CLAY RESOURCES

Wanganui Chronicle, Volume LXXXIII, Issue 19513, 16 February 1926, Page 11

CLAY RESOURCES

VALUABLE DEPOSITS IN DOMINION. INVESTIGATION ADVOCATED. In a paper, read at the Mining Conference at Dunedin, on the subject of clay resources of New Zealand, Mr W. Donovan ,of the Dominion Laboratory, stated that although tie clay industries were represented in New Zealand by 69 brick, tile, and pottery works, employing in 1923-24 1150 persons, and producing goods to the value of almost £600,000 per annum, comparatively little had been written concerning our national resources of clay. Kaolin, chemically, was composed almost entirely of kaolinite. The term was usually applied to a residual or primary clay, formed in situ by the decomposition of granite rock. Of true residuel kaolin no deposit had, to his knowledge, yet been reported in New Zealand. There were, however, some white sedimentary clays of the same chemical composition. Fireclays were plastic clays, usually containing from 50 to 70 per cent, of silica and 20 to 30 per cent, of alumina. Other constituents such as lime and alkalis, were present only in small amounts.

In chemical composition all clays were very similar to kaolin, being composed chiefly of kaolinite, but they differed in their high placticity and strong bonding power. The principal use was as the foundation material in the manufacture of all kinds of earthenware.

The best pottery was usually made from a carefully prepared blend of clay,, with felspar, ground flint, and sometimes bone ash. Until recently no deposit of felspar sutable for use in pottery industries was known in New Zealand, but Dr. Henderson, of the Geological Survey, had located on the Baton Road, Nelson, a mass of decomposed granite, which, on washing yielded 40 per cent, of clay passing 200 mesh. This material could replace felspar in the manufacture of porcelain. Another clay found at Puhipuhi of excellent colour and containing 55 per cent, of felspars could be similarly used. Flint had not been found in marketable form in New Zealand, but the glass from Mount Somers and from Takaka, which contained over 99 per cent, of silica and less than 1 per cent, of iron oxide, could doubtless be ground and used instead of flint. An exceedingly fine siliceous sand which occurs at Silverdale, near Auckland, should be equally suitable, and would require grinding. There were, however, some white clays which were natural blends of felspar, quartz, and kaolinite in proportions suited for the production of good porcelainware at moderate heat. Such were found near Whangarci (derived from the trachyte rocks), at Tadmor, and at Wyndham. At Glen Massey a white clay was found which though deficient in fluxing constituents gave hard, dense porcelain of good colour when fired at 1340 degrees C. Stoneware clays giving a coloured body, varying from cream to brown recording principally to the percentage of iron oxide present, were found at Kawa Kawa, Whangarei, Fairdown, Giles Creek, Empire Colliery (Reefton), Waikaiia, Terrace Colliery (Balfour). The clays most essential for New Zealand, at its present stage of development, were those suitable for the manufacture of building bricks, sewer pipes and roofing tiles. Bricks could be made from almost any clay, the properties being modified if necessary by a mixture with ojher clays, with sand, or with grog. This could be made from brick clays of fine texture. A terracotta clay was one of fine texture, giving a fine dense surface, containing sufficient fluxes to make it weather resisting without fusion, or small shrinkage, and of pleasing colour when fired. Portland cement was prepared by calciling an intimate mixture of 70 per cent, limestone, and 30 per cent, of clay, usually siliceous. Of late years Alumina cement had been developed, and was in demand because of its quick hardening properties. It was evident that New Zealand possessed abundant resources of clay and many varieties, and the clay industries would become increasingly important as the country developed. With the depletion of timber reserves the output of building bricks must increase in the near future. Hydro-electric development had created a demand for electric insulators and porcelain fittings, for the manufacture of which the Dominion had several suitable clays. They had the raw material for the production of sanitary ware of good quality. When in the future the local demand was sufficient to warrant the establishment of pottery works, suitable highgrade clay would be available. Until that time arrived the possibilities of an export trade might be considered. Mr J. C. Bradley (Westport) said that he realised the importance of having ini New Zealand material suitable for use in retorts. Each of the large works in Auckland, Wellington, and Christchurch was also on the lookout for the best material procurable, and if it could be shown that suitable clay was available in New Zealand, it would be tested and used. Mr P. C. Morgan said that great value undoubcdly lay in New Zealand’s clay deposits, but before doing anything with them, it was first necessary to go into the matter thoroughly and collect a large amount of information.