• Oxidized indigo is blue and insoluble

  • Reduced indigo is white and soluble

  • The urine vat is temperamental

  • 1744: Bergrat Barth discovers “indigo carmine” by dissolving indigo in sulfuric acid

  • This is the first “artificial” color

  • Indigo carmine is blue and water-soluble

  • Indigo carmine (FD&C Blue #2) remains one of only 9 FDA-approved food colors

  • Indigo carmine requires sulfuric acid