Black Grape Jam

Wash 1 kg of black grapes, remove them from their stems, and place them in a pot. Add 800 g of granulated sugar, close the lid, and let them sit at room temperature overnight. The next day, transfer the pot to the stove. Cook, stirring occasionally, and once the mixture comes to a boil, reduce the heat. If any foam forms on the surface, skim it off. Once the liquid begins to thicken, squeeze in the juice of half a lemon, boil for 5 more minutes, then remove from heat.