From what we've gathered from our various bread books, a crunchy crust on your loaf of bread is the result of moisture and high heat during baking. Moisture comes from the bread itself and from any moisture introduced to the baking environment (for instance, by spraying water in the oven or throwing in a few ice cubes).
It's hard to get the same high heat conditions in a home oven as in a professional baker's oven. We can come close by using a baking stone, which helps create an even distribution of heat, and then heating our oven as high as possible. Most artisan loaves with no eggs or dairy do very well at around 450°.
The combination of moisture and high heat allow for an initial rise (oven spring) and then the crust sets. The moisture on the outside of the loaf quickly evaporates, forming that hard, ultra-crispy crust.
The hairline fractures only form after you take the loaf out of the oven. The inside of the bread starts to contract slightly as the loaf cools, pulling on the crust. Since the crust is so hard, it cracks under this pressure instead of flexing.
Until we started baking the no-knead bread and the bread from Artisan Bread in Five Minutes, we had never seen this happen for ourselves. We think the fact that these recipes have such a high moisture content helped us get the kind of crispy crust that we'd read about.
Have you ever gotten this kind of cracking in your bread crust?
(Image: Emma Christensen for the Kitchn)