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Put a small plate into the freezer and wash some jam jars and lids thoroughly. Once clean, place the jars onto a baking tray in the oven on a low heat for 10 minutes in order to sterilise them.
In a preserving pan or large, wide saucepan, mix the sugar, lemon juice and blackberries, stirring the mixture over a low heat until the sugar has completely dissolved.
Now turn up the heat until the jam has reached boiling point and continue to let it boil for about 15 minutes. Place a teaspoon of the jam onto the cold plate, leave for a few minutes, and then push a finger into the side of the mixture. If wrinkles appear on the surface, then the jam has reached setting point. You could use a jam thermometer instead!
Let the jam rest in the pan for about 10 minutes and then pour it into the prepared jam jars. Place a wax disc on the surface of each jar and then seal with the lid. After your jam has cooled, label, date and store your jars somewhere cool and dark until they are needed.