Porcelain vs. Glass

PORCELAIN INSULATORS FOR OVERHEAD LINES

By C. Colne

Judging from your editorial of the 20th instant on "Porcelain Insulators for Overhead Lines" you seem to think the subject of much importance. It may, perhaps, interest your readers to discuss it a little further.

The requisites of a good porcelain insulator Mr. F. W. Jones says are, that it should be made "of simple earths only, both in body and glaze, and of the same material without any metallic oxides, the body and glaze to be vitrified together at a high heat. It must be resistant to all influences of heat, cold, dampness, acid or alkaline fluid or gases."

Not having read the authorities quoted by Mr. Jones - the works of Messrs. Pope and Preece and Sivewright - I am unable to judge whether the requisites for a good porcelain insulator agree with his views.

In comparing glass insulators with those made of porcelain we should first compare the component parts of each material. Glass used for insulators, the common green glass, is usually made up of silica (sand), lime and soda (carbonate or sulphate). The sand used usually contains oxide of iron, which colors the glass green. The lime used is more or less pure and often contains also oxide of iron. The practice here is to use carbonate of soda, while in Europe it is almost invariably the sulphate.

Silica, the principal component part of glass, represents from 30 to 80 per cent. of its weight, according to the nature of the glass. It is quite insoluble in water, and resists all acids except hydrofluoric acid. If glass contains a large proportion of iron its insulating virtues are also impaired. The same may be said of lime. Taking for granted that a dense or hard glass should be the best for insulators, glass manufacturers should always make insulators of glass rich in silica, with but little alkali and fuse it at a high temperature. Unfortunately the temptation for having glass which "works easy" is too great, and alkalis are often put in the batch in excess to save time and labor, and save the care of maintaining the furnaces at a high temperature.

If Mr. Jones is correct in his statement regarding the quality of a good porcelain insulator, that it should have a good smooth surface, be perfectly vitrified throughout so as to be homogeneous, impervious to moisture and free from flaws, I am sure good hard glass well vitrified and as free as possible from oxide of iron and alkalies should meet the purpose as well as porcelain.

Mr. C. H. L. Smith's commercial appeal to those using porcelain insulators to pay the proper price for a good article may be of value to him, but in my opinion there should be but little difference in the price of glass and porcelain insulators, the difference, however, to be in favor of the latter.

Good hard porcelain ("porcelaine dure" of the French), such as Mr. Smith says he is making, is usually composed of kaolin, plastic clay, feldspar, sand and chalk. These materials exist in abundance in the United States, as I had occasion to find when I wrote the report of the United States Commission on glass at the Paris Exposition of 1878. It is true, however, that their preparation consists more than the materials used in glass. The same impurities are found in them as those existing in glass materials. Hard porcelain materials being less fusible than those used for glass, a higher degree of heat is necessary to vitrify them. Vitrification, however, never attains the same degree as in glass. Hence, if thorough vitrification is required, as stated by Mr. Jones, glass should be superior to porcelain. Porcelain may be looked upon somewhat as a semi-fused opaque glass. The resistance to acids, moisture, heat, cold and alkalies in porcelain is about the same as in glass.

I do not quite understand Mr. Smith's statement that glaze is unnecessary in porcelain insulators, and that the only use of glaze is to keep them from soiling. Since he says that he has "thoroughly and carefully investigated the whole matter," I am a little diffident in disagreeing with him. It must be apparent, however, that if porcelain unglazed becomes more quickly and readily soiled, it is owing to its porousness, which is the result of a partial vitrification only. The glaze is a thorough glass enamel which covers the pores and prevents moisture from penetrating - a better vitrification than the porcelain itself. This, it seems to me, is quite necessary for good insulation. Neither do I agree with him that porcelain insulators are not used because of "first cost." I believe that our pottery manufacturers, if they are assured of a market, will find a way to make porcelain insulators of good quality at a slight advance upon the price of glass. As for the tariff question preventing the purchase abroad, I am glad it does, because we have everything here to make good porcelain insulators, and we should make them and keep our money at home. This may not be good Free Trade doctrine but it appeals to the national pocket, which oftentimes is a more sensitive article than the national heart. I should be pleased to see this discussion of insulators continued in your paper. I should especially be pleased to have an explanation from some electrician showing why porcelain is a better insulator than glass, and a positive definition of the requisites for a good insulator. I do not think that Messrs. Jones and Smith have been sufficiently explicit upon this point, as I have shown in a rough manner that glass possesses the qualities said to be needed, as well as, and somewhat better than, porcelain. Faites jaillir la lumiere electrique on this subject.