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1
Place rhubarb, strawberries, lemon juice, fruit pectin, and butter into a large kettle over medium heat.
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2
(Butter is optional but helps keep jam from getting too foamy).
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3
Stir the fruit mixture to help the juice start to form, and add sugar, about 1 cup at a time, stirring constantly until sugar is dissolved and the juice is starting to simmer.
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4
Turn up heat to medium-high, bring the mixture to a full rolling boil, and cook and stir for 1 minute.
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5
Skim off any foam that forms.
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6
Sterilize the jars and lids in boiling water for at least 5 minutes.
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7
With a jelly funnel and a soup ladle, pack the jam into the hot, sterilized jars, filling the jars to within 1/4 inch of the top.
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8
Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles.
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9
Wipe the rims of the jars with a moist paper towel to remove any food residue.
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10
Top with lids, and screw on rings.
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11
Place a rack in the bottom of a large stockpot and fill halfway with water.
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12
Bring to a boil over high heat, then carefully lower the jars into the pot using a holder.
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13
Leave a 2 inch space between the jars.
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14
Pour in more boiling water if necessary until the water level is at least 1 inch above the tops of the jars.
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15
Bring the water to a full boil, cover the pot, and process for 5 minutes.
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16
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool.
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17
To help the jam set, don't move or touch the jars until cooled.
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18
Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all).
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19
Store in a cool, dark area.
