Gather equal amounts of citrus fruits, water, and sugar. The ratio of sliced fruit to water to sugar should be about 1:1:1 — adjust up or down according to taste.
Using a mandolin or sharp knife, slice the lemons crosswise into thin rounds, discarding the seeds as you go.
Stack the rounds and slice into quarters.
Place the water & lemons in a large stainless steel or heavy-bottomed pot & bring to a boil. While you wait, fill a very large soup pot w/ water almost to the top & set to boil on a separate burner.
Simmer the mix for 25-40 minutes, or until the fruit is soft and turning opaque. Add sugar and bring back to a rapid boil, stirring often. Insert thermometer; stick dinner plate in freezer.
When the pot of plain water is at a boil, drop in jars & ladles & boil for 10 mins. Turn the pot off & toss in canning rings & lids, leaving them in the water until it's time to fill with marmalade.
Boil your marmalade until the candy thermometer reads 222. Take care that it doesn't boil over, and stir often so that you don't scorch the bottom.
To test if it's ready, put a tbsp of the mixture on your cold dinner plate & wait 30 secs. Tilt the plate. If it runs it needs more time. If it’s gel-like & wrinkles when you touch it, it's ready.
Use tongs to remove equipment from pot, drain well, and set on towel. Ladle mix into each jar and wipe excess with a damp paper towel. Put lids on top, add rings, and screw tightly.
Store in a cool, dark place for at least 24 hours while the seal is created. (You may hear the lids pop while this is happening.) Your marmalade should store for up to a year without refrigeration.
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