Testing the set: wrinkle, flake and thermometer, side by side
I had a kilogram of damsons in the freezer from September and a quiet Saturday, so I made a batch of jam purely to test the set tests against each other. Three methods, all checked at the same moments through the boil: the thermometer, the flake test and the cold saucer. The damsons went in frozen with 250 millilitres of water and simmered for 25 minutes until the stones floated, which I skimmed out with a slotted spoon, losing patience at about forty and leaving the rest for whoever finds them.
With 1.2 kilograms of sugar dissolved and the jam at a rolling boil, I tested at two-minute intervals from the four-minute mark. At four minutes the thermometer read 102.5, the flake off a wooden spoon ran straight off, and the saucer sample stayed liquid. At six minutes the thermometer read 104, which is the figure most books give for a set, the flake had started to hang in a drip before falling, and the saucer sample skinned but did not wrinkle. At eight minutes the thermometer read 104.5, the flake hung and then fell as a sheet, and the saucer sample wrinkled when pushed and held the wrinkle. I took it off there.
So on this batch the thermometer called a set about two minutes before it existed, which I think is the half-degree it reads low plus the fact that damsons are only moderately high in pectin. The flake test was suggestive but hard to judge; what counts as a sheet rather than a drip is not obvious until you have seen both, and even then it depends on how hot the spoon is. The saucer was unambiguous. It is also the slowest, since it needs a minute in the freezer each time, during which the jam carries on boiling, so I now pull the pan off the heat while I wait.
The jam itself is good: dark, sharp, with the almond note that damsons get from the stones. Seven jars. I have written in the book that the thermometer is for knowing when to start testing, not for deciding, and that the saucer decides. For fruit that is very high in pectin, like blackcurrants or quince, I would start the saucer tests at 103 rather than 104, because by the time it reads 104.5 those can be past the point.