When making jam or marmalade, you need to know how to test its setting point.
When you test, ALWAYS TAKE THE PAN OFF THE HEAT AS YOU TEST and TEST EARLY !!
1) Test with a sugar thermometer and remember, jam sets at a temperature of 220 degrees F.
2) Stir preserves thoroughly with a wooden spoon, turn the spoon round to cool the jam adhering to it, then hold the spoon horizontally. If jam has set, it will form a firm drop or flake on the edge of the spoon.
3) Put a little on a cold saucer, cool it, and if adequately set it should wrinkle and feel firm.
When making jam or marmalade do not continually stir. Wait until it reaches setting point, stir for a minute (this will get rid of quite a lot of the scum), then remove the remainder. A knob of butter or margarine put in will not only safeguard it from burning in a thin pan but will also help to prevent scum.