Watermelon ketchup goes well with French fries, hot dogs, whole roasted pig,
Puree the watermelon in a food processor in 2-cup batches. about 4 cups of puree. Press the puree through a strainer to remove fibrous material and seeds.
Transfer the juice to a 10-inch nonstick skillet, bring it to a boil over medium heat and reduce heat and simmer until thick, 1 1/2 hours, stirring occasionally. Watch carefully toward the end to prevent scorching.
During the last five minutes of cooking, stir in the vinegar, mace, cloves and cinnamon. Taste and add more spices if you like.