Instead of using plaster, you can pour resin in your clay mold. This will give you a very hard cast image.
Following the manufacturer’s instructions for the resin, pour the resin mixture into your textured clay dam. It will set up in approximately 6 minutes, and you can pull the clay mold off the resin. This clay is not reusable. The resin can be colored but when curing in the clay mold, some of the clay color will be absorbed into the resin. You may find that you like the back side of the resin piece as much as the textured front side. You can also paint it with acrylic paint.
Please note that there are different types of resin; some are toxic and others are less so. Be sure to follow all safety precautions including working in a well-ventilated area and wearing disposable gloves if recommended.
As the resin cures, the clay colors it. (Shown: the back side of a cast.)
Cast resin with a Red Oxide stain added.
Cast resin sprayed black.