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 color. If you wish to keep it a few days, take the juice when you have pressed out a teacupful, and adding to it a piece of alum the size of a pea, give it a boil in a saucepan. Or make the juice very strong and add a quart of alcohol. Bottle it air-tight.