1. Place a small ceramic plate in the freezer. This is your gel-testing plate and it needs to be very cold before you check the set.

2. Add the blackberries to a large, heavy-bottomed saucepan over medium heat. Use a potato masher or the back of a wooden spoon to crush the berries into a rough, uneven mash as they warm. Cook for 3 to 4 minutes until the berries are fully broken down and beginning to bubble, with some whole berry pieces still visible.

3. Add the sugar, lemon juice, and salt. Stir constantly until the sugar dissolves completely, about 2 to 3 minutes. The mixture will bubble steadily and begin to smell like warm, sweet fruit.

4. Increase the heat to medium-high and bring the jam to a full rolling boil, one that continues to bubble vigorously even when you stir. Stir frequently and scrape the bottom of the pan to prevent scorching.

5. Cook at a rolling boil for 12 to 15 minutes, stirring every 30 seconds. Watch for the color to deepen to a dark burgundy and the bubbles to become smaller, thicker, and slower. These are signs the jam is concentrating.

6. Test for the gel point: pull the cold plate from the freezer and drop a small spoonful of jam onto it. Wait 30 seconds, then push the jam gently with your fingertip. If it wrinkles and holds in place, the jam is set. If it flows back together, return the pan to the heat for 2 more minutes and test again.

7. Remove the pan from heat. Skim any foam from the surface with a spoon. Stir in an optional 1/2 teaspoon of unsalted butter to help reduce remaining foam.

8. Ladle the hot jam into clean jars, leaving 1/4 inch of headspace. Let cool completely at room temperature, then seal and refrigerate. For water-bath canning, process sealed jars in boiling water for 10 minutes.