Yesterday, I found a recipe online for a Finnish all-rye sourdough bread (scroll down the comments to hansjoakim, who translated this from the Finnish), which I have extracted and executed. Here it is, expanded with notes that I hope prove helpful to you:
Finnish Rye Sourdough Bread
Yield: Two round loaves, roughly 8" x 2" (20 cm x 5 cm).
Time: Prep, including all rises: at least 14 hours. Baking: 1 hour. Cooling: at least 8 hours. Sourdough will teach you patience.
Note: Not only mass-based-only, but almost all metric, to boot. And yes, that is a full kilo--two and a quarter pounds--of flour.
For the Sponge:
800 g room-temperature non-chlorinated water
450 g rye flour
50 g all-rye sourdough starter
For the First Sour:
350 g rye flour
For the Second Sour:
200 g rye flour
4 tsp salt
4 Tbsp caraway seeds (optional)
For the Shaping
more rye flour for dusting
In a large (4 qt/1 L) bowl, place rye sourdough starter and pour in water. Whisk briskly until starter has mostly dissolved, then start adding flour, continuing to whisk as you do so. When all 450 g of rye flour are added, mixture will be roughly pancake-batter consistency. Cover bowl with plastic wrap (or lid) and allow to rise in a warm place overnight. The batter will have more than doubled in bulk.
Stir the sponge briefly to collapse the bubbles, then stir in the next 350 g of flour. Replace the cover and return the sponge to the warm place for at least another two hours (mine went four). Dough will again double or more in volume.
Mix remaining flour, salt, and caraway seeds (if using) in a small bowl. Extract dough and, once again, stir briefly to collapse bubbles. Scrape into bowl of stand mixer, add flour/salt/caraway mixture, and mix on low speed (#2 if using KitchenAid) for twelve minutes, pausing occasionally to scrape down sides of bowl. Dough will look fairly shaggy and wet, especially if you're used to very smooth, firm, barely-tacky hearth breads. Replace dough in rising vessel, replace cover, and return to proofing chamber for at least another two hours (again, I went four). Dough will, once again, have doubled or more in volume.
Scrape dough onto a well-floured surface and cut into two equal pieces. Do not knead; rather, dust each piece liberally with sufficient rye flour that you can shape the dough without succumbing to Club Hand, and shape into two round loaves of roughly 880 g each. Dust these with rye flour (don't forget the bottoms), place on a baking sheet, and cover with a tea towel. Allow to rise until small holes are visible in the surface, at least one hour. The loaves may settle and spread slightly during this last proofing; ensure that they have not grown into one another. As this bread does not have a firm "skin", slashing is not necessary: a network of cracks will develop in the top instead.
If using a baking stone or similar, one hour before baking, place stone in oven and preheat oven to 445°F (230° C). Ensure that stone is well-oiled. NB: a stone is not called for in the original recipe, but I am fond of them.
Once loaves and oven are alike prepared, bake at 445°F (230° C) for approximately sixty minutes.
The original recipe does not cover a specific cooling regimen. However, I suggest the following, borrowed from Swedish limpa bread, of which this appears a close cousin:
When the loaves come out of the oven, wrap each one firmly in a tea towel and set upon a wire rack to cool v-e-r-y s-l-o-w-l-y: overnight is best.
Many rye bread recipes recommend altus brat (lit: "old bread")--the scraps of another loaf of rye bread--to be added to the current bread at the last stage. I will have to test this now that I have an all-rye loaf to test with.
I have yet to actually eat any of this, but it looks proper and sounds right when I thump it. *grin*
Sourdough breads keep longer, and better, than conventional yeast breads. If I had to hazard a guess, I would say that the lactic (et al) acid provided by the sourdough's bacteria do a happy little denaturing number on the proteins. Thus, a loaf I made for and I back on Thursday isn't stale yet--so, do not worry about it being day-old, or even longer, as long as the loaf remains sound.