Blue No. 1 is called "brilliant blue" and, as is typical of modern dyes, was originally derived from coal tar, although most manufacturers now make it from an oil base. Blue No. 2, or "indigotine," on the other hand, is a synthetic version of the plant-based indigo that has a long history as a textile dye. Indigo, which comes from the indigo plant (Indigofera), has been used for probably at least 4,000 years. There is a written recipe for dying wool with indigo on a Babylonian cuneiform tablet dated to the seventh century B.C. There is evidence that it was used in neolithic Europe and in pharaonic Egypt. It also comes from the woad plant (Isatis tinctoria), and was used by the Celts in Scotland to dye their faces.