In a wide, heavy-bottomed pot set over medium-high heat, combine all the ingredients and bring to a simmer. Turn off the heat and let the mixture cool, then transfer to an airtight container and refrigerate overnight or up to 5 days.
Strain the mixture through a sieve and into a wide, heavy-bottomed pot (save the rhubarb for later). Bring the liquid to a boil over high heat, stirring occasionally, until the liquid reaches 215°, about 12 minutes. Return the rhubarb to the pot and simmer, stirring occasionally to prevent scorching, until the jam lightly coats the back of a spoon and its temperature has returned to 215°, 10 to 15 minutes.
In a large pot of simmering water, sterilize seven 1/2 -pint jars. Once the jam is finished, use tongs to remove the jars from the pot and set upside down on a kitchen towel to drain. Turn off the heat and soak the lids and bands in the hot water. Turn the jars upright and pour the jam through a funnel to fill each jar with jam up to 1/2- inch from the rim. Wipe the rims clean with a kitchen towel and seal with the lids. Screw on the bands until snug.
Place a canning rack into the pot of hot water and return to a boil. Use tongs to lower the jam-filled jars into the pot, making sure there is enough water to cover the jars by 1 inch. Boil the jars for 10 minutes and turn off the heat. Leave the jars in the hot water for 5 minutes, then use tongs to remove them. Cool completely before storing in a cool, dark, dry place.