The subtitle of this book, "processes and principles," really says it all. The twelve chapters here are the twelve lectures Wittkower gave as Slade Professor of Fine Arts at Cambridge in 1970-71. His largest emphasis isn't on the merits of the finished works, but on how a particular sculptor worked and what rules or notions he relied on. He shows some of the tools that would be used (point or punch, flat chisel, claw chisel, hammer, rasps, drill or augur) from the Greeks onward. He spends most of his time on Michelangelo and Bernini. The Greeks only get perfunctory treatment. The anonymous medieval cathedral sculptors get substantial coverage.
Michelangelo said that sculpting entailed subtraction, and modelling was more like painting in that it entailed addition. What he meant was that the sculptor would create a model (or multiple models), in addition to drawings and sketches, of what he intended for the final work. Michelangelo's models were made by hand of clay (which became terracotta) or wax. So in making the model, you would be taking clay and shaping it with your hands and adding on bits of clay wherever necessary. Then a frame with points, and calipers to make sure you got all the distances right, would be used to "transfer" the design to marble. Michelangelo would draw his design/figure onto the marble and begin chipping away - subtracting from the block of marble. Vasari wrote that Michelangelo's method was as if the statue was lying supine under water, and gradually the water drained away so that the surface form of the statue was exposed bit by bit. In this way Michelangelo was "releasing" the human form trapped in the marble block. His preferred tool was the claw chisel; in many places on his works, the marks of the claw chisel can be seen. Where he didn't want the marks to remain, he would use abrasives to polish the surface.
Bronzes were done in a completely different way - from casts of the model. Liberated from the limits and difficulties of the block of stone, works in bronze could be "freer," says Wittkower.
Michelangelo created most of his works himself. For Bernini, working more than 100 years later, that would have been impossible. Bernini's commissions were enormous (e.g. busts of the 38 martyr popes for the chapels and nave of St. Peter's) and necessitated a large workshop. At one point he had all of the 39 highly skilled sculptors in Rome working for him. So while he designed the works, creating the models, the execution of them was left to others. Bernini was also willing to use more than one block of marble for a statue, which would have been unthinkable to Michelangelo and his Renaissance colleagues.
Some artists and theorists felt that sculpting was only stonecarving, not model-making. By that definition, Rodin didn't do much sculpting. He created hundreds of models which were then executed in bronze. In the early 20th century there was a new crop of sculptors, including Constantin Brancusi and Henry Moore, who were only interested in carving/sculpting, not in modelling.