Brushing egg wash onto pastry adds a golden color and sheen to the cooked pastry. Egg wash can also be used as a sort of glue to secure two edges of pastry together, as when making filled pastries, things like empanadas or other en croute recipes.
Egg wash can be used on other foods besides just pastry. Duchesse potatoes are usually brushed with egg wash before baking.
Egg Wash Recipe: The typical egg wash recipe consists of about two tablespoons of liquid for each egg. Using less liquid creates a darker egg wash. Egg wash is usually made with water, although milk can be used, too. Some egg wash recipes call for a pinch or so of salt as well.