According to Cennini, an artist should use nine layers of gesso, and they should be burnished until they appear like ivory.
Cennini and gesso
Cennino Cennini advised that eight layers of gesso should be applied brushing the gesso in a different direction each time and not allowing the coats to dry out too much before applying the next layer which is the final layer. On this note, according to Cennini, nine layers of gesso should be used.
To achieve the targeted ivory-smooth surface required in a painting with gilding, Cennini suggested rubbing powdered charcoal over the surface prior to the scraping process, so that any hollows would appear black against the flat white surface.
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