Score the peel of each orange and lemon into four lengthwise sections; remove the the peels with your fingers. Using a shape knife, scrape off and discard the white pith inside the peels. Cut the peels into thin strips; set aside. Section lemon and oranges; reserve juices but discard seeds.Advertisement
In a 8- to 10-quart heavy pot combine strawberries, citrus peel, citrus sections, reserved juices, pectin and butter. Bring to full rolling boil, stirring constantly. Add sugar. Return to full rolling boil, stirring constantly. Boil, uncovered, for 1 minute or until jam sheets off a metal spoon*, stirring constantly.
Ladle hot marmalade into hot, sterilized half-pint canning jars, leaving 1/4-inch headspace. Wipe rims, adjust lids. Process filled jars in a boiling-water canner for 5 minutes (start timing when water returns to boiling). Remove jars from canner; cool on wire racks. Allow to set at room temperature for 2 weeks before serving.
Dip a spoon into hot jam. Lift and hold the spoon sideways. If the jam streams off, it is too thin. As the jam thickens, it will drip. When the drips join in a sheet that falls off the spoon, the jam has set.