Heat the strawberries and lemon juice gently in the pan, stirring constantly to reduce the volume. Add the sugar, stir till dissolved and boil until setting point is reached. Remove the scum. Leave the jam undisturbed to cool until a skin forms on the surface and the fruit sinks (about 20 min.). Stir gently to distribute the strawberries. Pour into warm, dry jars and cover immediately with waxed discs. Tie down when cold.