The term "jam" is generally applied to fruit cut up in small pieces or mashed into a pulp, cooked with from three-quarters to its whole weight of sugar. Jam made from equal weights of fruit and sugar is always more reliable as far as the keeping quality is concerned, than that made from a smaller proportion of sugar. "Marmalade," in this country, usually signifies citrus fruits cut up into thin shreds. In America the term "marmalade" is applied to any kind of fruit treated in the same way as "jam," excepting that the fruit is rubbed through a sieve in order to remove the seeds.
In making jam it is a good plan to rub the bottom of the preserving pan with a little oil or butter, which will prevent fruit from burning; while it has also been found that if a little butter, say a tablespoonful, is added to jam while cooking, mould is prevented. Jams and marmalade will be found to be much better in quality if fruit is allowed to simmer gently until quite tender before adding the sugar. This is especially important in the making of jam from hard fruits, such as the quince, and also citrus marmalades. The latter should cook for at least 3 to 4 hours before the sugar is added, as the peel is exceedingly tough, and the sugar will tend to harden it.
If jam is boiled for too short a time, it will neither set firmly nor keep well, while if, on the contrary, it is cooked too long, it will become sticky.
All scum should be removed as it rises. When no more scum appears, put a drop or two of jam on a cold plate, and if at the end of a minute or two it has become too stiff to flow freely, it is cooked enough.