Thiourea Dioxide otherwise known as Spectralite or THIOX chemically reduces oxygen from Indigo Dye VAT when using natural indigo dyes to dye fabrics or fibres such as silk, wool, cotton etc.  Spectralite is approximately x 5 more efficient at reducing oxygen that the alternative which is known as Sodium Dithionite (HYDROS).
 
When dyeing cotton it is important to also use Soda Ash to ensure the correct pH (alkalinity) of the dye bath.
 
Thiourea Dioxide must be handled with care, as can be harmful and hazardous.  It must also be kept in ambient, and dry conditions.