68
China Mixtures
The following bone-china mixtures were made, the unwashed clay used in the ex- periments being ground to 120 mesh as in the case of the material used for the earthen- ware trials.
Mixture E
per cent.
Mixture F per cent.
Mixture G
Mixture H
per cent.
per cent.
Washed clay
30
25
Unwashed clay
.30
Bone ash
35
35
35
Cornish stone
35
35
Ball clay
5
18381
20
35
35
35
10
Test Pieces made from the above mixtures, and after drying fired at 1120°C. showed the following properties :--
Mix- ture E.
Mix-
Mix-
Mix-
ture F.
ture G.
ture H.
Firing temperature
°C.
1120
1120
1120
1120
Duration at maximum tempera-
ture
hours
6
6
6
6
Drying shrinkage (Linear)
per cent.
2.8
7.0
3.1
8.0
Firing shrinkage (Linear)
1.2
3.0
2.7
1.5
,,
Total shrinkage (Linear)
Porosity
5.0
10.0
5.8
9.5
"
37.3
33.9
36.9
31.9
*
JJ
nil
nil
nil*
nil
Warpage
Colour
* A slight tendency to crack.
white white whitet whitet
+ Ware somewhat "iron spotted".
All mixtures with the exception of G could be moulded easily and after firing yielded good hard biscuit-ware. Mixture G was not of a very plastic nature and a con- siderable amount of care was necessary in moulding, whilst the unfired ware was weak and somewhat difficult to handle without breakage. The fired test pieces made from this mixture were rather weak, and in some cases a number of fine surface cracks were pro- duced. Applications of lead and felspathic glazes to biscuits made from mixtures É, F and H were in all cases satisfactory.
Red clay
This sample consisted of 59 lb. of a moderately hard, reddish-brown mottled clay, only slightly plastic when mixed with water.
A preliminary washing test of the red clay showed that it contained about 25 per cent. of clay and 75 per cent. of residue, the latter apparently consisting of indurated clay, generally of a pink colour. In view of the small amount of plastic clay contained in this sample, and of the comparatively low price of terra cotta ware, it was not consider-
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