I'm not at all sure about this, but possibly the Antike were wooden statues and they had received 10 or 14 coats of gesso from previous students.

Dabei verwendet er beileibe keine kostbaren, teuren Materialien. Nein, aus Pappel- und Kistensperrholz, den billigsten Brettern, sind seine Objekte "geschnitzt". Und trotzdem sind sie es wert, wie Preziosen behandelt zu werden. Kirchmair hat ihnen Säcke schneidern lassen, zum einpacken und zuschnüren. Damit ihnen beim Transport nichts passiert. Das ist wichtig. Denn besonders strapazierfähig sind die "Bodenfrüchte" nicht, eher schutzlos und verletzlich.

Teilweise besitzen sie einen empfindlichen Anstrich aus einfacher Gipsfarbe mit Binder, teilweise sind sie naturbelassen.

http://www.antonkirchmair.de/pressetexte/jg_liegen.htm

Artists who are concerned with making permanent paintings should consider carefully the quality of all layers of their paintings. They should be as concerned about the layers they do not see (size and ground) as the layers they do see (oil colors, painting mediums and varnish).

Ground is the foundation of an oil painting.
Oil grounds have been for five centuries a simple mixture of chalk (in Northern Europe) or gypsum (in Italy), white lead and linseed oil. Chalk is a calcium carbonate, which naturally occurs in fossilized shell deposits, like the White Cliffs of Dover. The premium grade of chalk was "champagne" used to make "Paris Chalk." Today we also call calcium carbonate chalk "whiting." Like gypsum, chalk does not make a suitable white pigment because of its poor color and transparency. For making gesso, opacity is not an issue because so many layers are applied. However, an oil ground on fabric should be no more 1/8 inch thick so only a few layers can be applied. By adding a measure of opaque white lead, a thin and bright oil ground can be made.

Traditional Gesso
Gesso is Italian for gypsum (calcium sulphate dihydrate) which occurs naturally near salt deposits. Calcined gypsum, also called plaster of Paris, when mixed with animal glue, makes a luminous painting surface for fresco and for paintings on wood panels. Medieval painters applied as many as ten layers of gesso on wood panels. By painting alla prima with tempera on gessoed panels they could create "portable" frescoes. Gesso on wood panels makes a good surface for paintings that include burnishing and gilding techniques.

Gesso was not used as a ground for oil painting. The traditional primer for oil painting is an oil ground. Modern "gesso" was formulated by manufacturers of acrylic polymer primer about fifty years ago. Why they decided to call acrylic primer "gesso," a conservator suggested that the marketing department wanted to associate new acrylic primer with painting tradition so oil painters would use it. Whatever the reason, the name "gesso" continues to cause confusion.

http://www.gamblincolors.com/oil.painting.techniques/grounds...