From JOHN HOGGET
The curious reappearance of the original yellow colour to P. F. Larrington’s
cotton trousers after bleaching (Letters, 27 November) can be explained
by the differences in the molecular structures of the two dye types involved.
The majority of dyestuffs for textiles belong to the azo group, which
are cheap to produce and give good colour value but suffer the disadvantage
of being easily destroyed by bleach. Cotton, however, can be dyed with more
expensive vat dyes that form an insoluble, highly substantive pigment within
the fibre structure. The vat dye, usually an anthraquinonoid, has a high
resistance to bleaching.
Hence, the offending green dye from the tracksuit would be destroyed
by the solution of thick bleach, but the more robust vat dye remains intact
rendering the trousers their original pale yellow colour.
John Hogget Society of Dyers and Colourists London
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