To make a slow-melting ice block like those used in the recipes for Captain Radcliffe’s Punch and Regent’s Punch, you can freeze water in a halved milk carton for a square-shaped block or in a metal bowl for a round shape, or even in a Bundt pan for a decorative ring. You can also add fruit—such as maraschino cherries and sliced oranges—to the water before freezing it. Allow it to freeze overnight or until frozen solid (larger molds require longer freezing times). To release the ice, dip the bottom of the mold in hot water and invert.
Holidays on Ice: How to Make a Slow-Melting Ice Block