I finally got sick and tired of standing over a simmering pot of marmalade, stirring like an idiot.
Why do we stir the orange/sugar thing? Because if you don't it will stick to the bottom of the pot and scorch. By stirring each molecule of (future) marmalade gets just enough heat, but not so much that it will scorch.
Inner Voice: Ah...but doesn't an oven provide the same sort of even heat?
IV: And don't you also stick many things in the oven to cook that normal people usually put on the stove?
M: True that.
IV: So, then work on a method for making orange marmalade in the oven.
And so I did, and these are the results.
2lb Seville oranges 1 lemon, zest finely grated and juiced 6 c. water 3¾ lb. sugar (feel free to make this less sugar-y, the oranges from our tree -- I love saying that -- are unrelentingly sour and call for a ton of sugar)
Wash the oranges and lemon thoroughly. Slice the oranges into 1/8" (4mm) rounds (use a mandoline) saving the seeds as you go. Stack the slices and quarter them. Place the oranges into your jam pot. Put the seeds in a cheescloth/gauze/muslin bag and add. (These carry tons of pectin and you want that.) It should hold 8qt/8L. Add the lemon zest and juice and the water to the pot, set over high heat and bring to a boil, approximately 10 minutes. Once boiling, transfer to a roasting rack (to catch any unlikely spillage) in the dead center of your oven set at 250F-275F (120C-135C) which varies because all ovens are different. The idea is to have yourself a rapid simmer. Cook for 40 minutes or until the fruit is very soft.
While this is going on, put a saucer in your freezer. Boost the heat of your oven to return to a boil (in MY oven that's 350F/175C). Fish out the seed sack (that is NOT some lurid euphemism) and add your sugar and stir and cook, until it reaches 222-223F (105F-106F) on a thermometer -- I like a remote thermometer w. an alarm, for added idiot-proofness -- which in my oven takes 15 to 20 minutes. Since all ovens are different, the first time you make this, you may need to monitor the heat to prevent a boilover. (You also don't want to caramelize the sugar for a million different reasons.)
Test the set of your marmalade: drop a teaspoon of it on your chilled saucer and let it sit a half minute. Then tilt the saucer. The marmalade should be a soft gel, running slightly. If it's thin and "dripples*" keep cooking, checking every 5-10 minutes. If it resembles silicone, you've overshot your mark.
Your mantra should be: The pectin in the fruit needs to hit 222F/105C or so to do its magic.