1. Put the lemon slices in a large non-reactive pot. Cover with cold water and bring to a boil. Reduce the heat and simmer until the lemons are translucent, about 5 minutes. Drain. (Discard the water.)
2. Put the blanched lemons back in the pot. Tie the seeds from the lemons and kumquats securely in a piece of cheesecloth or étamine, and add the seed sack to the pot along with the sliced kumquats and water. Bring to a boil; remove from heat, cover, and let stand 24 hours. 3. The next day, put a small plate in the freezer. (Optional: If you have a candy thermometer, you can clip it to the side of the pot.)
4. Add the sugar and salt to the pot, and cook the marmalade for 30 to 45 minutes over medium heat. Remove the bag of seeds and when it’s cool enough to handle, squeeze the seeds in the cheesecloth using your hands or place it in a ladle and press it with the back of a soup spoon (being careful not to break the fabric and release the seeds) over the jam pot to so the pectin goes directly into the marmalade mixture.
5. Continue cooking until it has reached the jelling point, about 220ºF (104ºC) degrees, if using a candy thermometer. To test the marmalade, turn off the heat and put a small amount on the plate that has been chilled in the freezer and briefly return it to the freezer. Check it in a few minutes; it should be slightly jelled and will wrinkle a bit when you slide your finger through it. If not, continue to cook until it wrinkles slightly on the chilled plate when you nudge it.
6. Remove from heat, then stir in the kirsch (if using), and ladle the mixture into clean jars.