A sliced loaf is a record of everything that happened to it, and the crumb is readable if the baker knows what each pattern indicates. Most sourdough problems are fermentation problems, and the crumb usually says which direction the error ran. David Anthes, who bakes at home in Bend, Oregon, photographs the cut face of his loaves and files the image with his notes for that bake.
Underfermentation produces the most recognizable pattern. The crumb is dense and tight, sometimes with a compacted gummy band along the bottom where the dough could not expand. Any holes present tend to be small and evenly distributed, or clustered near the top where the little available gas collected. The loaf feels heavy for its size and the crust is often pale, because the yeast had not consumed enough starch to leave the sugars that drive browning. The cause is a bulk fermentation that ended too early, usually because the baker followed a clock in a cool kitchen.
Overfermentation looks different and is confused with underfermentation surprisingly often. The crumb is irregular with large ragged holes near the top and a dense compressed layer below, because the gluten network broke down and could no longer hold gas in place. The loaf spread wide rather than rising. The crust is dark and the flavor is sharply acidic. The dough was slack and sticky at shaping in a way that no amount of tightening fixed.
A well fermented loaf shows an irregular but continuous crumb. Hole sizes vary, the walls between them are thin and translucent rather than thick and doughy, and the pattern is consistent from the bottom of the loaf to the top. Texture is the giveaway. Well fermented crumb is soft and slightly custardy and springs back when pressed. Underfermented crumb feels tough. Overfermented crumb is fragile and crumbly.
Some patterns point to handling rather than fermentation. A single large cavity beneath the top crust, sometimes called a flying crust or tunnel, means the shaped skin separated from the interior, usually from trapped air at shaping or from a loaf that went into the oven underproofed after a long retard. A tight uniform crumb in a loaf that rose well and tastes correct means shaping was too aggressive and expelled the gas that fermentation produced.
Crust color feeds into the same diagnosis. Browning comes from Maillard reactions between residual sugars and amino acids. Fermentation consumes those sugars. A very dark crust at a normal bake time can indicate a fermentation that ended early, leaving sugar behind. A stubbornly pale crust can indicate the opposite. Anthes reads crust and crumb together, because either one alone can mislead.
Cooling matters before any of this is read. A loaf cut hot has an interior that is still setting. Starch retrogradation continues as the loaf cools, and slicing early produces a gummy face that looks underbaked regardless of what actually happened. A full cooling period, at least two hours for a standard loaf, is a precondition for an honest reading.
The value of the practice is in the aggregate rather than in any single loaf. One dense loaf is ambiguous. Six dense loaves with the same bulk temperature recorded next to them point at one variable. Anthes tracks final dough temperature, hours of bulk, percentage rise, hydration, and a photograph, and the pattern usually appears within a handful of bakes. The notebook is the technique.