1½ C (white) raspberries
½ C sugar
lemon juice to taste
Combine berries and sugar; refrigerate 24 hours, stirring occasionally. Macerating the fruit draws the liquid out. Just before cooking, add lemon - a little at a time, taste test as you go until the fruit flavor really 'jumps forward.'
Bring it all to a boil, dissolving all the sugar if it isn't already. At first, there's lots of liquid and it's lighter in color. If foam develops, remove from heat briefly to skim off foam. As it continues to cook, the volume will reduce, the color will darken. The bubbles start to get smaller, shinier and glossier and they form small 'wells' of bubbles. The boiling begins to sound more like a rustling.
If you want a seedless jam (jelly), now is the time to pass the jam through a strainer; then complete the cooking process.
As it nears 'done-ness' it's thick enough that as you draw a spatula across the pan it will clearly leave a track before syrupy-thick jam covers the bottom of the pot again. (see pics & jam making tips) The berries are broken down by this point and the jam is more cohesive, rather than chunks of berries in liquid.
The best test for done-ness' is to remove it from the heat, scoop out a small spoonful and put it in the freezer a few minutes -- just to bring it more quickly to room temperature -- to test the texture for spreadable consistency.
Pour or spoon jam in to sterile jars; seal.