Strawberries lend sweetness to this preserve, though enough classic marmalade bitterness remains to satisfy purists. The recipe comes from the Glass Rooster Cannery cooking school and farm store in Sunbury, Ohio.
Dip a spoon into hot jam. Lift and hold the spoon sideways. If the jam streams off, it is too thin. As the jam thickens, it will drip. When the drips join in a sheet that falls off the spoon, the jam has set.