Recipe

Equipment:

Large-sized pot
2 l hermetical glass jar
Cheese cloth, cloth towel, or any means of straining liquids

Ingredients:

1,5 l water
300 g rye bread (or 500g rye flour, though it'll be a pain to strain later on)
80-100 g sugar
10 g dry yeast
10 g raisins or any kind of dried berries
3 lemon slices (optional)

Before starting: use filtred water, or even bottled water. Clean your equipment really well, especially the glass jar.

Directions:

1. Boil water, toast the bread or flour until it's almost burnt, rinse the raisins with filtred water.
   
2. Once the water is boiling, add bread/flour, raisins, and 2 lemon slices. Take off the heat and set aside for 3 hours.
3. Remove all solids. Place a cheese cloth over your glass jar and strain the liquid. Carefully squeeze any pieces of bread that might remain.
4. Add yeast, sugar, some more raisins, and a slice of lemon if you want. Mix well and seal the jar.
5. Keep in a warm, dark place for 2 up to 5 days.
6. Strain in a glass bottle using a cheese cloth and refrigerate for at least 24 hours before serving.

Notes:
The ideal room temperature for kvas to ferment is 25°C, but anything in the range of 20°-32° will work fine.
If fermenting for more than 2 days, open the lid for a couple of seconds every day to prevent too much CO2 from building up.
Sugar can be substituted with honey (in which case cut the quantity in half), but this will result in more alcohol content as the honey will ferment as well.
Blueberries, mulberries, and mint are a nice addition for extra flavour. Add them to the strained liquid before leaving it to ferment.