In part II (Archeomaterials 1:2 (1987), 135-143 and 2: 1 (1987), 87) an experimental approach in elucidating the ancient processing of indigoid dyes was described. The most renowned dye of antiquity, Royal Purple, whose major component is 6,6-dibromoindigotin CDBI) was discussed. It was considered that only a more detailed knowledge of indigoid dye chemistry would allow reliable links to be made between specific chemical compounds and reactions on the one hand and the old description on the other. Experiments involving the reduction of DBI, not previously thoroughly investigated, have allowed a more detailed chemical explanation for the description in the old texts. -- ICCROM