Put a small plate in the freezer. Place berries in a nonreactive 10-quart stockpot set over medium-high heat. Using a wooden spoon, mix in 1/4 cup sugar with berries. Cook, stirring, until berries are juicy, 5 to 6 minutes. Stir in a third of remaining sugar until dissolved. Repeat until all the sugar has been added and dissolved, about 7 minutes total.
Bring mixture to a full boil; cook, stirring, 10 minutes. Continue boiling; use a stainless-steel spoon to remove foam from surface. Boil until most of the liquid is absorbed, mixture thickens, and temperature registers 220 degrees.on a candy thermometer, about 30 minutes.
Perform a gel test: Place a spoonful of jam on chilled plate, and return to freezer. Wait 1 to 2 minutes; remove plate from freezer, and gently press jam with fingertip; it should wrinkle slightly.
After jam passes the gel test, remove from heat. Pour warm jam into jars; seal, label, and refrigerate up to 4 months.