In a large, heavy-bottomed saucepan, combine the strawberries and sugar.

Using a potato masher, crush the berries until they reach the desired consistency for the jam.

Add the lemon juice and stir the mixture over medium-low heat until the sugar crystals are completely dissolved.

Increase the heat to medium-high and bring the mixture to a full rolling boil, stirring frequently to prevent scorching.

Add the butter to the boiling mixture to minimize the formation of surface foam.

Continue to boil the jam until it reaches the setting point of 220°F (104°C) on a candy thermometer.

Remove the saucepan from the heat and skim any remaining foam from the surface with a spoon.

Ladle the hot jam into sterilized glass jars, leaving 1/4 inch of headspace at the top.

Wipe the rims of the jars, apply the lids and rings, and process in a boiling water bath for 10 minutes to seal.