A friend who does restoration/conservation work on antiques and artwork has told me the method he follows when dealing with an "unknown": start with plain distilled water on a cotton swab, then spit, then water with a special liquid soap the experts use called Vipurex (I might substitute Murphy's Oil Soap). Only after that fails does he try solvents or abrasives, and it's always the least volatile and mildest abrasives first. Just sharing...