Crush blueberries one layer at a time.
Measure 4 1/2 cups crushed berries into a large saucepot.
Add the juice of one lemon.
Add sugar all at one time.
Butter will help prevent foaming.
Bring to full rolling boil, a boil that cannot be stirred down.
Add pectin, stirring constantly.
Return to boil and cook 1 minute, stirring constantly.
Remove from heat.
Skim off foam with metal spoon.
Ladle into hot sterilized jars, leaving 1/8 inch head space.
Clean rims of jars and cap with 2-part lids.
Process in boiling water bath for 10 minutes.
Remove from canner and cool.
Makes nine 1/2 pint jars.