Marble is the natural subject for grey wash. A statue is already a study in light and shadow with no color in it, which is exactly what this technique is for.
It is also the safest large scale subject: a statue does not have to be a likeness of anyone, so it can be pushed as large and as dramatic as the placement allows.
Because marble has no color to begin with. Everything that makes a statue readable is light and shadow, and that is precisely what grey wash renders.
As big as the placement allows. A statue reduced to a small panel loses the scale that made it impressive.
The face can. The snakes are what usually forces it larger, because they need to be separable from each other.
Not necessarily. Skin left bare around a statue reads as space, and space is what makes it look monumental.
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