Prepare the water bath canner (or other large pot) by rinsing it out, filling it about two-thirds full of water, putting in the rack, and put it on the stove. Heat over medium heat. 

Wash 6 half pint jars and lids. Have bands ready. Since this jam is processed in the water bath canner for 10 minutes the jars don't need to be sterilized, just washed in hot soapy water. You can put the jars in the canner to keep them hot. 

In a large stockpot, combine rhubarb, strawberries and balsamic vinegar. Crush them with a potato masher.

Whisk in pectin until is has completely dissolved. Bring to a boil over high heat. Stir constantly. Make sure the mixture comes to a full boil.

Pour in the sugar all at once and mix well.

Bring the mixture to a full rolling boil and boil for 1 minute.

Remove the saucepan from heat.

One by one, remove the clean jars from the canner and fill them with the hot jam. Leave a 1/4-inch headspace (the space between the jam and the top of the jar.)

Remove any air bubbles with a wooden spoon or thin spatula.

Wipe jar rims with a clean damp cloth. 

Place lids and screw bands on jars.

Put the filled jars back into the water bath canner and heat over medium-high heat until the water is boiling. 

Process jars in a boiling water canner for 10 minutes, adjust time for elevation.

After the 10 minutes, turn the heat off and let the jars remain in the canner for 5 minutes. 

Using a jar lifter, remove the hot jars from the canner and put them on a towel on the counter.

Let the jars cool overnight (12-24 hours). Don't mess them or tighten the bands.

The next day, remove the bands and check the lids to make sure they sealed. 

If any failed to seal, put it in the refrigerator to use first.

Wipe down sealed jars of jam and label them. 

Store in a cool, dark place. For the best quality, consume within one year. That being said, as long as the jars are sealed, they are safe to consume.