Indigo Dying is not a quick process. You dip the fabric in the indigo bath, let the pigment to stick on the fabric, then let it outside to oxidase so the color stays on the fabric, then dip it again to build up the color layer.
The process at the Zeugfäberei is 10min inside the bath, 15min outside to oxidase as one set, then repeat this for at least 6 times. If one wants to have darker blue, you can continue this for more times. The more you repeat, the darker it gets.
Unlike other dyestuff, indigo color is just staying on the fabric and not really going into the fiber itself. Even it looks very dark, almost black after the oxidation process, when you wash to take the papp out, some blue comes out. Also, one should bare in mind that color looks darker when wet. It is always a bit “disappointing” when you see washed and dried fabric… it is not as “dark” as I thought…
“Indigo is a challenging dye because it is not soluble in water. To be dissolved, it must undergo a chemical change (reduction). Reduction converts indigo into “white indigo” (leuco-indigo). When a submerged fabric is removed from the dyebath, the white indigo quickly combines with oxygen in the air and reverts to the insoluble, intensely colored indigo.”
https://en.wikipedia.org/wiki/Indigo_dye
I hope you can see the progress of dying cycle from the photos above. It often starts to get very deep after 3rd cycle, but then if you stop there it will be very light blue after washing it. Usually one needs to do 6 cycles, and for most of the Indigo Hyphae fabrics I did 7-8 cycles to get deeper blue.
After the dye process, you wash the fabric in water to remove papp and extra indigo that did not bind to the fabric. Then dip it in water with citric acit for 15min to neutralize the fabric (the Küpe is basic). Then wash in in water one more time to wash out the acidic water and extra indigo (through out the washing process, you will see the water getting blue). You can finish the washing process here, or put it in a washing machine and run a short wash program to make sure it is washed well. Hang in shade and dry it well.
The shade of blue comes from how many cycles it has been dyed/ condition of the Küpe/ how good the fiber of the fabric binds with indigo… it is never the same and you do not know which blue you get until you wash it.