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Method

1. If you don’t have a sugar thermometer, a few hours before you want to make your jam, put a couple of small plates or saucers into your freezer.

2. Wash the blackberries and add to a large, heavy-bottomed saucepan. Cook over a medium–high heat for 8–10 minutes, stirring occasionally, until most of the blackberries have broken down and the remaining berries are almost submerged in their juice.

3. Begin to add the sugar to the berries, a third at a time, stirring between each addition, then add the lemon juice. Bring the mixture to the boil, then turn the heat down slightly to a gentle rolling boil and simmer for 8–10 minutes, until the jam reaches 105C/220F on a sugar thermometer. At this stage the bubbles in the jam will have changed from large to small, indicating that most of the water from the fruit has evaporated.

4. If you don’t have a sugar thermometer, you can test if your jam has reached setting point using your pre-chilled plates. When you’re ready to test the set, remove a plate from the freezer and drop a teaspoon of jam onto it. Leave it for a moment, then run your finger through the jam. If the jam crinkles a little on the surface as though it’s beginning to form a skin, it’s ready to be jarred. If the jam remains runny and the line your finger has created disappears in the jam, cook it for a few more minutes and test again with your second chilled plate.

5. Carefully divide the jam between six warm sterilised jars and seal with sterilised lids. The jam will keep, unopened, in a cool dark place for 12 months. Once open, store in the fridge and eat within two weeks.