Boil plums in soda water (1 teaspoon baking soda to 1/2 gallon water) until plums start to crack. Drain, rinse and remove stones. Measure pulp. To 6 cups plum pulp, add 5 cups sugar. Boil 15-20 minutes. Add juice of 3 oranges plus rind and 1 3/4 cup walnuts. Boil 10 minutes more. Put into jars and seal.