Blackberry jam:
Day 1:
Sort the blackberries, then rinse them quickly with fresh water without soaking them.
Pour the blackberries, sugar, and lemon juice into a large cauldron or jam basin, bring to a simmer while gently stirring, then pour this mixture into a bowl.
Cover with plastic wrap and refrigerate overnight.
Day 2:
Pour the mixture into the cauldron or jam basin and bring to a boil while gently stirring.
Maintain the heat for 5 to 10 minutes, stirring constantly. Skim carefully.
Check the jam’s consistency by dropping a few drops onto a small cold plate: it should gel slightly.
Remove the basin from the heat, immediately fill the jars, close the lids, and turn the jars upside down.