Put the sugar into a copper or stainless steel preserving pan with 7 tablespoons of water per ounce of sugar. Boil and skim until the syrup is clear. Put in the ripe pitted plums. Cook over a high (but not fierce) heat until the syrup coats the skimmer: When you take the skimmer out of the pan, the attached jam will fall immediately. After a couple of minutes, you should be able to see the syrup gather on the edge of the skimmer and fall in large separate drops. Cook further over a very low heat for 7 to 8 minutes then pour into jars. Divide up the fruit among the jars little by little so that the jars are gradually hot. Seal immediately. Wait for 8 days before tasting.
