CLAYS
CHARACTERISTICS
OF CLAY
Plasticity
‑‑ 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.
States
of the Material --
1.
slip ‑‑ a clay in liquid suspension
2.
plastic ‑‑ clay that can be manipulated without cracking
3.
leather hard ‑‑ most moisture has left the clay, and it cannot be
bent without cracking
4.
bone dry or greenware‑‑ physical water has left the clay
5.
bisque ‑‑ preliminary firing of ware to harden it for glazing
6.
vitrification ‑‑ hard, glassy and non‑absorbent quality of a
clay body or glaze
7.
maturity ‑‑ the temperature or time at which a clay or clay body
develops the
desirable
characteristics of maximum non porosity and hardness; or the point at
which
the glaze ingredients enter in complete fusion, developing a strong bond
with
the body, a stable structure, maximum resistance to abrasion, and a pleasant
surface
texture.
Shrinkage
‑contraction
of the clay in either drying or firing; in drying, shrinkage
occurs
between plastic and leather hard (7‑15% for stoneware clay bodies. 20
25%
for porcelain clay bodies.) During this transitional period, work should dry
slowly
under plastic or in the damp room.
Drying
Defects ‑‑ warping and/or cracking caused by:
1.
forming imperfections ‑‑improper melding of seams or stressing of
the
material
creates points of weakness
2.
differential water ‑ all forming operations‑ even though starting
mix has
uniform
water distribution ‑ result in uneven distribution of water (i.e. water
at
bottom of thrown piece, outside edges of extruded form, outer surface of
pinched
form)
4.
non‑uniform drying ‑ such as top drying first, or side exposed to
wind, etc.
5.
moisture gradient ‑ result of drying process, where water leaves surface
before
replenished from interior
6.
drying too quickly.
Firing
Defects ‑‑ warping and/or
cracking caused by:
1.
improper support for cantilevered parts
2.
uneven temperatures in kiln
3.
discrepancy between clay and glaze rates of expansion
and contraction
4.
cracking due to ware being prematurely removed from the kiln
CLAY
AND CLAY BODIES
Clay is
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.
CLAY
TYPES
Individual
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.
Residual
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.
Sedimentary
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.
Kaolin:
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
plastic.
Ball
Clay: extremely fine‑grained, plastic, sedimentary clay, often including
much organic matter. Added to clay bodies to increase plasticity.
Stoneware
Clays: generally
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.
Fireclay:
high‑firing clay with varying characteristics of color, composition and
plasticity.
Earthenware
Clays: low‑firing
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.
Porcelain:
high
firing clay that is white in color. Also known as China.
CLAY
BODIES sold at STATE
Basic
Types of Clay Bodies:
1.
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
firing.
2.
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.
3.
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.
4
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.
*Clay
bodies sold in Tool Room (#272) on second floor:
Clay
strengths/weaknesses
wet color
Stonewares:
Soldate
60
excellent throwing body medium
hand building
light gray
Sculpture
mix 412
good throwing body excellent
hand building
creamy tan
(low grog)
Sculpture
mix
rough throwing
excellent hand building
creamy tan
(high
grog)
Immco
8-11 red
medium throwing
medium hand building
red brown
Immco
8-11 buff
medium throwing
medium hand building
discontinue
Sandstone
buff
medium throwing
poor hand building
yellow
B-Mix
white stoneware
medium throwing
poor hand building
cream
Porcelains:
Sleeper
porcelain
throwing
(see Reeves)
white
Reeves
porcelain
(see Sleeper)
hand building
white
Kai
porcelain
(see Sleeper)
hand building
discontinue