Throughout history, cities have been built with rock. Egypt's pyramids and Sphinx are made of limestone. India's Taj Mahal is built from marble. New York's Empire State Building is faced with granite and limestone. What makes a good building stone? It should be attractive and easy to work with—but not crack, crumble, or weather too easily.
Our Museum's Rocks
The Rock That Built Washington
This 100- to 125-million-year-old sandstone was quarried to build the White House and Capitol building beginning about 1790. It is an attractive building stone, although not a particularly durable one. In fact, some blocks cracked as they were hoisted into place and had to be replaced on the spot. But the rock was easy to carve. Most importantly, it could be readily transported to the new federal city by a short, navigable water route.
A City Made of Rock
Like all cities, Washington DC is made of rocks and rock products. Marble, sandstone, and granite can be cut, shaped, and polished to cover buildings. Concrete, crushed stone, and asphalt are all building materials humans produce from minerals, rocks, or petroleum. Construction materials such as brick, tile, plaster, wallboard, and steel are also made with rock-derived ingredients. Even the glass in windows is made with finely ground and melted sand.
Sandstone. Beach sand may one day harden into rock like this. The Smithsonian Castle is made of a red sandstone from Poolesville, Maryland.
Granite. This familiar rock makes objects that take great wear and tear. That's why many road and railroad beds, curbstones, cobblestones, monuments, statues, and tombstones are made of granite.
Marble. This marble was once limestone. It was transformed by heat and pressure within the Earth.
Concrete. The ancient Egyptians, Greeks, and Romans all used concrete, a synthetic rock. Heating limestone, clay, and silica to 1,425° C (2,600° F) makes cement, the glue in concrete. Combining cement, aggregate (crushed stone, or sand and gravel), and water forms concrete.
Asphalt. Asphalt is a tarlike hydrocarbon mixture that is solid or semisolid at room temperature. It is heated and combined with aggregate (crushed stone, or sand and gravel) to form the familiar material that paves most roads. Most asphalt is made by evaporating crude oil.
Gravel and Crushed Stone. Crushed stone (mainly limestone and granite), sand, and gravel are so essential to urban life that billions of dollars worth are produced each year. They're used in concrete, roads, and virtually all construction sites. Sand and gravel cover the paths of the National Mall.
The White House Gets a Facelift
This baluster and lintel were removed from the White House during the first major restoration of its exterior in 1980-97. George Washington directed that the White House be made of stone, as were all important European buildings at that time. Pierre L'Enfant, architect of the new federal city, contracted with the owners of Virginia's Aquia Creek quarry for their sandstone. But it was not durable. Can you spot the concrete patch on the lintel? It was one of hundreds of cosmetic repairs made before the White House restoration. Until then, iron sutures and braces joined cracked stones, sheet metal capped eroded carvings, and paint camouflaged everything. For about 190 years, cosmetic repairs such as these kept up the exterior appearance of the White House. Finally, however, the problems were too great. After scraping off 28 layers of white paint, modern stone carvers restored the exterior, using recycled sandstone from the Capitol's east front.
A Capitol Built of Stone
Today, the U.S. Capitol building includes different kinds of building stones from many places. But the original construction was with sandstone, transported by water from Virginia's Aquia Creek quarry. This weathered baluster once adorned the Capitol's East Front. Even layers of white paint couldn't keep parts of its surface from weathering away in layers, like those of an onion. These two carved brackets were nearly identical when installed in the 1790s. Over the next 160 years, many layers of paint were applied to protect the easily weathered sandstone. The one with more paint retains more of the original detail.
Our Museum's Rocks
Slate shingles cover the central dome and roof of the National Museum of Natural History. Slate makes good shingles because it splits easily into thin, durable sheets of solid rock. Why? It contains platelike mica minerals arranged in narrowly spaced layers. Several varieties of granite—a durable and beautiful igneous rock-face and trim the Museum. A bright green-and-orange granite, unakite, accents the Mall-side stairs. The green is the mineral epidote, and the orange comes from hematite inclusions in orthoclase.
