HYDROS ~ is the colour or oxygen remover used for indigo or woad vats.
Dithionite can replace Spectralite ( basically the same, but you would need less and it is more expensive).
A 10L Indigo Vat requests up to 50g of Hydros.
* Sprinkle over the Dye Vat to remove Oxygen - the Vat should turn a yellow green.
**Please be aware, that this is a chemical and to avoid a chemical reaction allways add Hydros to water and not Water to Hydros.
***Other words for Hydros: Sodium Dithionite, Thiourea dioxide, Thiox, Hydrosulfite Sodium.