Medium to Use for Sand Effects
(Grawin Opal Fields Australia)
The Australiam aborigines here use lumps of sandstone and other rocks found on gravel tracks.Then with a coarse rasp, grate rock in a collector bowl. Bag off the different colours into small plastic bags.
To use, place about a tablespoonful onto plate, couple of drops of water then add good dollop of wood glue (known as here as PVA Glue, mix to the consistency you wish to paint with. Dries quickly and adheres well.
NB: - this is a great tip because I find the rock/sand mediums sold commercially in little jars are really expensive. You don't need to go to Oz for your mediums either!
Just use whatever stones, soil, sand or gravel dust etc, happens to lie in or on the ground near you. If necessary, wash it through in cold water first or sift it through an old flour or tea strainer if you want. The process can be almost as much fun as the painting!