Bring a boiling-water canner 3/4  full with water, to a simmer. Wash jars and screw bands in hot soapy water; rinse with warm water. Pour boiling water over flat lids in saucepan off the heat. Let stand in hot water until ready to use. Drain jars well before filling.

Remove the colored part of the peel from oranges and lemons using vegetable peeler. Cut into thin slivers. Mix the peels, water and baking soda in large saucepan. Bring to boil. Reduce heat to medium-low, cover and let simmer for 20 minutes, stirring occasionally. Add the chopped fruit and juice. Cover and simmer an additional 10 minutes. Measure 4 cups of prepared fruit into a 8 quart sauce pot.

Stir pectin into prepared fruit in sauce pot  Add butter to reduce foaming. Bring mixture to full rolling boil on high heat, stirring constantly. Stir in sugar. Return to full rolling boil and boil exactly 1 minute, stirring constantly. Remove from heat. Skim off any foam with metal spoon.

Carefully ladle hot marmalade immediately into prepared jars, filling to within 1/4 inch of tops. Wipe jar rims and threads. Cover with two-piece lids and screw bands on.

Place jars on elevated rack in canner. Lower rack into canner. {Water must cover jars by 1 to 2 inches. Add boiling water, if necessary.} Cover; bring water to gentle boil. Process 5 min. Remove jars and place upright on towel to cool completely.

After jars cool, check seals by pressing middles of lids with finger. If lids spring back, lids are not sealed and refrigeration is necessary}. Yields {6} 1/2 pints.