Simple red plum seedless jam

In the distant 70s of the last century, plum jam in Ukraine was cooked on the street on a bonfire, it turned out much tastier than on the stove in the house. On the stove, it also turns out well, but I advise you, if you are engaged in preparations for the winter in the country, experiment a little and still cook jam on the fire, you will get excellent sweet food.
The jam recipe is very simple, although the jam is cooked in several rounds.
To begin with, the plums are washed and dried. If a plum stone is separated, then do not be too lazy to separate it. If the bone does not separate, then each plum should be pricked with a pointed match. Syrup is brewed from sugar and water (the foam from the syrup must be removed). When the syrup is ready, plums are lowered into it and the jam is cooked for 20 minutes, then removed from the heat and set aside until completely cooled.
Then the process is repeated three or four times until the required density is reached. After the end of the last cooking, the jam is laid out in jars and it cools down already in jars. After complete cooling, we close the jars with steamed nylon lids and take them out for storage in the cellar or in the refrigerator