Spiced Purple Plum Jam

1. Submerge 6 empty half-pint jars on a rack in a large pot of water. Cover and bring to a rolling boil, then reduce heat to low to keep jars warm until ready to fill.

2. Tumble plums, lemon zest, and lemon juice into a wide, nonreactive 8-quart pot. Whisk together sugar, pectin, cinnamon, nutmeg, and cloves in a small bowl, then stir into pot until combined.

3. Bring to a vigorous boil over high heat. Reduce heat to medium-high; cook, stirring frequently and adjusting heat as needed to maintain a gentle boil, until volume has reduced by at least one-third, 20 to 25 minutes. Remove pot from heat and check for set.

4. Working with one jar at a time, remove empty jars from canning pot. Using a wide-mouth funnel, carefully ladle jam into jars, leaving 1/2 inch for headspace. Use a clean wooden chopstick to work air bubbles out of jars. Check headspace again and add more jam if necessary to bring to 1/2 inch from the top.

5. Wipe jar rims, apply lids and bands (not too tightly), and return jars to canning pot; cover pot and return water to a rolling boil. Process 10 minutes. Turn off heat, remove pot lid, and let jars stand in the cooling water 5 minutes to help ensure a good vacuum seal.

6. Move jars to a folded kitchen towel or wooden cutting board to cool completely before checking seals. Any unsealed jars should be refrigerated and eaten promptly. Store sealed jars in a cool, dark place. They are shelf-stable at least 1 year.

Cook's Notes:

Do not use low-sugar pectin.

Jams thicken as they cool, so they need to come off the heat when they're still a little runny. But to set up properly, they also need to cook long enough to reduce the fruit's water content, which concentrates the sugar and allows it to gel and bond with the pectin. My favorite way to check for set is the saucer test: Tuck a few saucers into the freezer before you start cooking. When your jam smells sweet and looks glossy and thick and nearly molten, take it off the heat. Spoon a little onto a chilled saucer, freeze it for 1 minute, then nudge the jam with your fingertip. If it has formed a skin on the surface that wrinkles when you push it, the jam is ready. If your finger slides right through, it needs another 1 to 2 minutes of boiling.