THE MAGIC OF MARBLE

Sculptors and architects have an abundance of natural stone materials to choose from alabaster, travertine, granite, limestone and soapstone to name a few. But one medium has always stood out - marble. For the last 2,500 years, marble has been chosen for many of the greatest works of sculpture and architecture, from the Parthenon of Classical Greece to Michelangelo's Pietà and Washington D.C.'s Lincoln Memorial.

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Durable yet soft enough to be workable, marble occurs in both solid and patterned colors from snow-white to a rainbow of soft pastels. With its fine grain, marble takes a gleaming polish and can be worked in great detail.

Because marble occurs in massive formations, it can be quarried as blocks suitable for the largest sculptures and most ambitious architectural applications. To many sculptors, marble's most appealing quality is its slight translucency which imparts a subtle glow to the polished stone.

OUT OF LIMESTONE

For all its beauty, marble originates as drab limestone, a common sedimentary rock that forms through the accumulation of shells, coral and other organic materials. Limestone consists primarily of the calcareous carbonate minerals calcite (calcium carbonate) and/or dolomite (calcium magnesium carbonate).

Most limestone is dark-gray in color; higher grades containing more than 70 percent carbonates have lighter-gray colors. With its dull luster and poor polishing qualities, limestone is not a particularly attractive rock.

But because of its abundance and low cost, it is widely used for exterior building blocks. Its biggest use, however, is as the raw material for manufacturing portland cement.

When subjected to metamorphic heat and pressure, high-grade limestone undergoes a dramatic change. It first takes on a plastic consistency as its fine-grained, crystalline structure is destroyed and many of its impurities driven off. Then, with reduced heat and pressure, this plastic mass recrystallizes as marble with a substantially higher carbonate content and a structure of interlocked grains of translucent calcite and/ or dolomite.