Tyrian purple (6,6′-dibromoindigo) is the oldest known dye, and has been used since pre-Roman times in the Mediterranean region. In its reduced form in a vat, it dyes fibers a brilliant violet. Although this dye is stable in the solid state, the leuco-form used in vat-dyeing is unstable in light, giving leuco-indigo by debromination; fibers then become dyed a bluish shade. We examined how to prevent the influence of light on this dye in the vat. We found that light of 300-500 nm wavelength promoted debromination, and that good vat-dyeing could be achieved by excluding light of such wavelengths.( E-mail: sawada@ge.meisei-u.ac.jp)
