Wash your plums well. Cut open on one side and pick out the core, discard the core.
Cut into quarters, weigh your plums and place them into a large cooking pot.
Add the sugar, lemon juice, and 2 lemon halves to the fruits. Mix the content well.
Keep the pot with the fruits on the stove, cook over a higher heat setting until you can see a rolling boil (lots of foam too!).
Takedown the heat a bit and cook until the jam appears translucent and set. This can take 30 - 50 minutes approximately.
Pick out the two lemon halves and discard them.
Blend jam into desired consistency (this is optional).
Test if the jam is set by dropping some on an ice cold plate, the jam is ready if it doesn't run. OR test with a candy thermometer, it's set at 220 Fahrenheit/ 105 Celsius. If it runs, keep for some more time to cook on low heat and repeat the test until you have the correct consistency.
Drop some rum (or other liquor) into the jar lids. That will help in keeping the jam germ and mold growth free.
Fill your clean jam jars with the plum jam, close well with the lid and turn the jar upside down.
Keep them upside down for a few hours and then store them in a dry and cool place and keep away from direct sunlight. Keep in the fridge once opened.
1 Teaspoon = one serving, 100 Teaspoons is about 3 jars (roughly). Recommended use is 1 Teaspoon serving on a slice of bread.
If you live in a humid tropical climate, please store the jam in the fridge at all times.