Rinse the raspberries under cold water and let them drain well.
In a large pot, combine raspberries and sugar. Add lemon juice if using.
Stir the mixture over medium heat until the sugar dissolves and the raspberries start to break down, about 5 minutes.
Increase the heat to medium-high and bring the mixture to a boil.
While the jam is cooking, sterilize your jars by placing them in boiling water for 10 minutes. And most important, add clean kitchen towel at the bottom of your pot to prevent jars from cracking.
Stir constantly to prevent sticking and burning. Boil for about 15-20 minutes, or until the jam thickens.
You can test the thickness by placing a small spoonful on a cold plate—if it wrinkles slightly when you push it with your finger, it’s ready.
Carefully pour the hot jam into the sterilized jars, leaving about 1/4 inch of space at the top. Wipe the rims of the jars with a clean, damp cloth to remove any sticky residue. Seal the jars with lids.
If you plan to store the jam at room temperature, you'll need to process the jars in a water bath. Place the sealed jars in a pot of boiling water (make sure the water covers the jars by at least 1 inch) and boil for 10 minutes.
Remove the jars and let them cool completely overnight or 8-10 hours on a towel. As they cool, you should hear the lids "pop," indicating they've sealed properly.
