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.