Wash the raspberries and dry them well. Place a small plate in the freezer.
In a large pot (at least 5 quarts), combine the raspberries, sugar, and lemon juice. Stir well to combine. Let sit for about 30 minutes, stirring and mashing occasionally, until the raspberries break down and the sugar is partially dissolved.
Place the pot over medium-high heat. Attach a candy thermometer to the side of the pot. Bring the raspberries to a boil, stirring frequently. Reduce the heat to medium to keep the berries at a simmer.
Let the jam simmer for about 35 to 40 minutes, stirring frequently, until the jam is thick and syrupy and the temperature reaches 218 to 220°F. (Stir more frequently during the last 10 minutes of cooking. As the jam thickens, it is more likely to scorch on the bottom or sides of the pot.)
Check to see if it's ready by adding a tablespoon-sized dollop to the frozen plate. Return it to the freezer for 2 minutes, then check the consistency by pulling the back of a spoon through the dollop. Remove the jam from the heat if it's thickened to your liking or continue simmering until desired consistency.
When it’s done, pour the jam into sterilized mason jars and chill overnight.
Store in the refrigerator or freezer, or follow the directions provided by the canning jar manufacturer for how to process and seal your jam jars safely.
Notes
To sterilize the jars, place them in boiling water (the lids as well) and cook at a low boil for about 15 minutes. Remove them from the water with clean tongs and let dry and cool at room temperature on a clean kitchen towel. I like doing this regardless of whether I will be processing them in a water bath for long-term shelf storage or storing them in the refrigerator or freezer. Alternatively, you can wash the jars in your dishwasher using the sterilize setting.
Don't rush the simmering. It's important for the jam to simmer for at least 30 minutes to reach the proper temperature for the correct consistency. Natural pectin begins to activate at 218-220°F. Your jam will stay runny if it doesn't reach that temperature.
Stir more frequently during the last 10 minutes of cooking. As the jam thickens towards the end of the cooking time, it is more likely to scorch, so stir it frequently to prevent that.