• If you will be canning the jam, Prepare a canner, jars, and lids. Wash your jars and lids. Sterilize your jars in a large pot filled with water. Bring the water to almost a simmer over medium heat. Keep the jars in the simmering water over low heat until you are ready to fill them. Use a jar lifter to remove them from the water when you are ready to fill them, dumping the hot water back into the pot when you remove each jar from the water. Right before you are ready to can, put your lids in a small pot or bowl with hot, but not boiling, water. Put some additional white vinegar in a small bowl alongside a clean washcloth or paper towel. Keep the water in your canning pot at a simmer while you fill your jars. See The The All New Ball Book of Canning and Preserving for additional guidance.
• Place a saucer in your freezer with several spoons if you don't have a candy thermometer to test that your jam is set.
• Slice the rhubarb into 1/8 - 1/4 inch slices. I used the slicing blade of my food processor.
• Combine sliced rhubarb with sugar in a large glass or hard plastic bowl and let it macerate, covered, in the refrigerator for at least one hour and up to 48 hours. Letting it macerate longer will reduce the cooking time, resulting in a fresher jam.
• Transfer the rhubarb and all the juices to a large, wide pot or preserving pan.
• Add the lemon juice and bring the mixture to a boil over high heat, stirring occasionally.
• Reduce heat to medium-high to achieve a gentle, rolling boil. Reduce the heat to keep the jam from sputtering and splattering out of the pot.
  
• Stir frequently and reduce the heat if the jam sticks to the bottom of the pan.
• Cook until thickened and no longer watery, about 20 minutes. Toward the end of the 20 minutes, you will most likely need to stir constantly to prevent the jam from sticking to the bottom of the pot.

Test For Doneness

• Using a candy thermometer is the most reliable. Jams and jellies are done and ready to be canned when the temperature reaches 220ºF (104º - 105º C), which is 8ºF above the boiling point of water at sea level. If you are at a higher altitude, subtract 2º F for every 1000 feet above sea level. It is important to use your candy thermometer correctly. Make sure the tip of the thermometer is fully submerged in your jam or jelly, but keep it from touching the bottom of the pot. Hold the thermometer vertically and read it at eye level.
• Frozen Spoon Method. When you think your jam may be set, turn off the heat under your jam pan. Remove one of the spoons from the freezer and take a spoonful of jam. Set the spoon back in the freezer for 2-3 minutes until the back of the spoon is cool. Hold the spoon vertically. The jam is set if it 'sheets' off the spoon in one sheet instead of 'dripping' off the spoon in more than one drop.
• Frozen Plate Method. Turn off the heat under your jam pan when you think your jam may be set. Remove the plate from the freezer and put a small dollop of jam on the plate. Return it to the freezer for 2-3 minutes. Remove the plate from the freezer and run your finger through the jam. It is done if the trail your finger made is not immediately filled with jam. Additionally, look for the jam to 'wrinkle.'
• If the gel stage has been reached, skim off any foam.
• If the jam still needs to cook a bit more, turn the heat back on for 5 minutes before testing again.
• Once the jam is done, stir in vanilla if using.

To Water Bath Process

• Spoon your jam into hot sterilized jars, leaving 1/4 inch headspace.
• Remove air bubbles and wipe the rim.
• Center lid on the jar
• Screw band down until resistance is met, then increase to fingertip tight.
• Place jars in a canner, making sure there is at least 1" of water over the cans.
• Cover the pot and bring to a boil
• Boil for 10 minutes
• Turn the heat off, remove the lid, and let sit for 5 minutes
• Carefully remove jars with a jar lifter and place them on a rack where they can be undisturbed for 24 hours.
• Refrigerate any jars whose lids don't pop down.