This vat is based on the reaction between zinc dust and calcium hydroxide also known as calx or slaked lime. Consequently, it is possible to build up very dark, deep indigo blues without much difficulty using multiple dips.
Stainless steel measuring spoons do double duty.
Set the dye vat with water, calx and zinc dust. These two additives react and create a reducing environment in the vat, thus eliminating any residual oxygen in the water. After adding additives, let stand 6 hours with lid on. Mix up stock solution with indigo, alcohol, calx, zinc dust and water. Place the water into the 1-quart pyrex mixing cup.