How Porcelain Insulators are Made

IMPORTANT ROLE OF INSULATOR

Without Humble Piece of Porcelain There Would be No Electric Light In Its Modern Form.

Milady gazed 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 humble porcelain insulator – the Cinderella of the family – without which there would be no electric light in its modern form. Giving it no heed, she does not realize that the insulator is the product of not only years of patient study and research by trained scientists, but of the most careful and expert workmanship.

In Trenton there are 10 large factories specializing in the manufacture of electrical porcelain and giving employment to approximately 1,800 persons. These companies are the Connecticut, Cook Pottery company, Electric Porcelain Manufacturing company, Freeman Electric company, Imperial Porcelain Works, National Porcelain company, Star Porcelain company, Trenton Porcelain company and the Union Porcelain Works. Schenectady and Pittsfield are also great electrical porcelain producing centres as home of plants of the General Electric company.

It may seem that almost any one 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 1,350 degrees Centigrade, or 2,430 degrees, Fahrenheit, they will be within one-sixty-fourth of an inch to the inch of the dimensions required. It is not so simple. The big insulators, five and six feet high, must be accurate to within one-quarter of an inch.

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 of electricity without breaking down and allowing the current to pass through it, says the Trenton Advertiser.

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 in the form of a cream-like batter it goes to the filter presses, where most of the water is pressed out and it emerges as solid, damp cakes.

There are two methods of making porcelain insulators, the dry process and the wet process, and up to this point the procedure is the same. The dry process is used to making the devices to insulate the lower voltage currents – such as, for instance, building wiring equipment – while the wet process, which permits making 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 came 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 the powder is dropped into oiled dies – the dies are oiled so the powder will not stick to them – shaped according to the design of the article to be made, and the mixture is pressed into the form desired.

Long rows of machines, each stamping out a neatly molded switch base, cleat or whatever device it is engaged in making, 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, according to the article made. 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. It comes from the machine which completes this process 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 adaption 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.

The top becomes cone-shaped. 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 carefully designed specifications.

But suppose a carved conduit, or pipe, is wanted. How is it to be made?

The process is simplicity itself. Liquid porcelain mixture being poured into a plaster of Paris mould absorbs the water from the mixture adjacent to it, leaving a shell of porcelain next to the mould. The water in the fluid porcelain farther away from the sides of the mould 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 mould, the fluid is drawn off and the conduit is taken from the mould and dried.

To the uninformed it may seem an easy proposition to dry the porcelain. Nevertheless, great care has to be used in the process. 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 until the article is dried. Thus the outside is kept from cracking, and the drying process proceeds uniformly throughout the article.

Insulators made by the wet process method are, in some cases, five and six feet high, and upwards of two feet in diameter. The largest are not, however, made in one piece. Sections are turned out and then piled u as the housewife stacks a pile of plates, and the heat in the kiln, when they are “fired,” fuses them into one solid piece.

Even in the case of these large articles, the turning and the firing must be done so accurately, and allowance for shrinkage in drying and firing so carefully figured, that when the insulator is ready for inspection it is one-eighth of an inch of the dimension specified. 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; for glazing is the waterproofing, so to speak, 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 – a very serious consideration in the case of high voltage transmission lines and in various electrical devices.

The glazing consists of the same material as the porcelain with the addition of calcium carbonate or whiting, which causes 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 “saggers” – receptacles made of fire-clay resembling the casserole familiar to the housewife – and the “saggers” are piled in great columns inside the kilns.

The kilns are huge structures of 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 is started.

How do they know whether the temperature inside the kiln is high enough, or perhaps too high?

Again, the method is simple. 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 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 aperture in the kiln. The melting of the first warns the fireman 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, upwards 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, while in color and translucent in these sections.

After the porcelain has been bakes 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 dielectric or insulating strength. The larger insulators are subject 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.

Thus the innumerable porcelain insulators of many sizes and shapes and for a variety of uses are compounded, fashioned, fired and tested.

And why porcelain for this purpose?

Largely for two reasons. One is that it has a very high dielectric strength, due to its dense structure and its imperviousness to moisture. The other is that it is easily fashioned into the required shapes.

Furthermore, it is attractive in appearance when finished, is not as expensive as other materials, and is not as likely to break as glass, its nearest rival.