There is such a technique for processing gypsum scagliola. It takes its origin in Italy. Previously, with the help of such techniques, plastering of premises was made, as well as interior and art items.
Gypsum is closed not with water, but with glue composition. According to ancient recipes, a 10% adhesive solution was used. Now use a 3% solution. The glue is a retarder of solidification. Therefore, it is possible to work with gypsum for a long time. In addition, the glue is a strengthening binder for gypsum.
First we prepare a cast. To give the cast a certain shade, iron oxide pigments are used. To mix gypsum with pigments, it must be rewound so that the pigment is evenly distributed throughout the entire volume of gypsum. The amount of pigment should not exceed 10% of the amount of gypsum, otherwise the gypsum will lose in the fortress.
I use a sieve for rubbing. I rub it with a stone. Result look at the photo.