Strawberry Rhubarb Jelly

1-1/2 lbs red stalks of rhubarb
1-1/2 quarts ripe strawberries
1/2 tsp butter or margarine
6 cups sugar
6 oz. liquid pectin

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. Using a wide mouthed funnel, fill sterile jars, leaving 1/4 inch headspace. Adjust lids and process.