It is possible to clean a glass insulator. Mix a third of a can of Bar Keepers Friend (found in most grocery stores) with a gallon of water in a non-metal bucket. Put the insulator in the mixture, but be sure the water and the insulator are at the same temperature to avoid stressing the glass. Soak for 12 hours. Scrub the insulator with a nylon pad. Repeat if the insulator does not look clean.