salt and fabric dye

Hi Gang,
Just dyed a couple of T-shirts. Yes, good ol’ RIT fabric dye just like my mom used to use. The instructions have you add a cup (!) of salt and a dash of liquid dish detergent to 3 gallons of hot water. I’m guessing the detergent has something to do with lessening surface tension.
But what’s the salt for? A quick web search got me a couple of explanations that didn’t make much sense to me. (The fabric takes up the Na+ ions which in turn attracts the negatively charged dye. Huh? Why would the cotton take up only + ions?)
So I’ve come, as we all do, to get the straight dope.

Salt is a mordant. I’ve died a bunch of things. The salt softens the fibers and makes it take the dye easier.
I don’t know anything about ‘ions’ and such. But I’ve made natural dyes and the salt improves the color by density. It will work even better if you soak the cloth in the salt water mixture overnight.
Vinegar will do things to natural dyes too. Not sure how it would work with Rit.
Oooh, now I wanna tie-dye.

IANAChemist, but as I understand it, the basic idea is that there needs to be neutral charge on the fiber surface. For reasons I don’t really get, there’s an excess of negative ions in the combination of the dissolved dye and the fiber (it’s got something to do with cellulose).

In order to get equilibrium between the dyebath and the cloth, either you lose more negative ions from the fiber (that is, you get less of the dye taken up by the cloth, which means that your dyed color is fainter), or you add a source of positive ions (the Na+ ions in the dissolved salt) that allows equilibrium to be reached with more dye in the cloth (i.e., a deeper color = better dye job).

I morderd a few slugs with salt and died them.

I’m embarrassed to admit this actually made me laugh.

Also, thanks for the real answers. The ion thing makes more sense now.

Whether natural or not, the fibers are polymers and have the kind of “side branches” that in contact with distilled water tend to lose more H+ than they gain; the fiber itself becomes a negative ion, you can add something back to it to neutralize it but you can’t remove the negative ions from the cloth without breaking up the fibers. Depending on the type of fiber (wool and silk are proteins, polyamides; cotton and linen are “fiber”, undigestible carbs, polyeters*), the best mordant to use will be H+ or Na+, for which the sources which are cheapest and most likely to be available in any home happen to be vinegar and salt respectively. Using the wrong mordant will produce worse results re. dyeing, but it can also fragilize the cloth, it can break up the fibers (the technical term is hydrolyze).

  • No, not polyesters. In chemistry a little letter makes a lot of difference.