A simple and easy jam for first-timers. Remember to choose tart-skinned plums that are ripe but not squishy. Purple plums make a luscious, deep crimson jam. Use a very large pot for this recipe — at least 6 quarts.
Pit and chop (don’t remove skins) 2 kg plums, putting the fruit in a large pot as you go. There will be a lot of juice — keep this with the fruit. Rub the pits in a fine-mesh sieve until much of the residual flesh comes off the pits. Scrape this pulp into the pot, too. Boil the fruit with about a cup of water and the juice of a lemon until the mixture has reduced by around a third. If you have them on hand, include a couple of skinned apricot kernels too. (See below for more on this.)
While plums are reducing, pour about 1.8 kg white sugar into a large lasagne dish and warm sugar in a 250’F oven for 20 minutes or so. There will probably be a hard crust on the sugar; that’s OK — just crack it into pieces with a spoon or knife.
Once plums have reduced add the warmed sugar, stir until it has all dissolved, and bring mixture to a boil. Cook at a rolling boil for 4-5 minutes, carefully watching to make sure the jam doesn’t boil over. Reduce to an aggressive simmer after 4-5 minutes. (The mixture will be more likely to boil over as it heats up and reducing the heat slightly helps avoid this.)
There’s no need to stir the jam constantly, but do stir occasionally to make sure it’s not burning and sticking to the sides and bottom of your pan.
Wrinkle test for set at 12 minutes: pour a teaspoonful of jam onto a ceramic, glass or metal plate that has been in the freezer. Return to freezer for a minute and then remove plate and nudge jam with your finger. If it resists your finger and wrinkles, it’s ready. A slight wrinkle means jam will be a softer jelly-like set. A thick gel and firm wrinkle means jam is going to set really firm.
Pour into warmed, sterilized jars, seal, and can for 10 minutes in a boiling water bath.
* Preparing apricot kernels: Crack apricot seeds gently with a hammer (inasmuch as you can crack anything gently with a hammer…) Remove the white kernel and put into a small cup or bowl. Pour over boiling water and leave a couple of minutes. Pour over cold water (so you don’t burn yourself!) and lift out kernels. Rub them gently in your fingers — the outer skin should slough off.