Art. XLVIII.—On the Pottery Clays of the Auckland District.
[Read before the Auckland Institute, 20th August, 1883.]
In the year 1876, I had the honour of reading a paper before this Institute on “a few of the fire-clays of this district,” since which period the frequent opportunities I have had of examining the different deposits has led me in no way to alter my opinion as to the great value and extent of them, and since then I have seen and examined so many other beds of these alluviums that I make no apology for bringing before you to-night a few notes upon another series embraced in the very extended term, pottery clay. By this is meant that material of which the finest of our china and parian ware, and the roughest of our earthenware, are made; from the pure white kaolin obtained by the disintegration of the granite, to the rough ochreous clays and marls whose red and grey tints are so well known. Included with these is the terra cotta, a soft unctuous clay with a large percentage of peroxide or carbonate of iron regularly distributed. This, if of good texture and thoroughly admixed, will yield in the muffle a biscuit of the most delicate shades of redness; this is terra cotta, the use of which has increased very much of late in England, large houses being built entirely of it.
In dealing with the commercial values of heavy materials, such as clay, full consideration must be given to the removal of the same for shipping purposes, the carriage to market being always a serious factor. In dealing with this subject I have not been unmindful of this factor, and clays which are really of value on account of their texture or purity are passed over on account of the impracticability of bringing them to market.
Of the clays I have examined the great majority lack the whiteness of the true kaolin, and the biscuits from them vary from white to a full bright red. Those having light creamy tints to buffs are very plentiful, and I have no doubt that in many of these instances, when the beds are properly opened and worked, they will improve in uniformity and whiteness.
From Mongonui and Awanui I have received samples of excellent pottery clay, some being nearly white, but the majority of a cream tint. From Whangaroa I received through Mr. Will a sample of pipe-clay of pure whiteness, very unctuous and refractory, a nearly pure silicate of alumina. There is, I believe, a large deposit of this material. From the Bay of Islands I have obtained clays of fine texture, fairly white, perfectly free
from grit and fit for the potter without elutriation. In the Wade District, extending to Riverhead, we have a very varied assortment of clays, fit for all purposes of pottery material with the exception of the finer wares. From Mercury Bay I have received samples of fine, white, unctuous clays, but as to the extent of the deposits I have no information. In the Waikato, near Hamilton, there are some excellent clays, the biscuit of which is of a pure white. Some of these clays have been worked up into medallions, slabs, and ornaments by Mr. Wright, who has resided a long period in that portion of the district engaged on pottery work, and whose workmanship is in the highest degree ornate. Though these clays are so excellent they have not been proved as to extent, and I think it questionable whether they will be found free from iron in more than small deposits. And here we have the drawback at present of lengthy rail carriage to the centre of population in the district. At the coal mines, Kawakawa, Whangarei, Taupiri, and Miranda, we have the refractory clays accompanying the coal measures, but these will be of value only in the rougher articles of pottery, owing to the presence of iron pyrites in nodules or finely distributed, and shrinkage, from the amount of bituminous material present. I come now to the last of the locations where I have found any great extent of clays for varied purposes, and so situated as to be within reasonable distance of fuel and adjacent to this city where the articles may be brought to market and for export. I allude to the Drury and Papakura basin, overlying which are the plastic clays. Here, over several square miles of country, we have clays from the whitest material, giving a biscuit of great purity to yellow, grey and blue clays yielding biscuits from a light cream to a deep rich red, which will give a terra cotta of great beauty. I have analyzed some of these deposits, and here append the results:—
|Yellow Red Clay (terra cotta).||Blue Clay (pinkish creamy biscuit).|
|Oxide of Iron||9.7||6.3|
In reference to these deposits, I do not for a moment claim any discovery with respect to them, as Dr. Hochstetter, in a lecture delivered at the Mechanics' Institute in 1859, calls attention to these beds, advising the establishment of potteries for the manufacture of earthenware, and further states “remarkably suitable clays of every necessary variety have been shown to exist in the neighbourhood,” and also furnishes the results of two
bore holes put down by Mr. Ninnis at Drury, which show a depth of 69 and 64 feet respectively through clays of many colours from white to brown, the seams varying from 1–11 feet in thickness. A few months since I paid a visit to these bore holes, but there is little to see beyond the position in which they were sunk. To obtain a knowledge of these beds, however, it will be found, by skirting the Karaka estuary on both sides from the ferry to Papakura and Drury, that many fine sections can be obtained of the clays in situ, most of them having but little overburden, and stripping will not be so serious a matter as at the bore holes indicated by Dr. Hochstetter. In some places there are 30 feet of clays, in three or four distinct beds, differing slightly in their chemical composition though more or less varied physically. Some of them are very argillaceous, and a few arenaceous, while for variety I know of no district where it can be equalled. After carefully inspecting a number of these sections, I have found nineteen distinct characters, either chemically or physically, of which about twelve will be of value for pottery work. So pure are some of these clays as to require no elutriation,—as they are won from the cutting so they can go to the pug mill. Amongst them there is one terra cotta so rich in colour without any addition or treatment that this alone would be a valuable acquisition, and when we bear in mind the extent to which terra cotta ware is now coming into use, it will be seen that this is no fancy sketch.
In the future we shall undoubtedly see extensive works located in this portion of the district for the use of these valuable clays, and the working of these beds will be greatly facilitated by a small canal between the Manukau and Waitemata waters.
Before leaving this part of my subject a few words upon a peculiar clay from Kaitaia may not be out of place. Having received some samples from the north of fine unctuous pipe-clay, I was informed by Mr. Kelly, of Mongonui, that they used a similar clay to make the roads with in that part of the district, and a large sample was forwarded to me, at my request, by Mr. Houston, who states that after spreading upon the roads at first, the horses stick fast, and have great difficulty in making their way, but that it soon hardens, and, after once becoming so, no amount of rain will cause it to puddle again.
It has been found by later experience, however, not to have proved so valuable a material for road purposes as was expected. The surface becoming abraded in the summer, suffers from the strong winds, which remove a good deal, thus rendering it necessary to frequently repair. On examination this material proves to be a diatomaceous earth, the diatoms being intermixed with very finely divided silica. The exceeding minuteness of these fossil diatoms is so great that under a power of 2,000 diameters
the microscope only reveals their globular form. The finely divided state of this siliceous material is the cause of its bedding so firmly while wet.
In concluding, it will be interesting to review the extent of our importations into New Zealand of this class of goods during the last few years. Under this head we will include fire-bricks, china and parian ware, earthenware, drain pipes, and stoneware, the amount being as per invoice assessed for duty:—in 1877, £52,691; in 1878, £49,791; in 1879, £67,164; and in 1880, £34,951; to this last sum must also be added £1,050 for earthenware passed as free goods. When we add to these figures the cost of freight, breakage, and duty, the latter at 15 per cent., it will be seen that the subject I have brought before you is one well worthy a little consideration.