210 
The N.Z. Journal of Science and Technology. 
[May 
low-lying area between the Mannkan Harbour and the Tamaki foreshore 
at Whitford. These beds are regularly stratified, the consistency of the 
strata varying from a very plastic to a fine sandy nature. Also, in all 
localities there occurs a band of pure-white water-worn pumice interstrati- 
fied with the deeper clay beds. A similar pumice is found in much greater 
variety of texture and size in the rhyolite deposits of Ihumata, which, it 
is reasonable from geological evidence to suppose, were deposited by the 
Waikato when it flowed into the Manukau Harbour. It is probable that 
the material of the clays under consideration was brought down by the 
Waikato, and is the product of decomposition of the rhyolitic pumice 
occurring in the upper portions of the Waikato River. 
Methods of Analysis of Clays. 
Much attention has of recent years been given to the problem of 
analysing clays. As to the merits of the various methods in use there 
is much difference of opinion, but it is generally agreed that the mere 
chemical analysis of a clay into its constituent oxides is inadequate. 
When a clay is fired in a kiln, fusion and chemical action go on almost 
entirely on the surface of the particles. After the incipient fusion, or 
“ fritting,” the action is stopped. Under the conditions of fritting, it is 
the chemical composition of the smallest particles and of only the super¬ 
ficial layers of the larger particles that determines the temperature of the 
incipient fusion. It is obvious, therefore, that ultimate chemical analysis 
will not give the information required. Any successful method of practical 
analysis must state its results in terms of the mineral particles, both their 
chemical composition and their relative size being determined. 
It is still a common practice to attempt to calculate these mineral 
particles from the ultimate analysis, as is shown by the following comment 
of Searle : “ Some clays are almost destitute of feldspar but comparatively 
rich in mica, while others are the reverse, so that some means of identifying 
them is essential. When this is not used the curious result is obtained 
that German chemists calculate the alkalis, &c., to feldspar, whilst the 
French, following Voigt, calculate them to mica. English ceramic chemists 
appear undecided as to which course to follow, and some of them occa¬ 
sionally report notable amounts of feldspar in clays quite destitute of this 
mineral.”* 
To be satisfactory a method of technical analysis should fulfil the 
following requirements:— 
(a.) It should allow of reasonable accuracy in determining the size, 
nature, and amount of the extraneous mineral particles. If 
possible, it should also be generally applicable to all the clays 
occurring in any one district, for otherwise there is difficulty 
in comparing the results obtained. 
( b .) The information supplied should be in a form useful for controlling 
the actual manufacturing process. It should give a manufacturer 
definite information as to the behaviour and properties of his 
clays in the fire and in the other various processes of the 
industry. 
The first attempt to meet these requirements was the “ rational 
analysis ” of Seger (1876),t which is still the basis of the more modern 
* Searle, The Natural History of Clay, p. 143, Cambridge Univ. Press, 1912. 
f Fide Searle, loc. cit., p. 141. 
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