66
Partial analyses of the washed and unwashed clay are appended.
Washed.
Unwashed.
per cent.
per cent.
Silica
Si02
46.06
47.68
Alumina and Ferric Oxide
A1203
Fe208
}...
37.90
36.13
Lime Magnesia
Ca0
trace
Mgo
trace
Loss on ignition
14.73
14.06
It will be seen that there is little difference in chemical composition between the washed and unwashed materials, but there was a considerable difference in their physical characteristics, the washed material being very much more plastic.
Vessels made from the neat washed clay, after being fired to a temperature of 1060°C. for 6 hours, were strong and of a fairly good colour and free from "iron spots". Some of the test pieces, however, had cracked badly, and in all cases there was a strong tendency to warp.
Earthenware Mixtures.
A number of test pieces were made from each of the earthenware mixtures tabu- lated below. The unwashed clay used in these experiments was ground before use to pass a sieve having 120 meshes per linear inch.
A per cent.
B per cent.
0 per cent.
D
per cent.
i
Unwashed clay Washed clay
39
34
Flint
45
45
Cornish stone
16
16
Ball clay
5
91991
40
45
15
81999
30
45
15
10
These mixtures after firing should give earthenware bodies of approximately the fol- lowing compositions :---
A per cent.
B per cent.
C per cent.
D
per cent.
Silica
SiO2
76.2
76.2
76.4
76.5
Ferric oxide, Fe 203
Alumina .....Al203
}
19.2
19.2
18.7
18.7
Ca0, Mg0
4.3
4.4
4.6
4.7
Bases
2
K20, Na20