For centuries 'oiled charcoal' was prized for its ready adhesion to paper fibres, whereas ordinary charcoal particles tend to sit on the surface, remaining powdery and vulnerable to smudging. Oiled charcoal is produced by soaking pieces of vine charcoal in linseed oil. The oil is absorbed into the paper when the charcoal is used. The paper fibres around the charcoal lines darken leaving a slight discoloured halo. Over time the oil migrates, darkening the paper and making the haloes more pronounced. Eventually the areas of paper covered with the charcoal exhibit an overall brown tone, apparent in this drawing.