Diane Townsend Dry Ground
The dry ground is a pumice based chunk of material made to be applied to paper and rubbed in with your hand. The purpose of such a ground is to give a bit of "tooth" to a surface which will grab the pastel and make it smooth for layering.
They are available in Black and white, Sepia and Buff, Umber and Grey
You may tint the white or black DG with any pastel color and disperse it on the surface by just working it into the ground.
I suggest wearing a glove, otherwise you will have no hand left after doing this repeatedly! Once the surface is prepared just draw into it with any pastel.