Wash four 8-ounce glass jars and lids in hot soapy water, rinse them well, and keep them warm.
Combine the crushed strawberries, sugar and lemon juice in a wide heavy saucepan.
Bring to a full rolling boil over medium-high heat and cook for 18 to 25 minutes, stirring often and skimming foam, until the mixture reaches 220°F.
Remove the pan from the heat and let the bubbles settle for 2 minutes.
Ladle the jam into the warm jars, leaving 1/2 inch of space, cool uncovered for 30 minutes, then cover and refrigerate for up to 3 weeks.
