China Clay

China clays, also referred to as kaolins, are used to produce traditional ceramics when the color of the finished object and its high temperature performance are important.

Table 4 Typical compositions (weight percent) of some secondary china clays [3,22,23]

Location

SiO2

AlA

Fe2O3

TiO2

CaO

MgO

K2O

Na2O

H2O

Georgia

45.8

38.5

0.7

1.4

Trace

Trace

Trace

Trace

13.6

Florida

45.7

37.6

0.8

0.4

0.2

0.1

0.3

0.1

13.9

South Carolina

45.2

37.8

1.0

2.0

0.1

0.1

0.2

0.2

13.7

China clays generally contain large proportions of the mineral kaolinite, but can con­tain substantial amounts of other clay minerals. In all cases, the content of Fe2O3, TiO2, and other potential coloring impurities is low, resulting in bodies that range in color from white to ivory. China clay is found in both residual and secondary deposits. As detailed in Tables 3 and 4, the compositions of most china clays are slightly Al2O3 poor compared with the composition expected based on the mineralogical composi­tion of kaolinite (Al2O3 2SiO2 2H2O is 46.6 wt% silica, 39.5 wt% alumina, and 13.9 wt% water) due to the presence of impurities. China clays tend to have a moder­ate particle size (1-2 pm). Because of the particle size, china clays produce moderate plasticity during forming compared with other clays. Drying and firing shrinkage also tend to be moderate. China clays are used in many traditional ceramics, including pottery and stoneware, along with refractories and finer ceramics such as hard porcelains.