Crush the grapes in a large bowl using a potato masher or your hands. You want a chunky pulp, not a puree.
Pour the crushed grapes into a Dutch oven or heavy pot. Stir in the sugar, lemon juice, and salt.
Place the pot over medium heat (medium coals if using a campfire). Stir constantly until the sugar dissolves and the mixture comes to a gentle boil.
Reduce heat to low (move pot to cooler coals). Simmer uncovered for 20–25 minutes, stirring every 3–4 minutes to prevent scorching. The jam thickens as it cools, so don't over-reduce.
Remove from heat. Skim off any foam that rises to the top. Let cool for 10 minutes, then pour into a clean jar or container. Seal and let cool completely before storing in a cooler.
