Glass insulators on wooden posts were a common sight in the United States, providing electricity to homes and businesses for nearly a century. Insulators were protection for the line and the public, because glass does not conduct electricity. These insulators separated the electrical lines on poles and mounted on a wooden post. The moulds are a distinguishable shape and the glass is heavy. Most have a round base, some with "teeth" and some smooth. They have a top groove to hold the wire and often have embossed writing on the base. This makes the glass insulator difficult to clean, but it is possible.
Clean glass insulators like old bottles or Depression glass items. Use tepid or lukewarm water and do not change temperatures quickly to avoid cracking the glass.
Place a towel in a sink or pan in case you drop one of these heavy pieces while cleaning. Add the lukewarm water and detergent.
Work on one insulator at a time. Scrub the crevices with a toothbrush, working all around and on the underside.
Place equal parts of water and white vinegar in a pan larger than the insulator so you can cover the insulator completely. This mixture is suggested by the American Bottle website. Make sure the vinegar is room temperature or lukewarm before adding an insulator. Add the insulator and let it soak overnight. This natural product removes mineral deposits and is safer to use than the lime-remover bathroom products recommended by antique bottles websites.
Remove the insulator from the white vinegar bath and toothbrush any areas that have remaining residue. The residue should loosen with the overnight soak.
Tackle rust stains on old insulators with an oxalic acid product. These are available in the cleaning products at the grocery store with names like Zud and Barkeeper's Friend.
Use denture-cleaning tablets for stubborn stains, suggests a vintage bottle website. Coca-Cola is a cleaner recommended by the American Bottle website. Any etching or cloudiness that has become a part of the insulator may not clean. Insulators buried for years, like old bottles, will not clean by home methods but require a tumbler and the skill of a professional.