Wash, hull, and crush strawberries by hand with a potato masher. Using a food processor will destroy the natural pectin in the berries. Combine the strawberries and lemon juice in a large saucepan. Add butter or margarine and whisk in fruit pectin until dissolved. Over high heat, bring the mixture to a rolling boil and add the sugar. Stirring constantly, return the mixture to a rolling boil that cannot be stirred down. Boil for 1 minute, stirring constantly. Remove from heat and skim off foam.