It doesn't conduct electricity very much. It is weather resistant. It lasts a long time.
The most common insulation material for HV use remains porcelain. Synthetic materials are fast replacing porcelain as the insulation material of choice.
Porcelain
Most transformers are insulated with polymers or porcelain. Some transformers have an oil based insulation which must be kept dry to remain effective.
Porcelain is actually a poor conductor of heat, which means it is good at insulating and retaining heat. This property makes porcelain a good material for dishes and cookware that need to retain heat or keep food warm.
Bruno Martinoli has written: 'Isolatori di porcellana' -- subject(s): Electric insulators and insulation, Porcelain
In general, porcelain is not a conductor. It is considered an insulator. It is possible to make ceramics that will conduct, but the porcelain we normally find is a good insulator.
Ornl.com has said that blanket roof insulation is the best type of roof insulation. They are very helpful and good for insulating your roof. Ornl.com is good for roof insulation.
There Are Many Good Soundproofing Insulation Materials, but Insulation and Mass-Load Vinyl Seem to Be the Best. Mass-Loaded Vinyl and Insulation Are Good for Filling Gaps.
Porcelain
Because one of its main functions is to provide good insulation.
I do not understand why you want to remove the porcelain if it is broken off. Just remove & replace the spark plug which is no longer any good if the porcelain is broken.
I haven't seen a good epoxy that's thin enough for fine porcelain. I DO get good results from Elmer's China and Glass cement.