Used in the Natural Fermentation Indigo vat. Also used in some cotton dye processes, as an alkali to open up the fibres to better accept dye. “Mercerization” which straightens the cotton fibres and makes them both more lustrous and more dyeable, is essentially soaking the yarn is very strong sodium carbonate, then stretching and letting the yarn dry under tension.
This is pure Sodium Carbonate. “Washing Soda” available in the supermarket contains bleaches, dyes and perfumes and will not work in the Indigo Natural Fermentation vat.