Name: m corder

Message: What is the purpose of salt in the dye mix?  The recipe I am using calls for urea, salt and cold water dye (about 30 cups urea, 7 cups dye, and 1 cup salt per vat; then one cup of this mixture is mixed with 2 quarts of water for the dye used that day).


There is a page about salt in the FAQ on my site, "Do I need to use salt, in dyeing?", which includes the following:
"Contrary to some old wives' tales, salt is not a dye fixative and does nothing to make dye more permanent; however, it aids in the dyeing process by helping to drive the dye onto the fiber, out of solution, so that it is in the right place for any bonding to the fiber to occur."

The way salt works is by surrounding the fiber, which in water has a negative electrical charge. Most dyes also have a negative electrical charge (the two exceptions being basic dyes, which have a positive charge, and vat dyes, which are neutral). The salt stops the negative charge of the fiber from repelling the negative charge of the dye.

When reactive dyes are applied directly to the fiber in small quantities of water, as in the cases of tie-dyeing and dye painting, salt is not required because the dye is placed right next to the fiber, instead of floating around in a large quantity of water, as in vat dyeing. The salt is very important in dyeing with the large amount of water that is required for smooth, eve, solid-colored dyeing.

Your recipe is one that is unfamiliar to me. I have not heard of using such large amounts of urea, and I have not seen recipes for vat dyeing with cool water dyes that required any urea at all, except possibly one tablespoon per cup of the water initially used to dissolve the dye. A typical recipe for dyeing five pounds of cotton with cool water fiber reactive dye calls for using two and a half ounces of dye powder, twenty cups of salt, and two and a half cups of soda ash, in twenty gallons of water.

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