shock due to rapid firing process. Raku bodies remain porous
after firing. Most of the stoneware bodies sold at SFSU are suitable for raku.
bodies sold in Tool Room (#272) on second floor:
excellent throwing body medium
good throwing body excellent
excellent hand building
medium hand building
medium hand building
poor hand building
poor hand building