Put the slices of rye-bread in the oven (200 degrees Celsius) for about 45 mins, until they're dried.
Boil the 8 liters of water.
Crumble the dried rye-bread, put it in the boiling water for about 5 mins.
Let it the water, and rye-bread rest for 4 hours, covered with a tea-cloth.
Crumble the yeast, 15 mins before the 4 hours are over.
Mix the crumbled yeast with some sugar and the luke warm water.
Let it rest for 15 mins.
Filter the water-rye-bread mix in a kitchensieve.
Carefully extract all water from the rye-bread.
Wash, and peel the lemon.
Add the lemon-peel, the sugar, the yeast and the mint.
Stir the solution, and let it rest (covered) for 8 hours.
Sieve the solution (tea-cloth).
Put some raisins, a bit of lemon-peel, and a fresh leaf of mint in every bottle, close the bottles, and keep them in a cool place.
Ready when the raisins start floating.
Sieve the stuff one more time in a tea-cloth.
Put the Kvas in the fridge 4 hours before drinking.