Recipe from the book, Blue Ribbon Preserves, by Linda J. Amendt.
Before measuring the raspberries, sieve about 3/4 of the crushed fruit to remove the seed or the jam will be mostly seeds. While fresh raspberries usually make the best jam, frozen berries also produce a very good jam. A 12 oz bag of frozen raspberries will yield about 1 1/2 cups of crushed fruit or about 1 cup of seedless pulp.
4 cups crushed, fresh ripe raspberries (6 - 8 pint baskets)
1 tablespoon of strained lemon juice
6 1/2 cups of sugar
1/2 teaspoon unsalted butter
1 (3-ounce) pouch of liquid pectin
In an 8-quart pan, combine the raspberries, lemon juice, sugar and butter.
Over medium-low heat, stirring constantly, heat the mixture until the sugar is completely dissolved. Increase the heat to medium-high and bring the mixture to a full rolling boil, stirring constantly. Stir in the entire contents of the pectin pouch. Return the mixture to a full rolling boil, stirring constantly. Boil stirring constantly, for 1 minute. Remove the pan from the heat. Skim off any foam.
To prevent the jam from separating in the jars, allow the jam to cool 5 minutes before filling the jars. Gently stir the jam every minute or so to distribute the fruit. Ladle the jam into hot jars leaving 1/4 -inch head space. Wipe the jar rims and threads with a clean damp cloth. Cover with hot lids and apply screw rings. Process half-pint jars in a 200°F (93°C) water bath for 10 minutes, pint jars for 15 minutes.