Wash the grapefruits. Cut each grapefruit in half crosswise so you have eight halves.

Choose four of the halves and use a small spoon to scoop the pulp into a bowl. Remove any seeds, coarsely dice the pulp, and scrape any juice from the cutting board into the bowl. Set the chopped pulp and juices aside.

Take the remaining four grapefruit halves, remove any seeds, and slice each half into thin slices (as thin as you can safely manage). Use a sharp knife and take care—the fruit can be slippery.

Put the grapefruit slices into a large nonreactive pot or Dutch oven. Add enough additional water to the pot to fully cover the slices (this is extra water for cooking the slices — do not use the 5 cups called for later). Bring the water to a boil over high heat.

Reduce the heat to a simmer and cook the slices for 5 minutes. Turn off the heat and let the slices sit in the hot water until they are soft and translucent. When they are ready, pour the slices into a colander and discard the cooking water. Return the drained, cooked slices to the pot.

To the pot with the cooked slices, add the reserved chopped grapefruit pulp and any juices from step 2, 5 cups (1.4 L) water, 3 1/2 cups (700 g) sugar, and the juice of 1 lemon. Stir to combine and dissolve the sugar.

Cook the mixture over medium-high heat, stirring occasionally, until it reduces and thickens and the marmalade becomes syrupy. If using a candy thermometer, it should reach about 216–218°F (102–103°C). If you don’t have a thermometer, test doneness by placing a teaspoon of hot marmalade on a chilled plate: if it gels and wrinkles when you push it with your finger, it’s done.

Ladle the hot marmalade into clean jars, cover, and let cool to room temperature. Store covered; refrigerate after opening.