What does soda ash do before dyeing?

I soaked this lavender work shirt in soda ash water and wrung it out by hand.

It’s 100% cotton with white screen print graphics that I’m planning to overdye. The white ink stays white because it’s a synthetic material.

The soda ash step doesn’t look dramatic, but it makes a huge difference in how permanent the color will be.

It’s called sodium carbonate, but in the tie dye world, people refer to it as soda ash.

I usually mix one cup of soda ash per gallon of water and let the shirt get fully wet before squeezing it out.

Some people like to spin out the excess soda ash and pour it back into the main soaking bucket.