When you want an icing to dry hard, like on a British Christmas Cake, or to use it as a glue piecing together sugar sculptures, like those for the Mexican Day of the Dead, or for a gingerbread house, then Royal Icing is what you need.
My mother and I made a gingerbread house every Christmas. Her gluing system was massively difficult and dangerous. We used molten caramelized sugar. It was a race against the sugar burning, and then cooling. The advantage of caramelized sugar is that it is the same color as the gingerbread. We did not use royal icing for any part of our projects. We also had the problem that if it rained around Christmas, which it rarely did as we lived in Los Angeles, the sugar would get soft and our house would collapse!
Royal icing is something to make be feel. Following a recipe can easily lead to despair. Speaking from experience.
Royal icing is made of egg whites stiffened with powdered sugar. That’s it. The recipe ratio is three egg whites to 450 g (1 pound) powdered sugar, or 1 egg white per 150g (5.5oz) sugar.
Egg whites are more stable when whipped in a copper (or silver bowl). For an explanation, see Harold McGee, author of the authoritative Curious Cook. Either buy his book, or look for texts online. Lacking a copper bowl, you can achieve similar stability with cream of tartar.
There are many published recipes for Royal Icing calling for lemon juice. If you end up with a royal icing that is too stiff for your purpose, like you want to pipe some very fine white lines (or you added food coloring so the lines are colored) then you will need to thin down your icing. You can use water. If you will be tasting the sugar, then lemon juice gives it some flavor.
They key to success: Royal icing should be mixed very thick. In this case, “very thick” means that it is virtually impossible to mix this by hand, and unless you have a powerful electric mixer you can run into trouble. Once all the sugar is incorporated into the egg white you should see very crisp strong ridges from the action of the mixer. If you don’t see strong ridges, then add more sugar.
Use a stand mixer, or a very powerful hand mixer, like the one made by Breville, in the United States.
Powdered sugar tip. If you have a powerful blender, then you can easily make your own powdered sugar. Thus, if you have purchased powdered sugar, have used the packet, but still need more, then make it on the spot.