Pour the mixture into a 4-quart sauce-pan. Add the lemon juice and cook over medium heat, stirring frequently, until thick and jammy. You can test the doneness by dropping a teaspoon of the sauce onto a chilled plate and let it sit for 10 seconds. If it leaves a clear path when you run your finger through it, it's ready. This can take as long as an hour.

Pour into still-hot sterilized jars. At this point, you could seal and can the marmalade. Alternatively, cool completely, put the lids on the jars, and store in the refrigerator. The jam will keep in the fridge for up to three months.