Here is my quasi recipe for this bread
Stage 1, scald
In a 6oz mason jar I scald 3 rounded tablespoons of whole rye flour with some boiling water, mix it into sticky mass, add 1 teaspoon of white rye malt, mix well, cover and let stand in a warm place for about 2-3 hours
Stage 2, kvas (liquid preferment)
At this point the scald should be nice and sweet and of quite loose consistency. I place it in my bread bowl that has dried leftover dough on the walls and bottom. I add about 700ml of lukewarm water and mix well. I leave it to ferment in a lukewarm spot (for example oven with the light bulb on)
After a couple of hours there should be visible signs of fermentation: bubbles going up, slight foaming on the surface etc. This stage should go on until the kvas tastes pleasantly acidic. It may take anywhere between 24-48 hours
Stage 3, opara (preferment)
Now I add enough whole rye flour to obtain pancake batter consistency. I leave the bowl in a lukewarm spot until everything goes up and then falls back down. The opara should have a pleasant, acidic smell and taste. This stage could take 7-12 hours
Stage 3, dough
I add 4 teaspoons of sugar, a scant tablespoon of salt and enough whole rye flour to get a soft, but not too wet consistency. I work the dough with my fists for a couple of minutes until I'm happy with it (if you grab the dough in your hands it should be able to stay there and not flow down, it should somewhat keep its shape when you hold it) Now I smooth the ball of dough with a wet silicone spatula and let it ferment in a lukewarm spot until expanded and shows small cracks and broken bubbles on the surface. It can take 50-90 minutes. Now I roughly divide the dough with my fingers, form loaves in the air and place them in prepared pans. I smooth the surface with a spatula, trying to push the dough down and shape a nice dome on the top. Cover with a moist towel and let rise until expanded and broken bubbles start to show. It goes really quickly at this point, 30-50 minutes
Baking
I make a glaze using leftover dough thinned with water and brush the loaves. I dock the surface with a chopstick. Bake in preaheated oven 10 minutes at 475°F, 20 min at 425°F and 20 at 390°F or until brown on the top and sounds hollow when thumped on the bottom. 5 minutes before the end of baking I glaze the tops again. Right after taking from the oven I spray with some water
Let cool completely before slicing