Manufacture of electrical porcelain

ELECTRICAL PORCELAIN

The manufacture of porcelain for electrical purposes was recently discussed before the Birmingham and District (England) Electric Club by H. W. Brady. An abstract of his paper follows. That part dealing with the selection, preparation and mixing of the clays is interesting. The substances employed for manufacturing insulators are all clays, or closely related to the clays. They have been formed by the action of the weather on feldspathic rock, such as granite and porphyry. This process washes away practically all the materials except the aluminum silicate, which in the pure form is known as kaolin. This is the basis of all clays, and the several varieties of clays are due to other different substances included in their composition. Clays are classified in the following way: Strong clay is rich in iron, and used for making bricks. Mild or loamy clay contains gravel and sand, which must be washed out. The texture of this clay is loose, calling for an addition of chalk to act as a flux. Marl is a clay containing a considerable amount of chalk, which is useful for brick-making. Plastic or potter’s clay occupies an intermediate position between the colored-burning common clay and the white-burning china clay. The cause of its extreme plasticity is not certain. This is usually called ball clay, and is frequently colored by organic matter. Fire clay is simply highly refractory clay, and hence the best fire clay is the purest. Kaolin, or china clay, is the finest kind of clay. It is the most distinctive ingredient in the composition of porcelain. It is less plastic than the commoner clays, and when burnt becomes white and porous. In its finest form it is used for porcelain; in its less pure form it is used for ware intended to withstand very high temperatures. The latter is not true porcelain, as it exhibits no translucency. Kaolin, however, usually contains some impurities which must be removed by washing. It is generally allowed to stand in the open for some time to weather. For this purpose as large a face of the quarry as possible is exposed. It is easily crumbled when dry to a fine powder and takes up a large quantity of water, which makes it plastic and tenacious. When molded into shape and allowed to dry, it toughens and becomes feebly coherent. If fired, it shrinks about eight per cent and becomes hard, though it does not fuse. The formula for mixing clays for insulator pottery are usually secret, but the following is a mixture which is in use. This consists of three parts of china clay, twelve parts of ball clay, two parts of flint, two parts of Cornish stone, and four parts of feldspar. The preponderating portion of this is ball clay. The latter gives plasticity and ductility to the mixture, making it easy to work. The flint counteracts the shrinking and liability to crack and the distortion which would take place due to the fusibility of feldspar. Feldspar and Cornish stone are the fluxes, and both seem to be necessary for success, but neither one alone gives the desired result. It is not a little peculiar that the five ingredients called for above seem all to be necessary for successful porcelain-making, although they are all so similar.—Electrician (London), May 29.