Royal icing is a confection made of egg whites beaten together with sugar or confectioner's sugar until very stiff. Once the mixture hardens, it can hold up well at room temperature when used to create decorations for cakes or other desserts. Many bakers like to use royal icing for piped decorative elements, such as stars or swags. You can refrigerate royal icing before you use it. Once the cake is iced or decorated and the icing has hardened, however, refrigeration is your enemy. Condensation causes royal icing to melt.
Make your royal icing according to recipe.
Ice the cake and/or form piped or moulded decorations.
Place the iced or decorated cake in the refrigerator.
Remove the iced or decorated cake from the refrigerator to room temperature. Condensation will occur. The icing will begin to dissolve, as the structure of the stiff meringue breaks down. Your decorations will soften and may even collapse.
If you want to thin your royal icing, add a few drops of water at a time until you achieve the desired consistency.
Oil and fats will also cause royal icing to break down.