The bakers in our midst will know how challenging it can be to get a good grigne on their loaves. Here are a few thoughts on how to get it right!
Just before going into the oven, artisan bakers (and industrious home bakers!) will cut slash marks in the tops of their loaves. In the oven, these slash marks become the escape route for the trapped gases inside. The marks also act like an accordion, helping the crust expand evenly as the loaf "springs" in the oven. Without the slash marks, the gases would break through the weakest point in the surface, tearing the crust randomly.
There's not really a scientific reason to make a grigne - it just looks more aesthetically pleasing on the finished loaf! As you slash the surface of the loaf, cut at an angle instead of straight down - like slitting an envelope, as the great Peter Reinhart says. This creates a little shelf, or grigne, of dough that will separate from the loaf as it bakes, becoming crusty and brown.
Have you had any success getting a grigne on your loaves at home?
Related: How to Knead Bread Dough: The Video