The oxidized skin on the top of the vat must be removed by stirring the vat and removing the resulting “flower”
Indigo color
Oxygen reacts with the surface of the indigo vat to form a thin film which displays the metallic gleam typical of indigo. Beneath this skin the vat should be green in color
Dark Blue
To get dark blue, the item must be dipped multiple times.This paper is about to undergo a second dip
Green
The paper comes out of the vat green.Only upon exposure to air does the dye react with oxygen and turn blue
Second dip
Emerging from the second dip, this paper was first crumpled so that the color darkens where the fibers have broken along the crease line