Generally, you shouldn't wash raspberries because they so easily lose their fragrance, but if you want to wash them, dunk them very quickly in a large bowl of cold water and then immediately drain. Pour them onto a wide clean towel and gather it up to absorb any excess moisture.
In a wide-bottomed stainless steel saucepot over medium-high heat, combine the raspberries, sugar and lemon juice, and gently mix with a wide spatula. Bring to a simmer and pour into a glass or ceramic bowl. Let cool and press a piece of parchment paper to the surface of the fruit. Macerate at least 6 hours or overnight in a refrigerator.
The next day, chill a small plate for the jam test. Strain the fruit, reserving both liquid and solid. Pour the juice into a heavy saucepan and bring to a simmer. Skim all foam; cook for 5 minutes, until thickened. Add the solid raspberries back in and bring to a boil. Clip a candy thermometer to the side of the pot. Skim again and cook, stirring often, until the bubbles grow bigger, the juice looks darker and the jam measures 221 degrees on a candy thermometer.
Or do a jam test: Turn off the heat. Drop some jam on a cold plate. When the drop of jam doesn't run when you tilt the plate, or when it holds the track of your finger when you swipe it through the middle, the jam is done. It should feel sticky when pinched between your thumb and forefinger.
Pour the jam into sterilized glass half-pint jars, leaving 1/4 inch of headspace. Wipe the rims with a clean towel and cap with sterilized lids and screw tops. Tighten the lids just until you feel it catch -- no more. Process the jam for 10 minutes in a boiling water bath.