Frozen raspberries can be used, but the cooking time should be increased by a few minutes.
Jams and Preserves by Anneka Manning. Published by Thunder Bay Press.
Put two small plates in the freezer for testing purposes (you may not need the second plate). Place the berries and lemon juice in a large pan. Stir over low heat for 10 minutes, or until the berries are soft.
Add the sugar and stir, without boiling, for 5 minutes, or until all the sugar has dissolved.
Bring the mixture to a boil and boil for 20 minutes. Stir often and make sure the jam doesn’t stick or burn on the base of the pan. Remove any scum during cooking with a skimmer or slotted spoon. When the jam falls from a tilted wooden spoon in thick sheets without dripping, start testing for setting point.
Remove from the heat, place a little jam on one of the cold plates, then place in the freezer for 30 seconds. When setting point is reached, a skin will form on the surface and the jam will wrinkle when pushed with your finger. Remove any scum.
Spoon immediately into clean, warm jars and seal. Turn the jars upside down for 2 minutes, then invert and let cool. Label and date. Store in a cool, dark place for 6–12 months. Refrigerate after opening for up to 6 weeks.