1919.1 
Departmental Reports. 
217 
Fig. 3. 
At this stage it is important to know 
what effect pnmice sand such as occurs in 
these clays has upon the behaviour in the 
fire, whether fluxing or tempering. Clays 
Nos. 1 and 7 were selected, since No. 1 
has a low proportion of pumice in its clay 
fraction and No. 7 has a very large pro¬ 
portion (see Table II). To observe the 
behaviour of the clay fractions in the fire 
an inert tempering-material was required, 
and for this purpose the “ grog ’ 5 of the 
clay itself was chosen. This was observed 
by burning a sample of the clay to a higher 
temperature (cone No. 16) than the tem¬ 
perature of the experiment (cone No. 14), 
grinding it, and using a finely-sieved portion 
to temper the clay fraction of the same 
clay. No further kiln reaction or shrinkage 
could take place with the grog or between 
the grog and the clay fraction. The results 
are shown in Table VII. 
Table VII. 
— 
Composition of Sample : 
CF = Clay Fraction. 
SF = Sand Fraction. 
Shrinkage (Percentage 
of Original Volume). 
Porosity of 
Fixed Ware 
(Percentage of 
Total Volume). 
Air. 
Fire. 
a. 
CF No. 1, 50% ; own grog, 50% 
13 
70 
28 
b. 
CF No. 7, 52% ; own grog, 48 % 
19-5 
14 
23 
c. 
CF No. 1. 30% ; SF No. 1, 70% 
22 
0 
34 
d. 
CF No. 1, 30% ; pumice, 70% 
14 
32 
1 
e. 
CF No. 7, 30% ; SF No. 7, 70% 
16 
1 
32-5 
/• 
CF No. 7, 30% ; pumice, 70% 
10-5 
33-5 
0-5 
It is seen that the fine pumice in the CF No. 7 has increased the fire 
shrinkage of that sample as compared with CF No. 1 (see Table VII, a, b) 
and reduced its porosity. Further, by comparing c with d, and e with /, 
it is seen that the substitution of fine pumice (from the seam among the 
clay deposits) for the sand fraction turns a porous, fire-resisting biscuit 
from both No. 1 and No. 7 into something approaching a slag. It is 
evident that rhyolite sand, in the clay fraction at least, must be regarded 
as a flux. 
_ * 
The behaviour of the calcareous clay No. 14 (see Table II) is dif- 
j ferent from that of the preceding samples. The shrinkage in the fire 
is uniformly high, an interesting result in the light of the analysis 
which returns the alkali earths, &c., as occurring in both the clay 
fraction and the sand fraction. The results are shown in Table VIII 
and fig. 4. 
