Eugenol, the active ingredient in clove oil, is the culprit. Clove oil has been used to numb toothache pain since Medieval times. The ancient Chinese also routinely chewed cloves to freshen their breath before talking to the emperor.
Cloves are the unopened, dried buds of the myrtle tree. The bud oil is the premium product and is used as a food seasoning and in perfumes. The leaf oil is not usually used in its crude form, but is further processed to isolate the eugenol. Eugenol is still used as a topical analgesic by dental professionals, and is included in most over-the-counter toothache remedies. When mixed with zinc oxide, eugenol is a common base for temporary fillings.