When using charcoal and white conte on gray paper, do not blend the two. Allow the white conte to define lighter values, let the gray of the paper be a value, and use the charcoal to define the darker tones and blacks.

Notice that in the value scale the gray of the page is one of the mid-tone values. Where the value falls on your scale will depend on how dark or light the grey of the paper is.

If you choose to fill in the background with a darker tone, be certain to fill in the entire background– do not just make a dark halo around the still life.