Stone

Structural stone is natural rock used as walls, floors or even the roof of a building. On the University of Toledo campus, examples of structural stone include slate tiles on the roof of University Hall, limestone walls of U. Hall, the Fieldhouse and many other academic and residential halls, limestone of the Glass Bowl stadium, and marble slabs in many restrooms of U. Hall. The Pyramids of Giza are made of limestone. The Parthenon of Athens is marble. The walls and many buildings of Jerusalem are stone, as are Mayan and Aztec cities and pyramids, the city of Manchu Pichu, and stone dwellings of Ciudad Verde in what is now Colorado.

Dimension stone forms an integral part of the building, contributing to its support. Today, structural stone is frequently applied for decorative purposes only, as an exterior veneer, protecting the building from weather, serving the same function as vinyl or alumnum siding, only more durable and attractive.

Structural stone is durable and usually attractive. Although stone is very strong when resisting compression, stone is weak under tension and can be made to split if wedges can be inserted into a block or outcrop. The fact that rock breaks under tension limits the spacing of vertical supports in beam and column buildings such as Greek temples. A beam sags under its own weight, stretching the lower surface. If the beam sags too much and stretches that lower surface beyond its limit, tension cracks form. These cracks concentrate stress at their tips and propagate upward, dividing the beam into two pieces. Romans made widestread use of the arch and dome, engineering designs not found in the pre-Columbian Americas, to avoid this limitation in stone beams.

Iranian villagers use stones already broken from outcrop and tumbled down rivers as building stones. These rounded rocks do not fit together tightly. Some are split to provide flat matching edges, but there are always a lot of gaps and openings to be filled in by mortar or mud. Most structural stone, however, is extracted from quarries.

The most common kinds of rock quarried for structural stone are limestone, marble, slate, and intrusive igneous rock such as granite. Limestone and marble are composed of the mineral calcite, a relatively soft mineral. Iron tools are harder than calcite, so limestone and marble are easily cut by chisels. Granite is composed of harder minerals, mostly feldspar and quartz. However, some plutons are characterized by joints, natural fractures that are somewhat regularly spaced and in somewhat parallel sets. Joints are a result of stress or stress release. Rocks are more easily split along joints or along planes parallel to joints than along other directions. Quarry operators take advantage of this characteristic and split off rocks along jointing directions, even rocks hard enough to rapidly dull iron chisels. Slate is both soft and easily split (the diagnostic characteristic of slate is ‘slaty cleavage’, the tendency of a rock without layering or visible crystals to split along parallel planes) but is not very strong. However, a slate roof does not break down like asphalt or wood shingles and can last for decades - at least until someone walks across the slate shingles and damages them. Limestone and marble might also contain joints, and limestone often exhibits bedding planes that also provide natural weaknesses quarry operators can exploit. Other kinds of rock are used locally. Hardened volcanic ash (tuff), light in weight and soft enough to cut, is used in Honduras, and Manchu Pichu is made of andesite, a hard, dark volcanic rock found in such great abundance in the Andes Mountains that the rock was named after the mountain chain.

The cost of building from stone includes (a) cost of quarrying, (b) cost of processing (shaping and polishing), (c) cost of transportation, and (d) cost of masons and labor to set stones into place. Highly desirable and popular stone may command a premium, but costs must cover at least those expenses listed.

Egyptian pharaohs and Roman emperors transported stone great distance - across mountains and deserts, up the Nile River and across the Mediterranean - for buildings and statues. Several years ago, members of Congress were upset to learn that Italian marble was selected instead of Vermont marble for rebuilding a floor worn out after two centuries of foot traffic. Labor costs in Italy more than made up for the difference in transportation costs. Sidewalks and curbs in downtown Salt Lake City are being constructed from blocks and slabs of diorite, a granite-like rock with a chemical composition similar to andesite (the quarry is just a few miles from the new sidewalks). Around Dubuque, Iowa, many foundations and retaining walls are made from enormous blocks of dolomite, a rock similar to limestone. The basement walls of the house (built about 1900) in which I grew up are made of dolomite blocks.

Igneous rocks like granite are the most durable and among the most attractive when polished (index of polished slab images). Crystalline igneous rocks are also the most difficult to cut and polish. They are also slightly denser than limestone and thus most difficult and expensive to move. Marble polishes nicely and is easily cut (for a rock). Marble is composed of calcium or magnesium carbonate, minerals that react with acids. Marble exposed to a damp acidic environment in a modern temperate zone city quickly (within years) shows signs of deterioration. Even completely natural rainfall is acidic in nature. Marble tombstones in Civil War cemetaries are often so worn my weathering that names and dates are no longer readable. Limestone and dolomite are also calcium and magnesium carbonate respectively, reactive with acidic precipitation and fog. Limestone, however, is seldom polished so the first signs of weathering are not apparent.