I conducted an experiment to determine two things with respect to high-hydration sourdough bread:
(1) What does it look like when it's underproofed, perfectly proofed and overproofed?
(2) What is the difference between baking in an enclosed vessel v. on a baking stone with steam?
I made 5 loaves, each of which were about 385g as follows (~78% hydration):
-800g unbleached AP
-75g home-milled white, spring whole grain wheat flour
-75g home-milled red, spring whole grain wheat flour
-50g home-milled whole grain rye flour
-180g levain (100% hydration)
-21g fine sea salt
Mix flours and 710g water. Autolyze for about 1 hour. Mix in levain, salt and rest of water. Slap and fold until moderate gluten development. Bulk ferment for around 4 hours at 80-85 degrees (until grown by 50%) with 3 S&F every 30 minutes for first 1.5 hours. Divide, preshape and bench rest for 30 minutes, then shape into boules. All loaves were baked in an oven preheated to 500 degrees. When the loaves were placed in the oven, the oven temperature was lowered to 475 and the loaves baked with steam for 20 minutes. The steam was then taken away and the loaves baked for another 20 minutes at 450.
These are the ways in which the loaves differed:
(1) First bake: NO PROOF - immediately after shaping, Loaf One was baked in an earthenware pot, Loaf Two was baked directly on a baking steel. Steam was provided to the second loaf using lavarocks (with water poured over) at the bottom of the oven and sporadic spritzing with a spray bottle.
(2) Second bake: MIDDLE PROOF - Loaf Three was baked in an earthenware pot, Loaf Four was baked on the baking steel (same as first bake). These loaves were proofed at room temperature (about 80 degrees) for 25 minutes, and then in a warm fridge (about 60 degrees) for 35 minutes.
(3) Third bake: OVER PROOF - Loaf Five was baked in an earthenware pot. I did not use the baking steel for this bake because I already knew that the loaf would pancake. Loaf Five was proofed for 2.5 hours at room temperature (around 80 degrees).
Loaves One and Two Top View. I tried to score them differently, but I did a very rough job scoring for Loaf Two, which is evident in the bake.
Loaves One and Two Side View. The loaves seem to have similar oven spring, although Loaf Two baked darker.
Loaf Two. I was surprised that Loaf Two did not spread in the oven (because all my prior experiences trying to bake high-hydration SD on a baking stone resulted in spreading), but in hindsight this makes some sense as the loaf did not have a chance to relax after shaping.
Note that many of the holes inside are very small. These loaves felt quite dense.
Loaves Four and Three Top View. Loaf Four baked with a larger diameter due to spreading in the oven. In addition, the rings from the banneton are less evident in Loaf Four due to the spritzing.
Loaves Four and Three Side View.
Loaf Four baked higher on the side facing the oven door due to my spritzing on that side of the loaf.
Loaf Three. Loaf Four was much more open than Loaf Three, even though it was subject to the same conditions as Loaf Three. My assumption is that I handled Loaf Three more during shaping.
Comparison of Loaf One and Three (both baked in earthenware pot). The oven spring is about the same.
Loaf One has much more uneven hole distribution, with quite large holes near the outer edges/crust.
Loaf Three, on the other hand, has much more even holes. The smaller holes are bigger and the larger holes are smaller.
Loaf One crumb close up (middle of loaf).
Loaf Three crumb close up (middle of loaf).
Also note the shape difference between Loaves One and Three - you can see that Loaf Three wanted to spread more, but was prevented from doing so by the pot. (Loaf One, in contrast, is more of a triangular shape rather than the shape of the pot.)
Loaf Five Top View. This loaf baked darker because of the additional fermentation during the longer proof, which resulted in more caramelization of the crust. The overproofing is evident in the way the scores opened up (more of a stretching, rather than a tearing, apart).
Loaf Five Side View. You can see that the dough was really resting against the pot for the entire bake (where the rings end).
Loaf Five crumb close up. The smaller holes are much larger than the smaller holes in Loaves One through Four, and the size of the holes is generally more even.
This hole distribution seems more desirable for a sandwich bread cooked in a loaf pan, and I would not have considered this overproofed for such a loaf because the bread did not collapse, the bread is light and airy and there was some oven spring (but not enough for a blowout in the loaf pan). These results show me that slightly underproofing is preferable for this type of high-hydration, freestanding loaf than overproofing. In addition, a dutch oven or other kind of enclosed vessel provides support and more even steaming.
This is the earthenware pot in which Loaves One, Three and Five were baked. This is called a "ddukbaeggi" in Korean and comes in many sizes. (It also costs about $10 to $15 dollars per pot, which is much cheaper than a dutch oven!)