Pastels are chalk. It is dry pigment, mixed with a dry binder placed on a textured surface to hold the pigment to the paper. It doesn’t involve any liquid of any kind. Acrylics are a dry pigment, mixed with a liquid binder placed on pretty much any kind of surface that is properly prepared with gesso or some kind of acrylic medium. Water or liquid mediums are involved in the application of the paint. Acrylics cannot be put over top of pastels as the dry pigment doesn’t allow the acrylic paint to bind to the substrate. You can put pastel over top of acrylic ONLY if you use an acrylic ground medium specifically designed for pastels. It kind of feels like sandpaper after you apply it and it dries clear. You will have to put a fixative on the pastel if you are using it on a canvas substrate, and that is a whole other topic of discussion.
"Acrylics are a combination of pigments held in an acrylic Polymer Emulsion. Acrylic paints are a man-made water-soluble paint. Now take oil pastels. Oil pastels are a stick for drawing and painting that are made with a mixture of pigments that are then mixed in with a non-drying oil and waxed binder."
