Blackberry Bloom

A soda-fountain formula, written for someone standing at a marble counter with a draft arm and a syrup rack. Everything in it translates to a home kitchen with seltzer, but three of the terms need decoding first.

• Acid phosphate is the hingeThe "dash of phosphate" is not flavouring but the tart edge the whole drink hangs on: a dilute solution of phosphoric acid with calcium, magnesium and potassium salts, sold to fountains by the gallon and still made for bartenders today. It reads sourer and drier than lemon juice, without any citrus taste. A dash is roughly a quarter teaspoon; without it the drink is only sweet soda.
• "Floss" syrupFountain trade language for a syrup carrying a foaming agent, commonly an egg-albumen or similar solution worked in, so the finished glass builds a head. Plain blackberry syrup will work but will not foam in the same way.
• Solid sodaFountain draft arms gave a coarse, frothing stream and a fine or "solid" one. Solid soda means carbonated water drawn smoothly so the gas stays in the water rather than blowing off as foam. Pour cold seltzer down the side of the glass, not into the middle.
• Mineral glassA footed, slightly bellied fountain glass; capacities varied, but a tall glass of roughly 10 to 12 ounces gives about the right syrup-to-soda ratio.
• Shaved ice, not cubesShaved or finely crushed ice chills the soda on contact and helps hold the foam up. Cubes mix the syrup through less readily; the original calls for half-filling with shaved ice before the soda goes in.
• Proportion and yieldOne glass. An ounce and a half of syrup in a 10-ounce glass is about a 1:6 dilution — closer to sour than to modern soft drinks, especially with the phosphate in.
• Serve immediatelyThere is no mixing step at all; the rising soda is meant to do the work. Assembled and left standing, it goes flat and separates.