Icing
Royal icing is often used to decorate cookies because it dries hard. At the White House 2005 Christmas, Thaddeus DuBois, the White House Executive Pastry Chef at that time, decorated snowflake cookies with brushed and piped royal icing. In this case, as with many of the decorated cookies Dubois made for the president, his family and their guests, the traditional royal icing was used, a mixture of raw egg whites, powdered sugar, and a drop of lemon juice.
Due to health issues with raw egg consumption, such as salmonella, an egg-free variation of royal icing is sometimes alternatively used by decorators. Meringue powder is used instead of the egg whites to create stiffness. Pasteurized refrigerated egg whites are sold at grocery stores for a safer traditional recipe.
A sugar glaze made without egg whites and consisting of powdered sugar, water, corn syrup and flavoring (such as almond) is another popular choice for decorating cookies. To decorate a cookie with glaze, an outline is piped just inside the edge of the cookie. Then the design is filled by piping a line of glaze back and forth across the cookie, while staying within the boundaries of the outline.
The glaze must be applied quickly because royal icing crusts quickly, but it needs to be moist enough for the piped lines to melt into each, so that the design is smooth. When the icing may crust faster than a design can be filled, the design can be blocked off first into smaller sections. To block off the design, cookie decorators pipe the outline of the cookie as usual, but then section it off in smaller sections, filling them in one at a time. Empty nooks and crannies that the decorating tip didn't pipe into can be filled by carefully dragging a toothpick through the icing into any empty spaces.
Cookies can be decorated with more than one color of icing. This is accomplished by allowing the first color to dry completely (often for as long as 2 hours) before adding the second color of icing.
While the goal is usually to keep the colors separated when filling in a design on a cookie with icing (such as the white of Santa's beard from the red of his suit), sometimes the colors or bled together on purpose to create a design such as a spider web design. First a white outline is piped and filled in with white. Then using the black icing, a spiral from the center to the outer edge is piped. The web is created by dragging a toothpick in a straight line from the center across the spiral to the outer edge. The more lines, the more intricate the web. decorating bags,one filled with white icing and another with black, and both fit with small round tips.
Read more about this topic: Cookie Decorating