Another option is to press small objects into clay, pour plaster into the resulting shape, and remove the casting after the plaster has cured.
Before painting the plaster with acrylics, coat it with polymer medium to seal it. When done, a coat of GAC 800 or resin will give it a hard and shiny surface. Another option is to color it with encaustics. This will give your art a cloudy, waxy finish. In this case, you do not need to seal the surface as encaustics do not stick to nonporous surfaces.
Plaster with colored encaustics (added after casting) gives it a matte look.
Coat with GAC 800 or resin for hardness.