Marble Capital of the Ancient and Modern World

Few places in the world are as rich in stone history as Carrara in northern Tuscany, Italy.  Located where the Apuan Alps meet the Ligurian Sea, Carrara combines enormous marble deposits with nearby access to shipping on the Mediterranean.  This has made Carrara both a major center for marble quarrying and carving as well as a fabrication center for many other stones from throughout the world.
	Marble is limestone that has been heated, compressed and recrystallized into a denser form.  In Carrara, Jurassic-age limestones were compressed and deeply buried during collisions of the African and European plates.  The resultant marbles were later exposed by uplift and erosion of the northern Apuan Alps outside Carrara.
The Romans began quarrying marble near Carrara in the 2nd century BC.  Marble blocks were carted to the nearby port of Luni for shipment throughout the Mediterranean.  In Rome, Carrara marble was used in the Pantheon, Trajan’s Column, and many other structures and sculptures.
	Marble production declined after the fall of Rome but picked up again during the Renaissaince.  In the 15th century, Michaelangelo hand-selected blocks of the purest statuario marble at Carrara and had them carted to the coast and then barged up the Arno River to Florence where he carved his David.
Quarrymen (cavatori) and stone carvers (scarpellini) in Carrara have a long history of supporting  radical labor organizations.  Violent revolutionists, expelled from other European countries in the late 1800′s, migrated to Carrara and founded anarchist groups. Carrara is the birthplace of the International Federation of Anarchists.

Porphyry is an igneous rock similar in composition to granite.  Porphyry typically has large, light-colored crystals of quartz or feldspar floating in a reddish fine-grained matrix of smaller crystals.  The word “porphyry” comes from the Latin word for purple, which was the color of nobility to the Romans.  Porphyry was Imperial Rome’s most prestigious stone for columns, vases, alters, busts and other objects.  Imperial porphyry had only one source, in one of the most inhospitable places on earth.

Ancient Roman Quarry

The original Roman source of porphyry was Mons Porphyrites at Jebel Abu Dukhan in the eastern desert of Egypt, shown at right.  Here porphyry was quarried under appallingly difficult conditions, apparently by slave labor (probably Christians).  Once columns or other blocks were roughed out, they were ox-carted 150 km along the Via Porphyrites to the Nile at Qena.  From there porphyry was shipped downstream by barge and eventually by ship throughout the Roman empire.  Abandoned by the Romans in the 5th century, Mons Porphyrites is still very remote and difficult to access.

Stone from Mons Porphyrites can be found as panels in the Pantheon in Rome, as pillars in Baalbek’s Temple of Heliopolis in Lebanon, and in countless altars, basins, and sarcophagi throughout the ancient Roman empire

Modern Porphyry Quarries

Porphyry is now quarried in many countries including Italy, Argentina and Mexico.  It is prized for its great compressive strength and exceptional durability and for this reason it is most commonly used now as a paving stone.  Porphyry has been used as street paving in Europe for centuries.

Porphyry Paving Stones

Porphyry is produced as paving stones of various sizes and thicknesses depending on the intended use.  Paving stones typically have broken edges as shown here. Follow this link to see more modern porphyry products.

Not all stones are created equally. A stone’s resistance to weathering and staining depends on the minerals that compose the stone, the cement that binds the stone together, and the amount of open pore space (porosity) between the grains in the stone.

Crystalline rocks like granites contain resistant minerals with tight interlocking grains and very low porosity. Consequently, granites are some of the most durable building stones. Sedimentary rocks like sandstone and limestone vary greatly and are more difficult to evaluate. Some are very resistant to weathering while others will deteriorate much more quickly.

Stone durability can be evaluated in several ways. A close inspection with a rock hammer can give you a pretty good sense of hardness. There are also a variety of laboratory tests that can help. In addition, there are many stone treatments that enhance a stone’s durability and stain resistance.

Freeze-Thaw Damage

Repeated freeze-thaw cycles can damage some porous sandstones and limestones. Water that penetrates the pore spaces of a stone expands when it freezes and can cause spalling.  The best way to protect against freeze-thaw damage is to use a stone with a low porosity; however, this isn’t always possible. Fortunately, there are stone treatments that repel water and enhance the durability of stone. Products like ProSoCo H40 and H100 are water repellant treatments that strengthen stone by depositing silica into the pore spaces.

