The main ingredient was a dark-brown solution of ferric acetate called "kanemizu" (かねみず), made by dissolving iron filings in vinegar. When the solution was combined with vegetable tannins from such sources as gallnut powder or tea powder, it would turn black and become non-water-soluble, in the same manner that iron gall ink is produced. Coating the teeth with this liquid helped to prevent tooth decay and enamel decay. The dye had to be applied once a day or once every few days.