I want to know the answer to the following question:
Besides porcelain, paper and mica, what other non conductive material was used as an insulator?
All the background I'm provided with is that "Paper capacitors made by sandwiching a strip of impregnated paper between strips of metal, and rolling the result into a cylinder were commonly used in the late 19century; their manufacture started in 1876, and they were used from the early 20th century as decoupling capacitors in telecommunications (telephony).". Is that enough to answer the question?