69
ed that it would be economically practicable to refine the red clay by washing. No tests were therefore carried out with the washed material obtainable from this clay. The colour of the clay, moreover, renders it unsuitable for use in good quality earthenware or china mixtures, and for that reason also no tests were carried out in this direction.
Pottery trials.
The clay used in the tests was ground to pass a sieve containing 60 meshes per linear inch, but attempts to mould this material by means of the jigger and jolley machine were unsuccessful, the clay being practically devoid of plasticity and possessing very little binding power. In order to determine whether finer grinding would improve the pro- perties of the clay a further quantity of the material was ground to pass a 100-mesh sieve.
It was found, however, that although a considerable improvement in the working properties of the clay had been effected it was still lacking in plasticity and difficult to work, but with care it was possible to produce fairly good pressed pieces.
Further trials were then made with a mixture of 99 per cent. clay +10 per cent. ball clay, both clays being previously ground to pass a 100-mesh sieve. The addition of ball clay effected a great improvement, and this mixture worked well on the wheel, it being possible to mould small thin-walled vessels with ease.
Test pieces made from this mixture and fired at 1060 ̊C. gave the following results-
Maximum firing temperature
Duration at
,,
Drying shrinkage (linear)
Firing
J
55
Total
33
JJ
Porosity Warpage Colour
°0. Hours
1060
6
per cent.
8.0
1.5
>>
3"
9.5
"}
55
33.4
,
nil
light terra cotta.
The test pieces were hard and strong, wth a good "ring". It was noticed that an increase in the firing temperature to between 1120°C. and 1140°C. produced imperme- able ware of a dark brown colour, with no distortion in shape. This ware was strong and very hard, and if covered with a suitable leadless glaze would possibly be suitable for kitchen ware. The test pieces fired at 1060°C. could readily be covered with a lead glaze. It was also found that the ware was suitable for covering with a white slip, one application completely coating the vessels.
Summary and Conclusion.
(a) White Clay.
Good earthenware can be made from the washed clay, with or without the addition of ball clay, but this latter material renders the earthenware mixture rather more easy to work. It appears unlikely that the use of the unwashed clay for earthenware mixtures would be commercially possible owing to its tendency to crack and to the presence of surface defects caused by ferruginous particles.
Bone china may also be made from this material, the most successful mixture employed being one that included both the washed clay and ball clay, a strong biscuit of good colour being produced.