Porcelain is made with various clays and sand, and subjected to high temperatures to melt and fuse the minerals. Afterwards, a glaze is applied. The glaze itself is various melted sand compounds, which form a thick, glass-like coating. After cooling, porcelain does not conduct electricity, making it an ideal substance for insulators. Porcelain is also chemically inert, meaning most chemicals do not attack or react with porcelain.
Nearly all materials expand when heated, and contract when cooled. Porcelain is no different. Because insulators are mainly outdoors, they are subjected to wide temperature variations caused by the seasons. Over a period of years, because of the constant expansion and contraction, hairline cracks develop. John Marks, technical editor of "Rural Electrification Magazine," cited John Farquhar of Hi-Test Detection Instruments regarding the expansion-contraction problem. Farquhar stated the problem of hairline cracking is particularly significant when an insulator is mounted on an aluminum support, as opposed to an iron support. Aluminum expands more than iron, placing significant stress on the insulator, which causes hairline cracks. Hubbell Power Systems states hairline cracks in porcelain are difficult to detect.
Rain water is never pure water. It contains dissolved minerals, caused by air pollution. After the water evaporates off the surface of the insulator, these minerals form a film on the surface and seep into hairline cracks. Unfortunately, the minerals, such as iron, are conductive. The end result of pollution accumulation on the surface of the insulator is a short circuit when electricity flows across the pollution film. This effect is called "pollution flashover," and has been studied extensively by electrical engineering doctorates Vosloo and Holtzhausen at the University of Stellenbosch in South Africa. Pollution flashover is suspected for causing blackouts and brownouts.
Anecdotal accounts by working electricians state that porcelain becomes brittle in extremely cold weather, making it difficult to install. Further accounts state that during installation, if the insulators are bumped slightly or dropped, they will break easier in cold weather than in warm weather. As of 2011, no in-depth scientific studies have been done to investigate this claim.