Bring 2.5 gallons of water to a boil in a stockpot large enough to contain it, or distribute the water between two large pots. Look for a full boil before beginning the steep.

Run the bread through two toasting cycles at the darkest setting while the water heats, watching closely and using a suitable, well-ventilated area because heavy browning can create smoke. The slices should smell like deeply toasted crackers, not burnt ash.

Take the boiling water off the heat, immerse the toast and raisins, and cover the pot for an overnight steep of at least 8 hours. The bread will soften as the liquid develops an amber-brown color.

Lift out the soaked bread carefully and discard it. The softened slices may break apart, leaving small particles in the infusion.

Once the liquid has cooled to room temperature, combine 4 cups of sugar with 1.5 tablespoons of active dry yeast in a medium bowl, then stir this mixture into the bread infusion. The sugar should disperse through the liquid without leaving a gritty layer at the bottom.

Cover the pot with a lid or plastic wrap and keep it on the counter for 6 hours, stirring every couple of hours. Small bubbles and a pale foam ring may appear as the yeast becomes active.

Scoop out and discard the floating raisins, strain the liquid through a strainer or cheesecloth into the bottles, and refrigerate overnight with the caps loosely fitted. The strained kvass may remain slightly cloudy even after the bread particles are removed.

Tighten the bottle caps only after the kvass has chilled completely, then pour and enjoy it cold. The finished drink should have a malty aroma, sweet-tart flavor, and a gentle sparkling sensation.