Alabaster (also called satin spar) is a natural crystalline mineral of hydrated calcium sulfate mined near Volterra (between Florence and Sienna), Italy. Softer than chalk (3 Mohs scale of mineral hardness), its hardness of 1.5 to 2 makes a soft white gesso that is easier to polish to a smooth finish for gilding bases and painting grounds. Calcite or chalk, being a carbonate, effervesces on exposure to acids, whereas gypsum alabaster, when so treated, remains practically unaffected.
Recommended in Cennino Cennini's treatise on painting, Il libro dell'arte (often translated as The Craftsman's Handbook), to make "gesso sottile" for painting grounds.
Add to oil colors and mediums as a transparent filler and to give body or in tempera as a white filler.Source
Tuscan alabaster occurs in nodular masses embedded in limestone, interstratified with marls of Miocene and Pliocene age. The mineral is worked largely by means of underground galleries, in the district of Volterra.