67
The percentage of bases would probably be somewhat higher than indicated in the above table, as the amounts of soda and potash in the washed and unwashed lay were not determined.
The mixtures made with the washed clay worked well on the wheel and gave good results when pressed in plaster moulds.
Those made from the unwashed clay were somewhat less plastic and more care in moulding was necessary than when the washed clay was employed. It was found that the addition of the ball clay, as shown in table above, gave a great improvement to the work- ing properties of the mixtures.
The conditions of firing and the results of the tests carried out are tabulated as follows:-
Raw.
Washed. Mixture A. Mixture B. Mixture C.Mixture D.
Firing temperature
(Max.)
°C.
1100
1060
1100
1100
1100.
1100
Duration at maxi-
mum temperature.
Hours
6
6
6
6
6
6
Drying
shrinkage
(linear)
per cent.
6.3
7.5
4.1
5.0
4.3
5.0
Firing shrinkage
(linear)
5.8
7.5
4.9
3.1
5.0
2.5
>>
1
Total shrinkage
(linear)
12.1
15.0
9.0
8.1
9.3
7.5
71
Porosity
Warpage
44.6
36.1
34.5
32.9
86.8
32.9
Tendency
nil
nil
nil
nil
to warp.
Cracked
rather
badly.
Colour
White with iron spots.
White
White
White
White
White
with few with few iron spots. iron spots.
The fired ware was strong and hard, with the exception of that made from mixture C. Increasing the firing temperature of test pieces made from that mixture gave no im- provement in strength, but on the contrary a tendency of the ware to crack was obser- ved.
Glazing Tests on Earthenware. Biscuit were made from mixtures A, B, and D. was glazed with both opaque and transparent felspathic and lead glazes.
No difficulty was experienced in the application of the glazes, which adhered well to the biscuit. attempt to glaze biscuit made from mixture C was less successful, as the application of any glaze tended to weaken the body.
Stoneware
Impermeable pottery, similar to stoneware, was produced from mixtures A and B- by increasing the firing temperature to 1250°C. The total shrinkages of 11.2 per cent. and 11.8 per cent. respectively were not excessive for a semi-vitreous body of this nature. The ware was strong and a felspathic glaze maturing at a high temperature could be satisfactorily applied.