Sodium Dithionite otherwise known as HYDROS chemically reduces oxygen from Indigo Dye VAT when using natural indigo dyes to dye fabrics or fibres such as silk, wool, cotton etc.  
 
When dyeing cotton it is important to also use Soda Ash to ensure the correct pH (alkalinity) of the dye bath.
 
Sodium Dithionite must be handled with care, as can be harmful and hazardous.  It must also be kept in ambient, and dry conditions.