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