Recipe yields 5 half-pint jars.

Prepare for Canning

• Prepare the calcium water: Combine ½ teaspoon calcium powder (from the small packet in the box of Pomona’s pectin) with ½ cup water in a small jar. Screw on a lit and shake until well-combined. You'll have more than you need for this recipe. Store the excess in the refrigerator for use in making additional jam or jelly recipes.
• Wash the jars you'll use, making sure each is clean and free of nicks in the rim, which could impede sealing.
• Wash the lids and rings in hot soapy water. (If you're using non-Ball brand lids, prepare as suggested by manufacturer.)
• Place empty jars in a canning pot or large stock pot with enough water to cover by an inch or two, cover pot, and set on high heat. It can take awhile for the water to heat, so get it started before you begin making the recipe.

Make the jam

• Combine sliced rhubarb with ½ cup water in a small saucepan. Cook until rhubarb is softened. Use a potato masher or immersion blender to further crush the cooked rhubarb into small pieces if desired.
  1 pound rhubarb, ½ cup water
• Measure 2 cups of cooked rhubarb into a saucepan with the mashed strawberries. Add calcium water and stir well.
  2 cups mashed strawberries, 1 tablespoon calcium water
• Bring fruit to a boil over medium high heat, stirring occasionally. 
• Meanwhile, thoroughly combine sugar with the pectin.
  1 cup sugar, 1 tablespoon pectin
• When fruit comes to a full boil, slowly add the pectin and sugar mixture, stirring thoroughly for a minute or two to dissolve pectin.

Canning strawberry rhubarb jam

• Lift hot jars from the canner and drain.
• Ladle hot jam into half-pint or pint jars to within a quarter inch headspace. (The distance from the top of the jar to the jam.) A canning funnel makes this easy.
• Use a non-metal tool to remove any visible air bubbles.
• Wipe the jar rims to remove any jam that may have spilled. A clean rim is essential to a good seal.
• Set jar lids in place. Screw bands on finger tight.
• Use a jar lifter to gently submerge jars into the water bath canner. Be sure to place a wire rack in the bottom of the pot to prevent the jars from sitting directly on the bottom. Water should cover the top of the jars by an inch or two. The water will cool somewhat in reaction to the addition of the jars. Return the water to a low boil and then set the timer.
• Process in a boiling water bath for 10 minutes. For those at a higher altitude, add a minute to the boiling time for every 1,000' above sea level.
• Check seals. Lids should be solid and pulled down tight. (if they flex and pop, the jar didn’t seal; put unsealed jars in the refrigerator and use those first).
• Remove rings and wash outsides of jars. Store in a cool, dark place.