Before You Begin, prepare calcium water.To do this, combine ½ teaspoon calcium powder (in the small packet in your box of Pomona’s pectin) with ½ cup water in a small, clear jar with a lid. Shake well. Extra calcium water should be stored in the refrigerator for future use.

Wash and rinse jars, lids, and screw bands. Set screw bands aside until ready to use. Place jars in boiling water bath canner with a rack, fill at least ⅔ of the way full with water, and bring to a boil. Boil jars for 10 minutes to sterilize (add 1 additional minute of sterilizing time for every 1000 feet above sea level), then turn down heat and let jars stand in hot water until ready to use. Place lids in water in a small pan, bring to a low simmer, and hold there until ready to use.

Pick through blackberries, discarding any stems. If blackberries look clean, rinsing them is optional.

Place berries in a large bowl and mash them thoroughly ( a potato masher works well for this).

Place a mesh strainer over a large bowl, and transfer mashed berries into the mesh strainer. Press the mashed berries through the strainer (the back of a wooden spoon, as well as clean fingers, work well for this), so that the blackberry pulp goes through the strainer into the bowl below, while the seeds remain in the strainer.

Measure out 4 cups of the blackberry pulp. (If you have any left over, you can use it for something else.) Pour the measured pulp into a large sauce pan. Add calcium water and stir to combine.

In a separate bowl, combine the sugar and pectin powder. Mix well and set aside.

Bring blackberry pulp to rolling boil over high heat. Add sugar-pectin mixture, then stir vigorously for 1 to 2 minutes, still over the highest heat, to dissolve pectin. Return jam to a boil, then remove from heat.

Remove hot jars from canner and fill jars with jam, leaving ¼ inch of headspace. Remove trapped air bubbles, wipe rims with a damp cloth, and put on lids and screw bands, tightening bands only to “fingertip tight” (until resistance is met, and then just the tiniest bit more).

Place jars in the hot water, on the rack inside the canner. (Make sure jars are upright, not touching each other or the sides of the canner, and are covered with at least 1-2 inches of water). Place the lid on the canner, return the canner to a rolling boil, and boil for 10 minutes. (Add 1 minute additional processing time for every 1,000 feet above sea level.)

Remove jars from canner.

Allow jars to cool undisturbed for 12-24 hours. Then, confirm that jars have sealed. Remove screw bands from sealed jars, rinse off outside of jars if necessary, label jars, and store for later use.