Peel, wash and thoroughly shake free from water two pounds of spinach. Pound it raw and then press it in a strong towel, twisting the latter so as to extract all the green juice. Pour this into a saucepan, let it coagulate in a bain-marie, and pour it on to a serviette stretched over a bowl in order to drain away the water. Collect the coloring substance on the serviette, using a palette-knife for the purpose, and put into a mortar; mix with half its weight of butter, strain through a sieve, and put aside to cool. This green butter is always preferable to the liquid green found on the market.