The main use for salt, in dyeing, is when you are immersion-dyeing with a high water ratio, in a bucket, large pot, or the washing machine, when your goal is to get a single perfectly even color. Large amounts of water are good for level dyeing, but they also make it hard for the dye to get close enough to the fiber to react with it. That can mean that a lot of the dye gets wasted. If you add a lot of salt, it reduces the tendency of the negative charge of the fabric to repel the negative charge of the dye. The dye finds the fabric more easily, so it can react with it. This saves tremendously on how much dye you have to use, compared to not using salt at all, when you have a high ratio of water to fiber.
Salt reduces the solubility of dye. I like to add some salt to my dyebaths when doing
low water immersion dyeing (LWI), in order to increase the intensity of the markings. When you're using a low water ratio, as in LWI dyeing, dye painting, or tie-dyeing, you do not
have to add salt to your dye. Your colors may get darker if you add just enough salt, but adding too much salt will have the opposite effect to what you want. When you add salt, if you add it directly to your dye concentrate mixture, you can make previously dissolved dye come right out of solution. If this happens, your dye colors become lighter, instead of darker. It's something to keep an eye out for. Once I added a bunch of salt directly to the mixture I'd made of some turquoise liquid Remazol fiber reactive dye with water, and the dye precipitated right out. The results were as though I'd used only a tiny bit of turquoise dye. I had to do a second round of dyeing in order to get the turquoise into the design.
Take a look at my page about
using salt in dyeing. One of the useful tips there concerns the increase in the volume of salt you need to use if you use koshering salt instead of regular salt. One brand of koshering salt is much less dense than the other, so you have to use a lot more, if you're measuring by volume instead of by weight.
-Paula
