As far as I can tell, the ingredients described by Cennini[1] is close to the “traditional” preparation:

* Gypsum (Hydrated calcium sulfate)

* Zinc white pigment

* Clean tap water or distilled water

* Rabbit skin glue

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[1] Cennini, Cennino d'Andrea, The Craftsman's Handbook "Il Libro dell Arte," Daniel V. Thompson, Jr., trans. (New York: Dover Publications, 1960) pp. 69–74.