This recipe is changed from Bernardin's Rhubarb Concentrate, I changed it a bit to add raspberries for part of the Rhubarb originally called for.
Times are estimates. Directions assume you know how to can.
In a large stainless steel saucepan, combine rhubarb, raspberries and water. Bring to a boil over medium-high heat, stirring constantly. Reduce heat, cover and boil gently until rhubarb is soft, about 10 minutes. Remove from heat.
Transfer to a dampened jelly bag or a strainer lined with several layers of cheesecloth set over a deep bowl. Let drip, undisturbed, for at least 2 hours.
Meanwhile, prepare canner, jars and lids.
In a clean large stainless steel saucepan, combine rhubarb juice and sugar, Heat to 190F (88C) over medium-high heat, stirring to dissolve sugar, Do not boil. Remove from heat and skim off foam.
Ladle hot concentrate into hot jars, leaving 1/4 inch head-space. Wipe rim. Center lid on jar. Screw band down until resistance is met, then increase to fingertip-tight.
Place jars in canner, ensuring they are completely covered with water, but at least 1 inch. Bring to a boil and process for 10 minutes (adjusting for above sea level). remove canner lid. Wait 5 minutes, then remove jars, cool and store.
Notes: To reconstitute, mix one part concentrate with one part water, tonic water or ginger ale; adjust concentrate to taste.