Martínez Montañés (1568-1649) was the greatest Spanish sculptor of the Baroque era. His medium was wood; in fact, he was known as "el dios de madera" (the god of wood) because of his skill. Here and here are examples of his work.
Velázquez has among his paintings several portraits in which the subject stands before a background so neutral that it very nearly takes on the quality of a void--see, for example, his portrait of Don Pedro de Barberana y Aparregui. What is striking about this painting is that Velázquez, in his depiction of Martínez Montañés at work, makes it appear as though the sculptor has reached into that void, shaping out of it the rough form of the bust in the lower right.
Like a painter with brushes and paints before a blank canvas, the sculptor and his knives and chisels stands before the blank canvas that is a piece of wood; each coaxes an image from a blankness. I admit to not having thought this through too much, but it would seem that this take on the artistic process isn't that far removed from Milton's notion, in Paradise Lost, of God's having shaped the universe out of unformed matter (Chaos).