Place the rhubarb in a large glass bowl. Add sugar and stir to distribute. Cover and let sit for at least 8 hours stirring once or twice to redistribute sugar.
6 cups rhubarb, 2 cups sugar
The rhubarb will release a lot of liquid while sitting with sugar.
Place rhubarb and its juice into a nonreactive pot.
Add lemon juice.
1 Tablespoon lemon juice
Cook over medium high heat until mixture comes to a simmer, about 20 minutes. Stir often.
Reduce heat to low and continue cooking for another 10 minutes. Stir often to avoid burning.
Test jam on a plate. Place a small spoonful on a plate and let cool for one minute. Draw a line down the center with the back of a spoon. The mixture should remain separated. Tilt plate slightly - the mixture should only run a small amount.
Remove jam from heat and allow it to cool for 10 minutes. Transfer jam to a glass canning jar or other storage container. Cover and refrigerate.
Allow enough time for the rhubarb to release all of its juices
Use all of the juice along with the rhubarb for the jam
Use a non reactive pot for cooking the jam. A stainless steel or ceramic coated pot will work well.
Keep a close eye the jam while it cooks. Stir the jam often to avoid burning.