Sodium Hydrosulfite aka "Hydro"

The reducing agent used in indigo vats; pre-reduced, synthetic or organic.

This product must be ship ground only. So if ordering other items that you need quickly, mention it in the notes at check out. 

Jacquard:
Instructions
For use with Jacquard’s Pre-reduced Indigo: use 1 part indigo dye : 2 parts Sodium Hydrosulfite : 4 parts Soda Ash. (Use 1 gal/3.79 L of water for every 0.18/5 g of dye.)

For use as a color remover: use in equal parts with Soda Ash (1:1) at 3-5% by dry weight of fabric. Heat to 185°F/85°C, add fabric and continue heating. Color removal works best at a rolling boil. Keep fabric moving for 10 to 20 minutes or until color has left as desired. Wash with mild detergent in cool water.

Note: not all dyes are fully susceptible to color removal. Results may vary. Test first when possible.

CAUTION: Harmful if swallowed. Give water or milk if swallowed and call a physician. Avoid contact with eyes. Keep out of reach of children. Do not expose to excessive heat.