It’s the yolk of the egg that provides both the color and the binding power of an egg wash. The binding properties of yolk have been known to artists for millennia. “Egg tempera” was the medium all serious painters used from ancient Egypt to up until about the Renaissance when oil-based paints displaced it. Egg yolk goes on smooth, dries hard and glossy and sticks remarkably well to a variety of surfaces.
So if yolk is so great for color, shine and crust why not just use it by itself? The answer: because unadulterated yolk dries way too fast. That’s not a good thing since it gets goopy on the brush. Also you don’t want the surface of your pastry to turn taught or brittle before the pastry hits the oven, where it will expand, causing that inflexible surface to tear. The glaze needs some give, which is where the watery white comes in. Combined, egg white and yolk create a perfect all-purpose glaze. It’s golden, it’s shiny, it’s crispy…what’s not too love?
But what if you don’t love it? What if it’s not shiny enough? Too shiny? Not brown enough? Not flexible enough? Well then there are things you can add to bring it more into line with your personal aesthetic.
Water. Water dilutes the yolk still more than the white does by itself. What you have then is a slightly less glossy and/or golden glaze with more flexibility. It’s a nice thing for glazing a pastry or especially a bread that’s really going to increase its surface area in the oven. A brioche, say, or a parker house roll. A teaspoon of water per egg is good for most pastry applications, though bread glazes can take up to two tablespoons of water per egg.
Milk. Milk added to a whole egg glaze accomplishes much the same thing as water, except that the milk proteins, sugars and fats create more of a semi-gloss or even matte surface. As with the water-whole egg glaze, adjust the amount of liquid according to the surface increase you expect. A teaspoon is fine for the top of a piece of puff pastry, which will simply be pushed upward. The surface of a parker house roll will blow up like a balloon. More flex is necessary.
Cream. Cream is generally used in place of the egg white to create a deep, dark, extra shiny and mostly opaque glaze. The fat in the cream keeps the yolk from drying out during application, and the proteins and sugars brown up in the heat, giving a richer appearance. I find a cream glaze too thick for all but the most static, un-expansive applications, like short crusts (pie borders, lattices and such, or tart crusts). One yolk to one teaspoon of cream is fine. You can use up to three.
Salt. It’s said that adding a pinch of salt will amp up the gloss of a whole egg glaze. Me, I’ve never noticed much difference. I strongly suspect this is a kitchen myth. What a pinch of salt will do, as stated above, is create a thinner wash.
And that’s pretty much what I know about egg washes. What if you don’t like — or can’t eat — eggs but still want a glazed appearance on your baked good? Milk, cream or half and half are sometimes used as a glaze on breads or quick breads to give them a golden color. Here again more water means more flexibility and expansion without cracks in the finish. Use whole milk for a roll, heavy cream for a southern biscuit. Sugar sprinkled on a pastry (especially large crystal sugar) is very attractive in place of a glaze. Oil will also add sheen to the surface of a bread.