Scrape outer skins of oranges with blade of a knife to release citrus oils. Peel oranges, and grind rind; reserve pulp. Combine ground orange rind, 3 cups water, and soda in a medium size stainless steel saucepan; bring to a boil. Reduce heat; cover and simmer 5 minutes. Rinse well; drain.
Combine remaining 3 cups water and orange rind in saucepan; bring to a boil. Reduce heat; cover and simmer 15 minutes. Let stand in cooking liquid overnight. (Do not drain.)
Cut reserved orange pulp into quarters; remove and discard seeds. Place pulp in container of an electric blender; process until smooth.
Combine orange rind and liquid, pureed pulp, and sugar in a flat-bottomed kettle; slowly bring to a boil, stirring until sugar dissolves. Boil, stirring constantly, until mixture registers 220° on a candy thermometer or until mixture sheets from a cold metal spoon. Remove from heat, and skim off foam with a metal spoon.
Quickly ladle marmalade into hot sterilized jars, leaving 1/4- inch headspace. Cover at once with metal lids, and screw bands tight. Process marmalade in boiling-water bath 10 minutes.