Trim most pink and all of the green from watermelon rind. Cut into 1/2-inch cubes.
Dissolve 1/2 cup salt in 1/2 gallon of water. Add rind, cover, and allow to soak overnight. Drain and rinse well. Cover with clear water and allow to stand for 1 hour.
Prepare alum bath and ginger tea. Add alum to 1/2 gallon of water and bring to a boil. In another large pot, add ginger to 1/2 gallon of water and bring to a boil.
When alum bath is boiling, add drained watermelon rind and cook for 5 minutes. Drain and rinse. Then pour rind into ginger tea and cook until it returns to a full boil, about 10 minutes.
While rind is in ginger tea, prepare the syrup. Mix sugar and 1 1/2 cups water in a saucepan and bring to a boil. Simmer uncovered until rind is ready to be removed from ginger tea. With a slotted spoon or colander, remove the rind from the ginger tea, draining it completely, and place it in the syrup. Increase heat to bring the syrup to a boil and cook, stirring occasionally, until rind is translucent, about 30-40 minutes. (You may add a tablespoon or two of ginger tea if it gets too thick, though I didn’t find this necessary.)
Use a slotted spoon to transfer the rind to sterilized jars. Continue to boil the syrup until it is about the consistency of maple syrup. Carefully pour the syrup over the rind, filling the jars to within 1/2-inch of tops. Make sure the rims of the jars are clean, cover with sterilized lids, and tighten bands. Reserve any leftover syrup for use in tea or on pancakes or waffles. Allow jars to cool and check seals. (Lids should “pop” as they cool.) Refrigerate any jars that do not have a tight seal.
I’m no canning expert, so if you attempt this recipe, I recommend doing some research on canning first to make sure that the preserves will not spoil. Just to be on the safe side, I am keeping my jars in the refrigerator.