Prepare your jars and lids by washing in hot soapy water. Check for cracks or chips and discard any damaged jars.
Prepare a large hot water bath canning pot that is large enough to submerge all of your jars in at once. Fill Hot Water Bath canning pot half-full with water and add jars. Add more water if needed to allow for the water to be 2 inches over all the jars. Bring the water to to a boil then reduce the heat and allow the jars to sit in the simmering water.
Prep the fruit
Wash the strawberries, drain well and stem. Slice and measure until you have 4 cups of sliced strawberries. Crush the strawberries in a large bowl and set aside.
Place the blueberries in another bowl and crush, leaving some whole berries.
Add the 7 cups of sugar to a large pot.
Measure 4 cups of the crushed strawberries and 4 cups of crushed blueberries and pour the fruit into the pot with the sugar.
Turn the heat on low and add the butter to the pot to help reduce the amount of foam that will form on the top of the pot.
Stir to mix the sugar with the crushed berries.
Once most of the sugar has dissolved increase the heat to high and bring to a full rolling boil (a boil that doesn't stop bubbling when stirred), stirring constantly to prevent scorching.
Stir in pectin once heated to a full rolling boil. Return to a full rolling boil and boil for exactly 1 minute, stirring constantly.
Remove the pot from heat and allow to settle for a few minutes.
Skim off as much foam as possible with metal spoon and discard the foam.
Remove one jar from the hot water bath canner and place upside down on a soft towel to drain for a few seconds. Carefully flip the jar right side up using a towel to protect your hands from the hot jar.
Place the wide mouth funnel in the top of the warm jar and ladle the hot jam into the hot jar, filling to within ¼ inch of the top. Wipe the jar rim and threads with a damp paper towel.
Place a flat lid on top of the jar then screw a band on the jat just until finger tight. Place the filled jar on a folded towel as you continue to fill each of the jars with jam.
Once all of the jars are filled with jam place them on an elevated rack in your hot water bath canner. Carefully lower rack and jars into canner. (Water must cover jars by at least 2 inches. Add boiling water to the pot to bring the level up, if needed.)
Cover the pot; bring water to gentle boil and place a lid on the pot. Process the jam jars for 10 minutes. Carefully remove the jars from the hot water bath with a jar lifter and place upright on a towel to cool completely overnight.
After the jars are cool, check seals by pressing the middle of the lid with a finger. (If lid springs back, lid is not sealed properly and the jam will not be able to be stored in a cabinet or pantry. You can refrigerate any jars that do not seal to eat immediately.
Recipe Expert Tips
Makes about 7 half pints of jam.
If using an electric canning system follow the instructions from the manufacturer to prepare your canning jars.
Check jar seals after 24 hours.
Refrigeration is necessary for jars that have not sealed properly.
Label the jars: Write the name of the jam and the date the jam was made on a label and apply to the jar. Alternately you can write this information with a Sharpie permanent marker on the flat lid.
Jam jars that have sealed properly can be stored in a cool dry cabinet or closet for up to one year or up to 18 months depending on the flat lid used. Check with the manufacturer.