"Ganch" was an unknown word for me too, so i googled it and the first link was from Urban Dictionary... I'll not say what I found, it is inappropriate.

But then I searched some more, quite a lot, until I found this definition, from the Great Soviet Encyclopedia:

Ganch: Middle Asian name for a binding material obtained by heating rock containing gypsum (from 40 to 70 percent) and clay. An aqueous solution of pulverized ganch sets quickly (hardens) and is easy to mold. From the first centuries of the Common Era ganch was used as a material for plaster, volumetrical and plastic decor (carving, casting of lattices and other parts), and sculpture. Wet ganch can be cut easily and allows both bas-relief and high relief to be diverse and have fine details. Carved ganch has a pleasing white mat surface. A layer of slightly wet ganch serves as a base for wall paintings. In the Caucasus it is called gazha.