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Optical brighteners for dazzling white laundry
Whether made of wool or cotton, the pale yellow colour of textile fibres increases with aging as they absorb light in the blue and violet. How then can a fabric or piece of clothing be made to look very white ? By superimposing blue light to compensate for the absence of blue ? Washerwomen and cleaners of yesteryear thought of adsorbing a blue dye on the fibres thanks to the famous "blue bags" of Reckitt’s blue or Guimet’s blue (synthetic ultramarine blue), which have had their heyday. In fact, though laundry thus treated loses its yellow colour, it does not really look white but rather pale grey.
Another much more effective solution consists in impregnating the fabric with a dye that emits fluorescence in the blue following the absorption of light in the violet and near ultraviolet. Optical brighteners, the first patent of which dates back to 1935, are now an ingredient of every laundry detergent. Fluorescence adds brightness to whiteness, hence the well known catch phrase "whiter than white", formerly used in an advert praising the merits of laundry detergents, and used by a famous French stand-up comedian, Coluche, in one of his most memorable sketches.
Laboratoire de photophysique et photochimie supramoléculaires et macromoléculaires, CNRS-ENS Cachan
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