Crumb coating, also known as dirty icing, a cake, does not, as its name suggests, mean adding crumbs to the outer coating of a cake. Instead, crumb coating is the step that most pastry chefs take to make sure that all the loose crumbs on a cake are trapped in an initial coat of icing so that when you add the second (and actual) icing layer no crumbs mix with the icing and look bad.