DAY 1: Cut each nectarine half into 6 equal wedges. Place the wedges in a hard plastic or glass storage container. Pour the sugar evenly over the fruit, jiggle to help the sugar settle, and drizzle 8 ounces of lemon juice over the mixture. Do not stir. Press a sheet of plastic wrap directly onto the surface of the mixture, smoothing well to minimize air bubbles (this will help keep the fruit from browning as it sits).
Cover the nectarines and let macerate in the refrigerator for 3 to 6 days.
3 TO 6 DAYS LATER: Place a saucer with five metal spoons in a flat place in your freezer for testing the jam later. Remove the nectarines from the refrigerator and transfer them to an 11- or 12-quart copper preserving pan or a wide nonreactive kettle. By this time, they should have released a large quantity of juice, and most of the sugar should be dissolved. Stir the fruit well to incorporate any undissolved sugar. Taste the mixture and slowly add more lemon juice if necessary. You should be able to taste the lemon juice, but it should not be overpowering. Keep adding lemon juice until you are just able to detect its presence in the mixture.
Bring the jam mixture to a boil over high heat, stirring frequently with a large heatproof rubber spatula. Boil, stirring frequently, for 5 minutes. Remove from the heat and, using a large stainless-steel spoon, skim the stiff foam from the top of the mixture and discard. Mash two-thirds or more of the fruit with a potato masher to encourage it to break down. Return the jam to the stove over medium-high heat. Cook until the jam has thickened and become cohesive, 25 to 40 minutes, decreasing the heat slightly if the mixture starts sticking.
When the jam has thickened, test it for doneness. To test, carefully transfer a small representative half-spoonful of jam to one of your frozen spoons. Replace the spoon in the freezer for 3 to 4 minutes, then remove and carefully feel the underside of the spoon. It should be neither warm nor cold; if still warm, return it to the freezer for a moment. Nudge the jam gently with your finger; if it seems thickened and gloppy when you nudge it, it is either done or nearly done. Tilt the spoon vertically to see how quickly the jam runs; if it is reluctant to run, and if it has thickened to a gloppy consistency, it is done. If it runs very quickly or appears watery, cook it for another few minutes, stirring, and test again as needed. While you are waiting for the jam in the freezer to cool, skim off any white foam that appears on the surface of the jam in the pan.
Recipe from The Blue Chair Jam Cookbook by Rachel Saunders/Andrews McMeel Publishing, 2010.