Wash well and barely cover with water both ripe red wild sand plums and partially ripe pink plums. Boil until fruit is soft and liquid is bright red. Cool until warm only and strain through a clean white cotton sack or small pillow case to obtain clear pulp. free juice. Make jelly in proportion listed above. Bring strained juice to a boil, stir in butter to keep juice from boiling over sides of pan. Slowly stir in sugar, stirring constantly until mixture reaches 220 degrees on candy thermometer. Remove from heat immediately and pour into dry, warm sterilized 1/2 pint jelly jars, leaving approximately 1/2 inch at top of jar for expansion when jelled. Seal jars tightly.
Makes approximately 8 to 10 jars.
NOTE: You do not need Sure Jell for this jelly because wild plums contain enough natural pectin. Do not over cook because jelly will continue to jell while cooling in the jars.