Boil woad leaves - until liquid is sherry coloured: it changes quickly, but simmer for an hour.Add soda crystals - until alkalinity reaches 9-10. Use litmus papers from plant dye suppliers.Whisk liquid - until froth forms. A tiring 10-15 mins but liquid can also be poured from height from one pan to another making bubbles. (for a whisk break)Heat dye bath - to 50 deg again. Set aside 20 mins. (use cooking thermometer)Add desert spoon sodium dithionite - to remove oxygen. Add enough until dye bath turns limey green.Place silk in liquid - carefully without creating air bubbles. Submerge. Leave for 20 mins. Woad colouring occurs quickly after exposure to air, because the dye bath had the oxygen removed by sodium dithionite; re-exposure to oxygen turns it blue.Remove fabric - and rest. Watch it turn blue in air. (mine went turquoise on ahimsa cream silk using pond water)Rinse out - when colour as dark as will go, hang out to dry.