• Prep for testing - Place two small ceramic plates in the freezer so we can use them to test the jam later. Alternatively, you can use a candy thermometer to check the doneness of the jam.
• Prep fruit - Check the blueberries and remove any bad, discolored, or bruised ones. Give them a quick wash and drain in a colander.
• Combine the blueberries, sugar, salt, and lemon juice (and lemon zest) in a heavy-bottomed large pan or Dutch oven pot.
  2 lbs Blueberries, 2½ cups Sugar, 2 tbsp Lemon juice, ½ tsp Salt, ½ tsp Lemon zest
• Boil - Cook on medium heat until the sugar is dissolved. Then, turn the heat up and let the mixture come to a full rolling boil. Let it boil for two minutes on medium-high.
• Simmer - Then, turn the heat to medium-low. The jam should still be simmering, but at a low simmer, not bubbling. Use a potato masher to mash the fruit, so you have a smoother, jam-like consistency.
• Thicken - Continue to simmer for about 20 to 30 minutes more, stirring often so the jam does not stick to the bottom of the pan. The jam will continue to thicken and deepen in color as it cools.

Testing the jam

• Test the jam - Remove one of the ceramic plates from the freezer. Place a teaspoon of jam on the cold plate. Set aside 2 to 3 minutes, and then test with your finger. Simply move or push the jam with your finger. If it wrinkles or crinkles, it means the jam is ready. If not, let it cook for a few more minutes and test it again. Alternatively, you can use a candy thermometer - the jam should reach 105 C / 221 F.
• Remove scum (Optional) - Just when you are ready to remove the jam from the heat, add a teaspoon of butter and stir well. Butter helps dissolve the foam that accumulates on top of the jam.
• Fill jars - Ladle the almost hot jam into warm sterilized jars, leaving 1/4 inch space from the top. Use a clean, sterilized knife or spatula to move the jam a bit – this will remove any air pockets, small air bubbles, or a large bubble.
• Close the lid - Before you tighten the lid, place a piece of wax paper on top. Wipe the jar rim with a clean paper towel. Then, top the lid. Alternatively, use the lid with screw-on rings with the canning jars.

The canning process (if you plan to can the jam)

• Sterilize the jars - Sterilize 4 x 8 oz (250 g) jars by washing them in hot, soapy water or cleaning them in the dishwasher with a gentle cycle. Then, place them in the oven for 20 minutes at a low 284°F / 140 °C.

  Note - Leave them in the oven until you are ready to use them (you can turn the oven off). Don’t forget to wash and sterilize the bottle lids as well.
• Canning - Place a rack in the bottom of a large stockpot. Fill half the pot with water for a hot water bath. Bring the water in the pot to a boil on high heat. Lower the jars over the rack, leaving enough space between the jars. Bring the water to a boil again. Cover the pot and process/simmer for 15 minutes.
• Cool - Then, carefully remove the jars from the stockpot, place them on a kitchen towel to absorb any excess moisture, and let them cool completely.
• Store - Press the top of the lid to ensure the seal is tight. The lid should not move. Store in a cool, dry place.