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1
Divide plums halves into two equal portions. In a non-reactive mixing bowl, mix first group of plum halves with 1 pound 4 ounces of sugar. Cut the remaining plum halves in half to make quarters. In a separate bowl, mix plum quarters with remaining 12 ounces sugar. Cover both bowls tightly and let stand at room temperature overnight.
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2
In a large pot, scrape plum halves along with any sugar and syrup and cook over medium heat, stirring only enough to prevent scorching, until softened, about 15 minutes. Pass plum halves through a food mill fitted with the smallest-hole plate; scrape any plum left on the mill into the plum puree. Mix plum puree into quartered plums and stir in lemon juice. Scrape plum mixture into a wide jamming pot.
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3
Heat plum mixture over medium-high heat, stirring as needed to prevent scorching, until jam starts to foam, about 15 minutes. Continue to cook, stirring minimally and lowering the heat if necessary to prevent scorching, until foaming has subsided, about 15 minutes longer; scrape any foam off jam surface with a stainless steel spoon as needed. Continue to cook after foaming has subsided until bubbling has slowed and jam looks glossy, 5 to 15 minutes; lower heat as necessary to prevent scorching.
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4
When plum quarters have broken down and jam has thickened, begin testing for doneness: Turn off heat and set a small spoonful of jam in the freezer for 5 minutes. Jam is ready once it holds together and doesn't run off the spoon when tilted. If jam is too runny, return to heat and cook, stirring frequently and repeating the spoon test every 5 minutes, until jam passes test.
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5
Eat right away or can, following canning instructions. Canned jams can be stored in a cool, dark place for up to one year.
