Welcome to the world of woad, or pastel as it is known when grown in the
traditional south western parts of France. A very humble plant that
grows less enthusiastically in my Sydney back yard than it did on the
farm near Toulouse, but is giving me enough to fiddle with. This colour was achieved by a very simple process which started out in my garden pot looking like this:
Here's a technique I'd been meaning to try for years, described in Michel Garcia's book "Couleurs Vegetales". It uses just the fresh leaf of one of the indigo giving plants, and a mallet. Gotta love that. No chemicals, no mordants. Just some soap to transform in magic.
Place some thick cardboard down on a firm wooden surface. Then place fabric on top, and fresh indigo leaves on the fabric. ( First i tried silk, but a stronger fabric like hemp gave much better results.) Then cover again with fabric and tap lightly until the green chlorophyll shows through the fabric.
Leave the fabric in the sun for about half an hour, where it may begin to change to blue already. After that,
rince with hand soap and watch the green transform into the blue/turquoise of indigo.