EGG WASH

An egg wash is used to make pastry (such as a pie crust) shine, and aids in browning.  An egg wash is also used to bind parts together (such as when patching holes or cracks in your crust, or to seal the edges of the top and bottom crusts together, or to add fancy cut-outs), or to moisture-seal the bottom crust when blind baking so it doesn't get soggy when the pie filling is added.

An egg wash is just beaten egg and maybe a little bit of liquid (usually milk or water), which is mixed together and brushed (or "washed") onto the pie dough's surface with a pastry brush. 

Be careful not to brush it on too thickly, or it can dribble down between your crust and the pie pan, essentially gluing them together.  And if it's too thick and goopy, whatever you're trying to seal won't seal, it will just get mushy.

“'Why' and 'How' are words so important they cannot be too often used.” Napoleon Bonaparte

EGG WASH RECIPE: 

The basic rule of thumb is 1 tsp of liquid per egg.  You can experiment with adding spices to your egg wash, such as cinnamon or nutmeg, for added flavor and added color.  The part of the egg you use and liquid you add to your egg wash will determine the finished look of the pie crust.

For this result:	Make an egg wash with:
Shiny	whole egg, salt
Faint Shine	whole egg, milk
Matte, golden-brown	whole egg, water
Shiny, golden-brown	egg yolk, water
Shiny, golden-brown	egg yolk, salt
Shiny, dark	egg yolk, cream
Crisp, pale	egg white
Matte, golden-brown	heavy cream or half & half
Faint shine, golden-brown	olive oil