On Wed, 1 Nov 2000 RSRICHMOND@aol.com wrote:

> I believe that in the textile industry direct cotton dyes were largely 
> replaced by reactive cotton dyes (such as the Procyon series of dyes) quite a 
> few years ago.

   Reactive dyes, which combine covalently with -OH groups of cellulose,
   were introduced in the mid-1950s (ICI's Procion dyes, soon followed
   by CIBA's Remazol series). They are, of course, completely resistant
   to washing, but they can still fade. Direct dyes (introduced in the
   1880s) attach to cellulose by non-covalent and non-ionic forces, and
   are much less resistant to washing. They are nevertheless still made
   in large quantities because they are cheap and, being bis- and 
   poly-azo dyes, provide a lot of strong colour per gram. Application
   is technically simple; dyes that you can buy for home use on
   cellulose fabrics are direct dyes.

   According to the Colour Index (1996 CD-ROM), the dye we know as
   sirius red F3B (C.I. 35780, Direct red 80) is made by 25 different
   manufacturers, with as many different trade-names. It was first
   made in 1922. Congo red (C.I. 22120, Direct red 28) was discovered
   in 1884 and is still manufactured by 16 companies. (The C.I.
   doesn't generally bother mentioning producers of dyes for use
   only as biological stains because the quantities involved are
   trivial by comparison with other uses.)

   It's interesting that reactive dyes have never caught on as
   histological stains. This is because they combine with pretty
   well everything in any tissue. Several fluorescent reactive
   dyes are used as intracellular markers, especially in 
   neurobiology, and fluorescent labelling of antibodies, avidin
   etc is a kind of reactive dyeing.

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1