I dug out a couple of nos octal sockets , pure white ceramic insulator ,unglazed , marked 'Frequenite' Clix pats ,so Ediswan clix , nice quality items machined contacts ,but from being in a box of other assorted bits for years the ceramic had picked up all kinds of metal deposits on the surface , after soap and water I tried Arm n hammer baking soda tooth paste and an old tooth brush ,took a little time but gradually ground away all of the surface contamination, back to whiter than white again .