[Trade Journal]
Publication: Scientific American
New York, NY, United States
vol. 124, no. 11, p. 205, col. 1-3
Making. Electric Insulators of Porcelain
By Ralph Howard
MILADY gazes admiringly at the china that reflects the light from the electric dome over the dining room table, and wonders how such marvels of ceramic art can be wrought. But she gives no thought to the bumble porcelain insulator—the Cinderella of the family—without which there would be no electric light in its modern form. It may seem that almost anyone could mold a porcelain wall-switch base, a cleat used in wiring buildings, or some of the simpler forms of insulators. But when it comes to making them so that after the shrinkage that results from subjecting them to a temperature of upward of 2,642 degrees Fahrenheit, they will still be within the dimensions required, it is not so simple. Nor is size the only thing that has to be considered. The porcelain must be so mixed, molded and fired that it will withstand, in some cases, hundreds of thousands of volts without breaking down and allowing the current to pass through it.
Porcelain is made of potters' clay, ball clay, feldspar and flint, the two latter being in the form of fine powder. These are mixed with water, the coarse particles strained out, and the resulting cream-like batter goes to the filter presses where most of the water is pressed out and it emerges as solid, damp cakes.
There are now two methods of procedure, the dry process and the wet-process. The dry process is used in making the devices to insulate the lower voltage currents—such as, for instance, building-wiring equipment—while the wet process, which permits making a denser structure, is employed for insulators for high-voltage power-transmission lines, and in transformers, oil switches, etc.
The making of wall-switch bases, cleats, and other small porcelain insulators is a fascinating process. The damp cakes that come from the filter press are dried until just enough moisture is left to press them. Then they are reduced to a loose, damp powder, and this is dropped into oiled dies shaped according to the design of the article to be made. Long rows of machines, each stamping out a neatly molded piece at the turn of a lever, mold thousands each in the course of a day. The insulators are taken from the press and placed in neat rows on pallet boards and dried by steam anywhere from a few hours to several days. Then the little rough edges or burrs are removed and the insulator is ready for glazing.
In making wet process porcelain, the cakes from the filter press are given a kneading that eliminates the air and mixes the mass together. 'From the machine which completes this process it comes in the form of a cylindrical mass, ready to be fashioned into any desired shape.
Some is put into a plaster of Paris mold and pressed into the pattern wished. Another piece will be placed on a horizontal wheel operated by a motor—an adaptation of the ordinary potter's wheel. When it is set on the wheel it is just a plain, ordinary looking cylinder of yellowish porcelain. But under the manipulation of a skilled operator it quickly takes on a graceful appearance as shown on our cover. The touch of a little implement cuts a series of grooves, or perhaps a continuous thread, in the side, and ribbon-like shavings of porcelain fly from the -article being turned, crumbling as they fall. When the work is done, what was a mere cylinder has become an accurately measured insulator, conforming to careful specifications.
Where a curve conduit or pipe is wanted, the process is simplicity itself. Liquid porcelain mixture is poured into a plaster of paris mold shaped like the outside of the conduit desired. The plaster of paris absorbs the water from the mixture adjacent to it, leaving a shell of porcelain next to the mold. The water in the fluid porcelain farther away from the sides of the mold is not absorbed, and hence the mixture remains in a liquid state. When the walls of the conduit are sufficiently thick, which depends on the length of time the mixture has been in the mold, the fluid is drawn off and the conduit is taken from the mold and dried.
Great care has to be used in the drying. The porcelain must dry evenly, and therefore not too rapidly. Steam is turned into the drying machines to insure sufficient humidity. Gradually the amount of steam is decreased and the heat is increased. Thus the outside is kept from cracking and the drying process proceeds uniformly throughout the article.
Insulators made by the wet process are, in some cases, five and six feet high, and upward of two feet in diameter. The largest are not, however, made in one piece. Sections are turned out and then piled up as the housewife stacks plates, and the heat in the kiln, when they are "fired," fuses them into one solid piece.
Milady is very proud of the glossy appearance of the dinner set on her dining table. But the glaze that gives it a glossy sheen is not there merely for looks. The cups would not hold coffee or tea, and the plates would be soaked with grease were it not for the glazing that renders the more or less porous porcelain impervious to moisture.
And that is exactly the reason why porcelain insulators are glazed. While moisture might not penetrate them extensively, it would cling on the minute rough edges of an unglazed surface and tend to create a film of water which would impair their insulating properties.
The glazing consists of the same material as the porcelain with the addition of calcium carbonate or whiting, which cause the glaze to melt at a lower temperature than the porcelain. It is applied, in the form of a thick liquid, in several ways. Small articles are coated by means of an automatic sprayer which resembles a huge atomizer and sprays the glaze over them. Others are sprayed by hand, and still others are dipped into the glazing fluid.
After glazing the porcelain is placed in receptacles made of fire-clay resembling the casserole familiar to the housewife, and these are piled in great columns inside the kilns—huge structures of brick about fifteen feet in diameter and sixteen feet high, beneath which is a furnace. When the kiln is full, the door is walled up with brick and fire clay, and the fire started.
Three cones, called pyrometric cones, are made from the same materials as the porcelain. The proportions are so changed, however, that the tip of one of these cones will bend over at 20 degrees Centigrade less than the melting point of the porcelain, while the third requires 20 degrees more to melt it than the porcelain. These cones can be seen through a small apperture [sic] aperture in the kiln. The melting of the first warns that the heat is approaching the desired degree. When the second one melts, the fire is carefully manipulated so that the heat will not rise to a degree that would melt the third.
The intense heat, upward of 1,360 degrees Centigrade, melts the glaze, which runs evenly over the outside surface of the porcelain, and vitrifies the latter, making it a hard, dense, impervious mass.
After the porcelain has been baked for the requisite length of time and the contents of the kiln have cooled sufficiently, the articles are removed and tested. Not only must the insulators meet specifications as to size—and that, as indicated, takes very careful work—but they must be tested for their insulating strength. The larger insulators are subjected to tests at from 60,000 to 300,000 volts.
The testing is one of the most interesting phases of the work. The spectator sees a row of insulators to which wires are attached. The operator throws on the current. Down one of the insulators goes a spitting blue line of "fire" with a crackling noise. The visitor's face tingles from the electricity in the atmosphere, and there is a pronounced odor of ozone from the oxygen liberated in the air.
