Lightly whisk the egg whites, lemon juice and glycerine if using in a large bowl. Gradually work in the sifted powdered sugar. Continue beating until the icing is stiff, smooth and glossy. For flat coating, the icing should stand in soft peaks, so when you lift the spoon out of the bowl the icing stands in a peak with a gentle curve. Cover the bowl of icing with clean damp dish towel or plastic wrap and leave for about 1 hour before using to allow any air bubbles to dispense.