1. Place 12 clean 8-ounce canning jars right side up on a rack in a boiling-water canner. Fill the canner and jars with hot water, about 1-inch above the tops of jars. Boil jars over high heat for 10 minutes. Remove and drain hot sterilized jars one at a time, reserving hot water for processing filled jars. Place jars on a wire rack set over a rimmed baking sheet. Bring a large saucepan filled with water to a boil over high heat and reduce to a simmer, add clean lids and lid rings. Simmer for 10 minutes; do not boil, as this may cause problems in sealing jars. Drain lids and rings; set aside.

2. Place a saucer and five metal spoons in a flat place in the freezer.

3. Fill a large pot with water and bring to a boil over medium-high heat. Add tomatoes and cook for 1 minute. Drain and, when cool enough to handle, peel over a large heatproof mixing bowl; discard skins.

4. Place a cutting board on a rimmed baking sheet to collect juices. Coarsely chop tomatoes and transfer to bowl along with juices. Add sugar and lemon juice to bowl and stir to combine. Transfer to a large, wide, nonreactive saucepan. Place mace in a fine-mesh stainless-steel tea infuser with a firm latch; add to saucepan.

5. Bring tomato mixture to a boil over high heat. Add salt and reduce heat slightly; skim foam from surface. Continue to cook, watching the heat, until jam thickens and no longer seems watery, 30 to 45 minutes, scraping bottom of the pan frequently with a heatproof rubber spatula and decreasing heat as liquid begins to reduce and stirring constantly during the final 15 to 20 minutes of cooking.

6. To test the jam for doneness, carefully place a half spoonful on one of the frozen spoons. Return to freezer for 3 to 4 minutes. Remove from freezer and feel the underside of the spoon. It should neither be warm or cold. If it is still warm, return to freezer for a minute or two.

7. Nudge jam gently with your finger; if is thick and gloppy, it is either done or almost done. Tilt spoon vertically; if jam runs very slowly it is thickened and done. If it runs very quickly or appears watery, continue cooking a few minutes more, stirring, and re-test until done.

8. Remove mesh tea infuser from saucepan and skim any remaining foam from surface.

9. Reheat water in the canner until it reaches at least 180 degrees, within 10 minutes of filling the jars. Place filled jars into the canner one at a time, using a jar lifter that is securely positioned below the neck of the jar. Keep jars upright at all times.

10. Add more boiling water, if needed, so that water covers jars by at least 1 inch. Increase heat to high and cover. Once water begins boiling, heat jars for 5 minutes. Turn off heat and gently transfer jars to a wire rack set over a rimmed baking sheet and invert, spacing each jar at least 1 inch apart. Avoid placing jars on a cold surface or near a cold draft.

11. Let jars sit undisturbed until fully cooled, 12 to 24 hours. Do not tighten ring bands on the lids or push down on the center of the flat metal lid until jar has cooled completely.

12. Once jars have cooled completely, test to make sure each jar is completely sealed. Press down on the middle of the lid with a finger. If lid springs up when finger is released, the jar is unsealed. Store sealed jars in a cool place for up to one year. If any of the jars are unsealed, store in the refrigerator and use within several days. Always refrigerate jam after opening.