Prepare the water bath and jars: Fill a large stock pot or canning pot about 2/3 full with water and place a rack in the bottom. Begin heating the water over medium-high heat so it will be near boiling when the jam is ready. Submerge clean jars (and keep lids and bands hot in the water or in a separate hot container); keep jars hot (hot but not vigorously boiling) until ready to fill.

Prepare the guavas: Remove stem and blossom ends from 1 1/4 pounds (1 kg) pink guavas. Coarsely chop the guavas and place them in a blender with 1 tablespoon lemon juice. Puree on low speed to avoid pulverizing the seeds.

Remove seeds and skins: Press the guava puree through a fine-mesh sieve or run it through the fine screen of a food mill to remove seeds and skins. Transfer the finished guava puree to a large, heavy saucepan.

Prepare the strawberries: Hull and coarsely chop 1 pound (280 g) strawberries, then lightly mash them with a potato masher. Add the mashed strawberries to the saucepan with the guava puree. (Optional: you may blend or food-mill the strawberries with the guavas for a smoother, looser jam.)

Add acidity and calcium: To the fruit in the saucepan add 3 tablespoons lemon juice, 1/4 teaspoon citric acid, and 2 teaspoons calcium water. Stir to combine.

Bring fruit to a boil: Heat the fruit mixture over medium-high heat and bring it to a full rolling boil, stirring occasionally.

Mix sugar and pectin: While the fruit is heating, whisk together 1 1/2 cups (300 g) granulated sugar and 1 1/2 teaspoons Pomona's Universal pectin in a bowl until evenly combined.

Add sugar/pectin and finish boiling: Whisk the sugar/pectin mixture into the boiling fruit, stirring continuously until the mixture returns to a full rolling boil. If desired, add 1 teaspoon butter to reduce foaming, or skim foam from the surface.

Jar the jam: Remove the saucepan from heat and ladle the hot jam into the hot jars, leaving 1/4-inch headspace. Wipe jar rims clean, center lids, and screw on bands fingertip-tight.

Process jars: Place filled jars in the boiling water bath and process for 10 minutes. Remove jars and set them on a wire rack to cool completely. When cool, check seals; any jars that did not seal should be refrigerated and used within 3 weeks.