Before you begin, sterilise your jars. I put the jars and lids through a dishwasher, then into a low oven to ensure they are completely dry. You can, instead, boil the jars for several minutes in a large pot, then remove to a tray and into the oven to dry. Be very careful not touch the inside of the jars or lids- you do not want to contaminate the jars after sterilising.
Stir the mixture occasionally and continue removing the scum from the surface until the jam reaches 105 degrees. At this point, remove a spoonful carefully from the pot onto a small plate. Leave the jam on the plate for a minute to cool, then run your finger through the centre to see if it has set. It shouldn't be runny, nor should it be solid: just a soft set jelly. The line you create with your finger should remain without the jam on either side oozing back into the centre. If it is still very runny, keep boiling and testing until it reaches a desirable consistency. The whole process may take up to half an hour, depending on the quantity of jam you are boiling. Immediately pour the jam into the sterilised jars- the heat will further ensure that the jar and the jam is free of bacteria. The jam and jars will be very hot- take care not to burn yourself. Using a tea towel, screw the lid carefully onto the jar. Leave the jam to cool thoroughly, then store in your pantry. They are ready for use as soon as the jam has cooled. The jam should keep for considerable time, so long as everything has been thoroughly sterilised. Enjoy with some fresh scones and double cream for a delectable afternoon tea. Bon appetit!