Marble sculpture

Marble has been the preferred material for stone monumental sculpture since ancient times, with several advantages over its more common geological "parent" limestone, in particular the ability to absorb light a small distance into the surface before refracting it in subsurface scattering. This gives an attractive soft appearance which is especially good for representing human skin, and which can also be polished.

Of the many different types of marble the pure white ones are generally used for sculpture, with coloured ones preferred for many architectural and decorative uses. The degree of hardness is right to carve without too much difficulty, but still give a very durable result, if not exposed to acid rain or seawater.

Famous individual types and quarries include from classical times Parian marble from Paros, used for the Venus de Milo and many other Ancient Greek sculptures, and Pentelic marble, from near Athens, used for most of the Parthenon sculptures, and by the Romans. Carrara marble from northern Italy was used by the Romans, and very extensively up to recent decades, when the pure white statuario grade more or less ran out. This was used by Michelangelo and other Renaissance sculptors, and later exported, including to America.

Marble is a metamorphic rock derived from limestone, composed mostly of calcite (a crystalline form of calcium carbonate, CaCO₃). The original source of the parent limestone is the seabed deposition of calcium carbonate in the form of microscopic animal skeletons or similar materials. Marble is formed when the limestone is transformed by heat and pressure after being overlain by other materials. The finest marbles for sculpture have no or few stains, though natural stains can be incorporated into the work itself.

Among the commonly available stones, only marble has a slight translucency i.e. subsurface scattering that is comparable to that of human skin. It is this translucency that gives a marble sculpture a visual depth beyond its surface and this evokes a certain realism when used for figurative works. Some types of marble also have the advantage that, when first quarried, it is relatively soft and easy to work, refine, and polish.^{[citation needed]} As the finished marble ages, it becomes harder and more durable. Preference to the cheaper and less translucent limestone is based largely on the fineness of marble's grain, which enables the sculptor to render minute detail in a manner not always possible with limestone. In contrast to limestone, marble is also extremely weather-resistant. As a result, surface changes due to the immediate environment are not always visible to the naked eye. This feature can pose challenges when dating ancient works.

Marble sculptors must be careful when handling their materials, as the stone can absorb skin oils and develop yellow or brown stains. While more resistant than limestone it is subject to attack by weak acids, and so performs poorly in outdoor environments subject to acid rain. For severe environments, granite is a more lasting material but one which is far more difficult to work and much less suitable for refined works. Compared to metals such as bronze, furthermore, marble is inflexible and vulnerable to fracturing. This drawback means that sculptors must incorporate specific supporting features into the sculpture to prevent collapse. In Thomas Ridgeway Gould's The West Wind, for example, he poised the figure's short and slender ankles delicately upon the balls of her small feet. This would not be possible without the deliberate decision by Gould to distribute almost all of the marble's weight to her massive, flowing skirt. Unlike bronze sculpture, this statue is not hollow; her drapery is one solid block of marble.

Sculptors usually begin by knocking off, or "pitching," large portions of unwanted stone. A suitable tool for this task is a point chisel, which is a long, hefty piece of steel with a point at one end and a broad striking surface at the other. A pitching tool may also be used at this early stage, which is a wedge-shaped chisel with a broad, flat edge. The pitching tool is useful for splitting the stone and removing large, unwanted chunks.

The sculptor may also use a mallet, which is similar to a hammer with a broad, barrel-shaped head. When the mallet connects to the tool, energy is transferred through the tool, shattering the stone. Most sculptors work rhythmically, turning the tool with each blow so that the stone is removed quickly and evenly. This is the "roughing out" stage of the sculpting process. While a mallet provides the force needed to fracture the marble, it must be used accurately. The smallest miscalculation can ruin the intended sculpture and even injure the sculptor.