POTTERY RAW MATERIAL.

Evening Star, Issue 3840, 15 June 1875, Page 3

POTTERY RAW MATERIAL.

As before observed, plastic clay and coal beds constitute the base of all pottery manufactures. What that clay is will now be illustrated. All clays that are composed of sufficiently fine material to mould with the hand or on the potter’s wheel, under the application of his skilled handicraft, may be called “plastic clay.” The purest of these are called potters or pipe-clay. These are often a varietv of kaolin, or porcelain earth, mixed with silicious sand, carbonate of lime, magnesia, and other substances; those used for pottery purposes are usually grey white, pure white, blue, bluish white, and black. Potter’s black clay bums white, its coloring material being owing to carbon ; some potter’s clay, however, of red, yellow, green, or blue colors become yellow and red when burnt. These varieties are used In other brapchesof the art of pottery. Kaolin or porcelain clay is found in beds or veins, and is of various colors, as white, grey, blue, &c., and is stated to be the product of the decomposition of “ orthoolase felspar,” or of granite, porphyry, or other rocks containing this mineral. The kaolin of China consists of 71.15 parts of silex, 15.86 of alumina, 1.92 of lime, and 6.73 of water; that of Cornwall, according to “ Boaae,” of nearly equal parts of silica and alumina, with one per cent, of magnesia. It is found in Otago : the commoner varieties of ♦ pottery clay abound in various localities in this Province, such as Tokomairiro basin. Gypsum, or plaster of Paris, is also extensively used ip connection with this art, and is to be found in the northern and southern districts of Otago, but in what quantities remains to be developed. _ Felspar, which is chiefly used in the production of glasses, and whose importance to clays have been noticed, is also to bo obtained in commercial quantities in Ota a*. Its composition la essentially of silica and alumina, with potash and soda. "Varieties of this mineral abound in different parts of Otago, of which “obsidian,” an amphorus felspar, is found on the Kakanui ranges. That most used in the pottery art is orthoclase or common felspar. Its chemical composition is as follows Silica, 64.70; alumina, 15.20; potash, 7.14; soda, L 4; lime, L 3; iron, 1.2. Obsidian contains the next largest percentage of potash ; —Silica, 72.84; alumina, 4.11; potash; 6,10; soda, 3.5 ; lime, 1.4; iron, 1.3. This family of minerals, under various distinctive names, is used in the manufacture of all kinds of pottery ware. The glazes used for pottery are composed, t «large extent, of the same materials used for the bodies of ware, and are generally adopted by affinity to them. Carbonate of potash is largely used in preparations of cobalt for smalts and other things, while hydrate of potash is employed in the purification of clays, as an absorbent of free silica. Crystalised soda and nitre are used in preparations of enamels and glazes. As before stated, these minerals exist in the waste wood of our forests as potash, in the unused wealth of the ocean as common salt, and in saline deposits on our coast as nitre. Tin, zinc, copper, iron, autimony, cobalt, manganese, silver, gold, platinum, cronium, lead, uranum, iridum bismuth, etc., are some of the coloring materials employed. A flint, which enters largely into the composition of pottery ware, is found in Otago in the cretacious formations. Specimens have been obtained in the northern district and , south and north Canterbury ; also in central and southern Otago. Chert (quasi quartz) is used for maxing mixing mills, in which fie calcined materials used for bodies, glazes, and enamels are ground to their required consistency; and owing to its affinity to felspar and other material, it is much sought after. A variety of it abounds in this # Province, at Oamaru and Shag Point ; while large masses of grey, yellow, and white chert are procurable between Waihola and the _ Ocean Beach. Any practical mind will see in the foregoing information that which becomes the bone and sinew of a country in its development, as all these materials must be obtained bv means of some description of labor, in mining, quarrying, or manufacturing.