Glossary
Royal icing
A hard-drying icing made from egg whites or meringue powder and powdered sugar, used for cookies and fine piping.
Royal icing is a hard-drying icing made by beating powdered sugar with either egg whites or meringue powder until it forms a smooth, pourable or pipeable mixture. Once dry, it sets into a firm, almost brittle finish, which is why it is the standard choice for decorated cookies and fine piped detail rather than for frosting a whole cake.
Consistency changes what the icing can do. A stiffer mix holds piped lines, dots and lettering in place without spreading, while a thinned-down flood consistency is poured or spread to fill in an outlined shape and settles into a smooth, glossy surface as it dries, which is the basis of most decorated sugar cookies.
Look at drying time when planning a project, since royal icing needs hours, sometimes closer to a full day, to dry hard enough to stack or package cookies without smudging. A common mistake is moving decorated pieces too soon, or piping fine detail with icing that is still slightly too thick to flow cleanly out of a small tip.