Wash the skins of the oranges and lemons well. Cut off the ends of the oranges and lemons and discard. Then cut the oranges and lemons in half crosswise, then into very thin half-moon slices; discarding any seeds. If you have a mandolin, this will make the process faster. My mandolin did fine with the lemons, but not the oranges. I ended up using the slicing blade on my food processor for the oranges.
Using a large stainless steel pot, place the sliced orange and lemons (and their juices) into the pot. Add 8 cups of water, cover with lid, and bring the mixture just to a boil, stirring often. Remove from heat and stir in the sugar until it is dissolved. Cover and allow to stand at room temperature overnight.
The next day, bring the mixture back to a boil. Reduce the heat to low and let simmer, uncovered, for 2 hours.
While the marmalade is simmering, wash the jars in hot, soapy water and rinse. Sterilize the jars by boiling them 10 minutes, then keep the jars in hot water until they are used. Keeping them hot will prevent the jars from breaking when filled with the hot product. Wash and rinse all canning lids and bands. Treat the lids as directed by the manufacturers. Lids can be used only once.
Temperatures at Various Altitudes:
Sea Level – 220 degrees F.
1,000 ft – 218 degrees F.
2,000 ft – 216 degrees F.
3,000 ft – 214 degrees F.
4,000 ft – 212 degrees F.
5,000 ft – 211 degrees F.
6,000 ft – 209 degrees F.
Remove from heat and immediately skim off any foam that forms on the top and discard. The foam is the result of bubbles from the boiling jam coming up through the boiling jam.
If you want to be sure the Orange Marmalade is ready, place a small amount on a plate and refrigerate it until it is cool but not cold. If it is firm, neither runny nor too hard, it is done. It will be a golden orange color. If the marmalade is runny, continue cooking it and if it is too hard, add more water.
Pour or ladle the Orange Marmalade into the clean, hot Mason jars leaving 1/4-inch head space. Wipe the rims thoroughly with a clean damp paper towel and seal with the lids.
Store in the pantry for up to one year.
* Choose fresh whole oranges and lemons that are smooth-skinned and heavy for their size as that means the fruit is full of plenty of juice and flavor.
Orange Marmalade Recipe: https://whatscookingamerica.net/karencalanchini/orange-marmalade-jam.htm