James Pass - Durability of Porcelain as an Insulator

[Trade Journal]

Publication: The Electrical Engineer

New York, NY, United States
vol. 20, no. 394, p. 490, col. 1-2





In THE ELECTRICAL ENGINEER of Sept. 24th there is an interesting article by Mr. A. E. Dobbs, questioning the durability of porcelain as an insulator, and which comment I am glad to see because it calls attention to a condition of things which is worthy of thought and investigation by all who are interested in the permanence of electrical insulation.

It can hardly be questioned that porcelain, when skillfully compounded and carefully made, is an excellent and durable insulator. The recorded experience with this material for aerial lines in England, where it is used in preference to all other materials, should have some weight. It seems also to be well established, so far as laboratory tests can establish such claims, that porcelain, when well made, is a better insulator than glass, especially for high voltages. But how are we to know that the insulator which looks well and lasts well today will still be a good insulator five years hence?

Laboratory tests do not enlighten us on this point and appearance is no indication whatever. Of course, certain gross faults may easily be detected; such as an imperfect vitrifaction, etc. But to make a porcelain body which is perfectly vitrified and yet has considerable strength to resist fracture requires experience and technical skill; and to make such a porcelain and cover it with a perfect glaze that will not crack in time has from the dawn of the pottery industry taxed to the utmost the skill and ingenuity of the practical potter.

This cracking of the glaze is called among potters "crazing" and is generally known by that name; perhaps from the fact that its unwelcome and unexpected appearance has driven so many good men to the brink, if not into the slough, of mental despair.

The glaze used on porcelain is quite peculiar and there is little resemblance between the formulas for this glaze and those for ordinary glass. A porcelain glaze must attach itself very firmly to the body, when heated to near the melting point of that body, and it should have as nearly as possible the same co-efficient of expansion as the body which it covers. The exact adjustment of glaze to body is difficult to attain, the glazes having usually a larger coefficient of expansion than the bodies. That is, they are affected to a greater extent by changes of temperature; and if the difference is considerable, it will be seen that the glaze is in a state of tension. Sometimes, when the adjustment of glaze to body is very bad, a piece of ware will be found glaze-cracked, or "crazed," when it is taken from the kiln—for the difference of contraction begins as soon as the glaze, in cooling, loses its viscous property, becomes brittle and this is cumulative as the temperature is reduced.

A very serious feature of this "crazing" is that unless the adjustment is very bad as mentioned above, the "crazing" may not take place at once but may occur at any time. It usually occurs within two or three years after manufacture, according, principally to the more or less uniform expansion and contraction of body and glaze, but also influenced slightly by other conditions; and this increasing liability to "craze" we will endeavor to explain.

It is well known, especially to manufacturers of thermometers, that there is a tendency in glass to continue contracting slightly for some months after manufacture; so that a thermometer, if filled and graduated soon after the tube and bulb have been formed will gradually read high, usually not exceeding one degree. This tendency to continue contracting after manufacture is quite marked in glaze that is suitable for porcelain; and if at time of manufacture the glaze is under sufficient tension, though not enough to cause fracture at the time, the increasing strain brought about by this further contraction of the glaze causes "crazing" sooner or later. A piece of porcelain ware which has done good service in the first year of its life may be rendered useless in the second year, or later, by "crazing;" and a porcelain insulator which looks well, even lasts well in the laboratory, may be of little value for insulating purposes within a short time after manufacture.

"Crazed" porcelain should never be used as an insulator, for when subject to moisture the fissures in the glaze fill with water by capillary action, forming excellent channels for leakage of current. It should be a source of regret that such large quantities of defective porcelain have been used in the construction of electrical devices. We know of reputable manufacturers of electrical apparatus who have assembled and sent out in the market to do service in permanent installation tons of this defective porcelain; we have seen again some of the same articles tat had evidently done service in a damp place and every crack in the glaze was filled with verdigris.

It is certain from the observations of the writer that there is a much larger percentage of "crazing" in porcelain manufactured for insulation than in that which is used for table-ware ; and the reason for this is not hard to find. First, there has been a considerable amount of amateur effort in this branch of potting. Second, everything passes inspection which is sufficiently accurate mechanically to admit of its being assembled. Third, price seems to be the only thing considered by the people who buy porcelain for electrical purposes.

Some manufacturers of electrical supplies deliberately abandon porcelain which they know by experience to be of good quality and purchase cheaper material; and even though it be "crazed" when it comes to their hands they assemble and send it out for use in first-class construction work, where it is accepted because it is "porcelain," without any regard to its qualities.

Good porcelain can be had and at reasonable prices as compared with the cost of porcelain for other purposes; but poor porcelain that is "crazed," or is liable to craze, or is imperfectly vitrified, will be used because it is cheap and "passes inspection." It should be required that all porcelain bear the manufacturer's name, and a rigid inspection should prevent the use of "crazed" porcelain in new work or its continued use in old work. This, we think, would in a short time give us a better insulating material and one that could be depended upon.


Keywords:Pass & Seymour
Researcher notes: 
Supplemental information: 
Researcher:Elton Gish
Date completed:December 25, 2008 by: Elton Gish;