Precision of pH for Ferrous and Fructose Vats and Indicators of Under or Over Reduction

Q: I have seen a lot of commentary online about the necessity of getting the pH of a ferrous/fructose vat just right. Can you provide any insight on how much I need to be concerned about the precision of the pH? And how can I tell if my vat is properly reduced, is it by the color of the vat?

A: I've found that a lot of literature is over-fixated on precise pH measurements of a vat. Indigo vats actually have a lot of flexibility so long as your pH is not too low because the indigo won’t properly dissolve in a vat that is too acidic. For a home and hobby dyer, working with a ferrous or fructose vat, I've never seen any issues related to a pH that is too high (unless you are working with silk because the alkaline environment can really break down silk fibers quickly).

You really don’t need to worry about over-reduction (having a pH that is too high) in a vat reduced by iron or fructose. The only vats I've ever seen that are over-reduced are those that have an industrial reducing agent like thiourea dioxide and lye. They will turn pee-yellow to pale yellow/white and literally strip indigo off of well-dyed pieces. I have never seen a ferrous or a fructose vat do that.

As for under-reduction (having a pH that is too low), the color of your vat is not an indicator of under-reduction as different reducing agents color the liquor differently. Generally, what you are looking for is the transparency and darkness/lightness of the liquid rather than the color: if the liquid is transparent and not mid to dark green or blue, it is going to transfer any available indigo pigment well. If the liquid in your vat is opaque and dark that is an indicator that the pH is too low and it needs more reduction. If you try to dye in an under-reduced vat your blues will appear dull.