Glaubers salt

A chemical additive that slows how fast dye strikes, allowing for smoother blends.

I haven't used this salt much in my own process because I'm usually ice dyeing, but I have heard that it helps with getting blues, especially turquoise, to be more vibrant.

It's often something people mix when they're doing liquid dye in squirt bottles. Some people doing large batch tie dye will mix up what they call chemical water, which has salt, urea, and sometimes thickeners in the water pre-dissolved, and then they'll portion out enough for each dye color.