Hmmm, this may be a bit more technical than anyone really cares, but sodium chloride (table salt) is rarely a 'mordant'.
So, what is a mordant you ask? From the
Compendium of Chemical Terminology by the International Union of Pure and Applied Chemistry (IUPAC),
(see
http://goldbook.iupac.org/M04029.html)
A mordant is a "Substance that fixes a dyestuff in or on a material by combining with the dye to form an insoluble compound...". That is, a mordant does a chemical reaction with the dye and forms a new compound with the dye that does not stay dissolved in solution (the solution likely being water). Most all mordants are either acid or alkaline like tannic acid or alum.
As for RIT dye specifically, salt does not react with the dye, so technically, it is not a mordant.
Be that as it may, table salt is still quite useful. Why you may ask? Table salt more readily goes into solution (dissolves in water that is) than the RIT dye. As such, it pushes the RIT dye out of solution. With the dye out of solution, it is then available to attach itself to something else (like a cotton shirt).
Originally Posted by Farlander
I might also add that I have used the dyes sold by theatrical suppliers quite sucessfully for tye-dying and other types of resist painting. They are used in painting scrims (transparent curtains, when the light is from the front, they appear opaque, when from behind, they nearly disappear, giving a "gauzy" appearance to the scene behind it).
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