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Now that I have a small first collection of pigments to play with before new plants can be tested next summer (I do have some old dried things I can try too), there are multiple ways to use them. They need some kind of binder, although I suppose you could just soak them in water. Alcohol? But even watercolours have binders added to add intensity to the colour as well as make it stick to your paper.
You can use oil, egg, honey*, gum, shellac, wax, milk, spit! or buy readymade binders for a variety of mediums. Even an acrylic binder which I may just have to test, although I’m leaning towards wax and shellac since I plan on working with that anyway.