Take fresh spinach or beet leaves, and pound them in a marble mortar. If you want it for immediate use, take off the green froth as it rises, and mix it with the article you intend to colour. If you wish to keep it a few days, take the juice when you have pressed out a tea-cup full, and adding to it a piece of alum the size of a pea, give it a boil in a saucepan.