Instructions:
- Sterilise all jars and lids. Place one small plate in the freezer to chill for testing the set later.
- Combine blueberries, sugar, and lemon juice in the preserving pan. Let stand for 30 minutes (maceration) to draw out juices.
- Place the pan over medium heat. Stir gently until the sugar is completely dissolved (the mixture should look clear). Bring to a rolling boil.
- Once boiling rapidly, use a spoon to carefully skim off any greyish foam (scum) that forms on the surface and discard it.
- Increase heat to maintain a vigorous, rolling boil that cannot be stirred down. Stir frequently to prevent scorching. Cook until the jam reaches the setting point (check using the chilled plate test).
- To test the set: Drop a teaspoon of hot jam onto the chilled plate, return it to the freezer for 1 minute. Push the jam; if the surface wrinkles, it has set. Remove the pan from the heat.
- Allow the jam to cool for about 5 minutes to help distribute the fruit evenly. Carefully ladle the hot jam into the sterilised jars, leaving ½ inch of headspace.
- Wipe the rims clean, secure the lids tightly. Process in a boiling water bath for 10 minutes for shelf stability, or allow to cool completely for refrigerator storage.