Sodium Dithionite (also called Hydros, sodium hydrosulphite or sodium hydrosulfite) is widely used in Europe as a safe alternative to Spectralite for chemical dyeing with woad & indigo.
You can substitute Sodium Dithionite where Spectralite is mentioned in the text but you will need to double the quantities because Sodium Dithionite is not as strong. For example, use 30 grams of Sodium Dithionite where 15 grams of Spectralite was specified.
Sodium Dithionite is weighed and sealed in a polythene bag. The bag is then labelled, placed in a padded envelope and posted to you First Class.