"Take any amount of fruit (1kg maximum at a time), and half as much sugar. Put fruit and sugar in a pot and from now on you only need patience and the right method" my grandma said. She never allowed gelatine enriched sugar in her kitchen, by no means.
Well, the secret was in the right method, which meant you had to spend 30 tiring minutes, a wooden spoon in hand, stirring through the hot mixture in an uncountable number of eights. 15 minutes while simmering on the hotplate, 15 minutes while set to rest on the cool surface of the workplace. 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 ... . An endless row of tiring eights.
To keep tiredness within a limit it's a good idea to sit down next to the pot during the whole process. This way you only will suffer from arm tiredness... But it's worth the effort. Even when the finished jam sometimes comes out a bit runny - always depending on the fruit's natural content of pectin, the flavour is unbeatable pure and fruity.
When you prepare jam from only 500g fruit/ 250g sugar, the stirring time decreases to 20 minutes.