Travis Spencer, age 13, of Erie, Pa., for his question:
WHY IS MARBLE SO SMOOTH?
Marble is any limestone that is hard enough to take polish. It is so smooth when cut into slabs or blocks because it has been made hard and strong by heat and pressure beneath the surface of the earth for millions of years.
Marble, in the geological sense, is limestone that has been metamorphosed or changed. Ordinary limestone is made up of fragments of shells or irregular grains of calcium carbonate. But in marble it has been changed by pressure and heat to a mass of crystals grown firmly together. Metamorphism has made marble more uniform in hardness and grain.
The finest marble is white and is called statuary marble.
All marble is composed of crystals of the minerals calcite or dolomite, which when pure are perfectly white. Colored marble results from the presence of other minerals or small amounts of staining matter mixed with the calcite or dolomite.
Black, gray, pink, reddish, greenish and many kinds of mottled and banded marbles are used in the designs of buildings. The color of red marble is due to tiny particles of hematite between calcite or dolomite crystals. Serpentine marbles are principally green and yellowish silicates. Flossiliferous marbles are limestones which are full of fossil shells.
In marble quarries a machine called a channeler cuts a series of channels or slots in the face of the rock. These slots are often eight to 12 feet deep and 60 to 80 feet long. Although marble is never destroyed except by burning at extremely high temperature, poor quarry methods may shatter or damage it. Because of this, explosives cannot be used. The blocks or slabs are lifted out carefully with large derricks.
A great toothless saw is used to cut the rough stone, while a stream of water containing sand runs over it. The friction of the steel blade and the sand cuts the marble into the desired sizes. Sometimes a wire saw is used instead of a solid blade.
After cutting, pieces of marble are then placed on a circular rubbing bed and held stationary. Sand and water flow over the rotating bed surface, rubbing away the marble to an even level.
Next, the marble is smoothed further by grinding with carborundum and finer abrasives under a stream of water.
Finally, the last step of fine polishing is done by a mixture of tin oxide and oxalic acid applied to the marble surface by means of a buffer wheel. It can then be highly polished.
Marble is most often used for buildings, monuments, interior decorations (such as fireplaces, floors or stairs), statues and table tops.
In the United States, marble has been used since colonial days. The largest American marble quarry is in Vermont. This state has often led in marble production. Other important marble producing states are Alabama, Georgia, Virginia and Wyoming.