Add ammonia until the Liquid is yellowish in colour. Add sodium dithionite to deoxidise the liquid. Stir gently without splashing until a bronze scum appears. Put in the hot wet wool then remove it and shake in the air. Dip it again if not blue enough.
Indigo is a fast dye that fades very little in sunlight or in washing.
These are chemical keys to enable the dye to hold fast on the material. Alum is the most common and is dug out of the ground as a mineral salt Mordants are dissolved in water and the material simmered in the vat for about 15 minutes. The material is taken out of the vat, rinsed, and allowed to drip dry and cool. It is then ready to take up the dye.