Using a potato masher, mash berries until fairly smooth.

Measure 5 cups berries into a medium-size heavy saucepan.

Stir in pectin and butter, if using.

Bring to a full boil, stirring constantly.

Add sugar and return to boil, stirring constantly.

Boil hard for 1 minute.

Ladle jam into clean jars and process for 10 minutes in boiling-water bath.