Place a saucer with five metal teaspoons in a flat place in your freezer for testing the jam later.?Bring a medium kettle of water to a boil, then carefully drop the tomatoes into the water to loosen their skins. Leave the tomatoes immersed for 1 minute, then drain them in a large colander. When they are cool enough to handle, peel them over a large heatproof mixing bowl, discarding the skins. Place a cutting board on a rimmed baking sheet and chop the tomatoes into medium pieces. Transfer the tomatoes and their juices back into the mixing bowl. Add the sugar and lemon juice, stirring well to combine. Transfer the mixture to an 11- or 12-quart copper preserving pan or a wide nonreactive kettle. Place the mace into a fine-mesh stainless-steel tea infuser with a firm latch and add it to the mixture.
Bring the jam mixture to a boil over high heat. Add the salt and decrease the heat slightly. Skim off any surface foam with a large stainless-steel spoon. Continue to cook, monitoring the heat closely, until the jam thickens and no longer seems watery, 30 to 45 minutes. Scrape the bottom of the pan often with a heatproof rubber spatula, and decrease the heat gradually as more and more moisture cooks out of the jam. For the final 15 to 20 minutes of cooking, or when the jam starts to visibly thicken, stir the jam gently and constantly to prevent burning.
To test the jam for doneness, carefully transfer a small representative half-spoonful 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 runs very slowly, and if it has thickened to a cohesive consistency, it is done. If it runs very quickly or appears watery, cook it for another few minutes, stirring, and test again?as needed.
When the jam is ready, remove the mesh tea infuser. Skim any remaining foam from the surface of the jam.
Recipe from The Blue Chair Jam Cookbook by Rachel Saunders/Andrews McMeel Publishing, 2010. Photo Credit: Sara Remington, 2010