There are many ways to prepare an indigo vat, some use soda ash and spectralite, some use some use sulphuric acid, some use iron and some urine. For this indigo vat, I am using a fructose base. You can also use ageing fruit instead of fructose sugar.
The fructose indigo vat was developed by Michel Garcia. The addition of the fructose sugar acts as a reducing agent to the Indigo. The sugar removes one of the oxygen molecules from the indigo making it soluble in water.
The addition of the Calcium Hydroxide (slaked or hydrated lime) changes the pH from an acid to a base. The proper pH to get good colour on wool should be about +9 and for cotton and cellulose +10.