When making a glacé or royal icing with pure icing sugar, don’t let it sit around uncovered as it will start to set on the top and add lumps to your icing when you stir it (as this one has!). If you aren’t going to use your icing straight away, remember to cover the surface closely with plastic wrap and/or the bowl with a damp cloth to prevent a hard layer developing on the top. This way, it will stand around very happily waiting for you for at least a couple of hours. read less