Testing - (Wild) Plum Jam

http://foodviva.com/jam-recipes/plum-jam-recipe/
https://www.nigella.com/ask/how-do-i-know-if-my-jam-has-reached-setting-point

1 - We have loads of wild plums growing on the Valley Home Farm. They look like cherries, but are just small plums. The first tree to ripen is at the drive-way, just about the time the blackberries are just about to ripen. The trees are unkempt and, I believe, could produce a lot more fruit. As it is we got about 5 lbs out of it. The first item on the menu is jam. Straightforward jam! Now, I don't like my jams too sweet, yet I know sugar is required to make jams gel properly. Here's my first Frankenstein.

2 - 2021 - we no longer have loads of wild plums, but I managed to snag some cherry plums anyway. The key to making a nice soft jam or jelly is the heat! If using less sugar, you have to let it boil longer, until a candy thermometre reads the right temperature - 220F. That seems to be the big secret.

2 Tbsps vinegar
1 kg ripe plums
750 g granulated sugar
juice from 1 lemon

1. Fill a large bowl with water and add the vinegar. Soak the plums in the vinegared water for 20 minutes.
2. Pit the plums. I use this home-made method because I have a thing about using simple and available tools.
3. Pour the sugar and lemon juice over the plums and mix thoroughly to cover the plums. Leave to macerate in the sugar overnight.
4. Heat the macerated plum mixture over medium heat until it begins to simmer, stirring occasionally to help the sugar melt.
5. When it starts to boil, clip on a candy thermometer, reduce it to a rapid simmer and continue to cook, uncovered, stirring occasionally, until the thermometer reads 220F/105C. This can take upwards of 30 minutes, less if you add more sugar, longer if you have less sugar.
6. To test that your jam is ready, put a dollop on a frozen plate and put back in the freezer for 1 minute. If, when you run your finger through the sweet puddle, it leaves a clear trail that doesn't fill in, your jam is done!
7. For long-term storage, sterilize the jars and lids in boiling water. 
8. Leave 1/2 inch space at the top of the jar when filling it. 
9. Screw lids on until not quite as tight as possible - screw on tight, then loosed by 1/4 turn.
10. Put each jar on a rack in a large pot of boiling water. Jars must not sit directly on the kettle bottom. 
11. Once the pot returns to the boil, begin processing (boiling) time of 10 minutes if using half-pint or smaller jars (235 ml or smaller), 20 minutes for larger. 
12. After the time has elapsed, remove jars to a cloth-covered counter or baking sheet, away from any drafts. Do not move for at least 12 hours — 24 hours is best — so that the jars fully seal and the jam sets.