Caustic Pencil 40% 1'sA caustic pencil is typically made from a substance called silver nitrate. The percentage mentioned, 40%, refers to the concentration or strength of silver nitrate in the pencil. In this case, a caustic pencil with 40% silver nitrate would have a relatively high concentration of the chemical. Caustic pencils are used for medical purposes, primarily for the removal and cauterization of warts, skin tags, and other small skin lesions. When applied to the