Canning method
Wash and cut rhubarb into 1-inch pieces and blend or grind. Wash, stem, and crush strawberries, one layer at a time, in a saucepan. Place both fruits in a jelly bag or double layer of cheesecloth and gently squeeze out juice. Measure 3-1/2 cups of juice into a large saucepan.
Add butter and sugar, thoroughly mixing into juice. Bring to a boil over high heat, stirring constantly. Immediately stir in pectin. Bring to a full rolling boil and boil hard 1 minute, stirring constantly. Remove from heat, quickly skim off foam, and fill sterile jars, leaving 1/4-inch headspace.
For more information on how to sterilize jars see "Sterilization of Empty Jars" . Adjust lids and process as recommended in
Processing: Process Hot, Half-pints or pints, boiling-water canner, 0 - 1,000 ft - 5 minutes.
Altitude-adjusted processing table: Table 1. Recommended process time for Strawberry-Rhubarb Jelly in a boiling-water canner. / Process Time at Elevations of / Style of Pack - Jar Size - 0 - 1,000 ft - 1,001 - 6,000 ft - Above 6,000 ft / Hot - Half-pints or pints - 5 min - 10 - 15