Publication: Electric Journal
Pittsburgh, PA, United States
G. I. GILCHRIST and T. A. KLINEFELTER
This article deals with a phase of the electrical field which is still in the "rule of thumb" stage in a great many respects. A resume of existing literature on the subject shows data of varying value, and much haziness in general. The electrical porcelain manufacturer, the ceramic engineer, the designing engineer and the operating engineer all view the problem from different angles. The object of this article is to bring about a better understanding between production and operating engineers, particularly that the latter may understand the manufacturing standpoint. To get this standpoint clearly, it is desirable, first of all, to outline the manufacture of electrical porcelain step by step; next, the data obtained from an investigation of porcelain mixes and ingredients and their influence on design; finally, the design itself.
THE foundation of porcelain manufacture is the mixture of materials which, when burned, makes the porcelain. Electrical porcelain, as far as its main ingredients are concerned, is like other porcelain, china ware or chemical ware. The proportions are different and the treatment a little different.
A mere listing of the various components of the high grade wares and the widely different localities from which they come will enable one to understand how the porcelain maker is hampered by variable factors.
SELECTION OF MATERIALS
The three main factors, or ingredients, are flint, feldspar and clay, and there are many different kinds of each ingredient to choose from. In the choice of flint, there is flint from the pebble, which is an amorphous form; flint from quartzite, the crystalline form, or just plain, pure sand, ground up. And then, again, it may be dry ground or wet ground. There are numerous feldspars, and a great variety of clays.
In the case of flint the porcelain maker wants a finely ground or powdered material which is clean and pure. The ordinary flint is very finely ground quartz rock, almost pure silicon dioxide. In appearance, the ground material might be mistaken for ordinary white flour. As the material is abundant, there is no trouble from adulterants or impurities, and so the user is generally concerned only about the fineness of its grinding. This is important, so a shipment of flint is usually checked up for fineness of grain and moisture content.
The same statement applies also to feldspar shipments so far as the usual routine testing is concerned. However, due to its importance and the wide variation of feldspar quality, other tests must be made, and the material checked from time to time. The feldspar is the active agent in the mix during the burn, and, if adulterated with flint or with an inferior feldspar, an otherwise high grade spar may be very inefficient in producing the degree of vitrification desired. So a chemical analysis is frequently of service in determining the quality of the material. A much used test is to melt some of the feldspar in the kiln and obtain an idea as to its quality by examination after the burn.
A still better method, which was used during the investigations here described, is to make the feldspar into standard cones, such as are used in determining kiln temperatures. These cones are made