http://www.accademia.org/explore-museum/artworks/michelangelos-prisoners-slaves/

Michelangelo is famous for saying that he worked to liberate the forms imprisoned in the marble. He saw his job as simply removing what was extraneous. This endless struggle of man to free himself from his physical constraints is a metaphor of the flesh burdening the soul.

All the unfinished statues at the Accademia reveal Michelangelo’s approach and concept of carving. Michelangelo believed the sculptor was a tool of God, not creating but simply revealing the powerful figures already contained in the marble. Michelangelo’s task was only to chip away the excess, to reveal. He worked often for days on end without sleep, keeping for days his boots and clothes, as reported in Vasari’s chronicles about Michelangelo’s passion and talent. One can clearly recognize the grooves from mallet and pointed chisel on the marble surface used in this initial stage. Unlike most sculptors, who prepared a plaster cast model and then marked up their block of marble to know where to chip, Michelangelo mostly worked free hand, starting from the front and working back. These figures emerged from the marble “as though surfacing from a pool of water”, as described in Vasari’s “Lives of the Artists”. The method was to take a figure of wax, lay it in a vessel of water and gradually emerge it, noticing the most prominent parts. Just so, the highest parts were extracted first from the marble.


https://commons.wikimedia.org/wiki/Category:Atlas_by_Michelangelo#/media/File:'Atlas_Slave'_by_Michelangelo_-_JBU_02.jpg

https://commons.wikimedia.org/wiki/Category:Atlas_by_Michelangelo#/media/File:'Atlas_Slave'_by_Michelangelo_-_JBU_01.jpg
By Jörg Bittner Unna - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=39154773

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https://medium.com/@nilsaparker/the-angel-in-the-marble-f7aa43f333dc

Every block of stone has a statue inside it and it is the task of the sculptor to discover it.

I saw the angel in the marble and carved until I set him free.

For Michelangelo, the idea was already there, inside the hunk of stone, whether by divine providence or his own imagination. His eyes and hands were merely the vessels by which that idea—the art—was brought forth into the physical world as he or God (or both) originally intended.

...

By sculpture I mean that which is fashioned by the effort of cutting away, that which is fashioned by the method of building up being like unto painting.

It was this statement, in a 1547 letter to Varchi, that stopped me in my tracks the other day. In one sentence, Michelangelo not only summed up the critical distinction between sculpture and painting, but he pinpointed the essence of the editing process in the context of the written word. Sculpture, like editing, is about chiseling away at the unnecessary, at the external, in pursuit of the truth and beauty within. Painting, like writing, is the process of adding layer upon layer to a flat surface until there is something where before there was nothing. 

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https://www.artyfactory.com/art_appreciation/visual-elements/form.html

At the start of the 20th century African masks were collected by artists like Picasso, Matisse, Derain and Modigliani who were inspired by their bold visual impact and unusual sculptural form. Picasso, in particular, saw the potential of incorporating their striking imagery into his own work in an attempt to revitalise the tired tradition of figurative art. Although these artists had no great appreciation of the cultural values of African masks, they sensed that the creative approach of the tribal artist opened the door to a wider exploration of the relationship between the form, medium and content of an artwork. The result was a cross-cultural fertilization of the 'classical' with the 'primitive' which gave birth to Cubism, Fauvism and Expressionism as well as influencing some of the more mystical elements of Surrealism.

The form and content of an African mask were alien to the conventions of academic realism and as such, they offered a new freedom of expression to the contemporary artist. For example, mask imagery was often multi-layered, combining human and animal features in one design to symbolize an ecological balance. Masks were also a vigorous statement about experience, more than they were a response to what the artist had observed. Modern artists recognized the creative potential in this conceptual approach and were eager to explore it in their own work.

Tribal masks were mostly used in ceremonial dances as a channel of communication between the natural and supernatural worlds. Wood, the most common material used for making masks, was chosen not only for its abundance in the forest but also for its quality as a spiritual medium. Some tribal artists would take time to pay their respects to the life-force of a tree, requesting its permission to be cut down and used to make a carving. They felt that the tree had its own soul and that its wood was the natural host for the spirit of their work. This kind of holistic approach to carving invested the form of a mask with a certain integrity that reflected contemporary ideas about 'truth to materials' where the sculptor respects the natural properties of wood or stone and allows them to show through in the finished work.

The adoption of these so-called 'primitive' conventions by modern artists actually widened the parameters of artistic creativity and encouraged a more experimental attitude to the development of the form, medium and content of an artwork.