Tart Blackberry Soda Syrup

Put blackberries, sugar, filtered water, and tartaric acid into a nonmetallic jar. Stir gently with a wooden spoon to combine and cover tightly. Let stand for 24 hours, stirring occasionally, to allow sugar to dissolve.

Strain mixture through cheesecloth without pressing on berries (discard berries). Divide blackberry liquid between sterilized bottles or jars, cover tops with clean cheesecloth, and secure with kitchen twine or a rubber band. Store in a cool, dark place for 3 weeks, until acid is thick and syrupy and slightly fizzy.

To serve, add 2 to 3 parts seltzer or water to 1 part acid and serve over ice.