When using charcoal and white conte on gray paper, do not blend the two but instead use each separately to define different values - white conte for lighter tones, gray paper for mid-tones, and charcoal for darker tones and blacks. The gray paper provides a mid-tone value that falls in the scale depending on how dark or light it is. If filling the background with a darker tone, be sure to cover the entire background evenly rather than just making a dark halo around the subject.