Salt Damage

Salt damage can occur in porous stones that are exposed to salty water from surface splash, salty coastal air, or from soil moisture. When salty water penetrates a stone, salts crystallize in the pore spaces and can cause spalling and discoloration. Salts can enter through the face of the stone and mortar, by getting behind poorly mortared stone, or by wicking upwards from shallow soil moisture. Stone treatments such as Prosoco Salt Guard repel water and salt; however, they can only be applied to the face of a finished stone wall or the surface of a floor. They are breathable treatments and can not protect stone from water that gets behind or wicks up from beneath the stone.

Oil and Stain Repellants

Oil and food stains can penetrate some stones (particularly sandstones and limestones) and can be very difficult to remove. Oil and stain repellants, like ProSoCo SLX100 and Stone Masonry and Tile Protector (SMTP), penetrate the pore spaces and help stone resist staining. The image at right shows the difference in stain penetration between treated samples and an untreated control.  Water repellants also reduce water penetration and therefore reduce the potential for freeze-thaw damage.

Color Enhancers

Sometimes the natural color of a stone isn’t exactly what the client, architect, or designer desires. In many cases, color enhancers can be used to deepen the color of stone as well as provide water and stain repellancy. Different enhancers provide varying amounts of color change.  Shown at left are the color changes produced by  ProSoCo Gloss ‘N Guard, Gloss ‘N Guard WB, and Paver Enhancer.

Stone Cleaners

In most cases stone should be thoroughly cleaned after installation. The most common cleaner used on masonry stone is muriatic acid diluted with water. Muriatic acid removes mortar stains and other calcium deposits as shown in the image at right. However, muriatic acid is generally not appropriate for limestones and marbles because the acid can etch or discolor the stone. Other cleaners, such as ProSoCo’s Safety Klean, are safer to use on limestones and marbles. It is always best to test muriatic acid, or any other cleaner, on a stone sample before use.

Hod carriers (hoddies) are the lowest laborers on a masonry crew.  They deliver brick and/or stone to the masons.  They do it differently in Bangladesh than in the U.S.

Petra, an 11th-century ruin in Jordan carved into a sandstone wall

Sandstones are one of the primary types of stone used as masonry stone.  They occur in many settings on all continents and have been used as a building stone since the earliest days of recorded human history.

Geology of Sandstones
Sandstones are generated by the erosional breakdown of pre-existing rock.  The pre-existing rock may have been igneous, metamorphic or sedimentary, depending on the location.  Sandstones are most commonly created when mountains are uplifted, which leads to the erosion and subsequent deposition of sandstones as well as shales and siltstones.  Most sandstones are deposited as marine sediments (in the ocean); however, they are also deposited in rivers and lakes as well as by wind (sand dunes).

Sandstones used in the building stone industry are composed mostly of quartz grains with lesser amounts of other minerals.  Quartz is more resistant to weathering than most other minerals and it tends to survive and be concentrated by the erosion and transportation processes that deposit sandstone.   The grain sizes of sandstones range from barely visible by the naked eye to large pebbles.  Geologists break sandstones down into many categories (arkose, greywacke, conglomerate, etc.); however, in the stone industry they all fall under the category of sandstone.

All sandstones were initially deposited as loose sand in the water or on the surface of the earth.  As the sand is buried by later sedimentary deposits, it undergoes compression and cementation.  Compression, due to the weight of the overlying sediments, pushes the sand grains closer together, reducing the amount of open pore space.  Cementation occurs as groundwater circulates through the buried sand deposit and precipitates out minerals in the pore spaces between the sand grains.  Most sandstones have been buried thousands of feet below the surface by younger sediments, only later to be exposed at the surface again by erosion of the overlying rocks.  In general, the deeper a sandstone has been buried, the more it has been compressed and cemented.  The more well-cemented a sandstone is, the more desirable it is as a building stone.

Sandstone Colors
Sandstones range in color from nearly white to almost black, although most are in the gray/tan/brown range.  The color is determined by the types of mineral grains that comprise the sandstone and the type of cement that binds the sand grains together.  Light colored sandstones are usually predominantly composed of quartz grains and are cemented with either silica or calcium carbonate.  Darker sandstones usually contain a wider variety of mineral grains. Red sandstones, which are common in the southwestern U.S., get their red color from iron oxide cement.

Berea Sandstone
Sandstones have been quarried commercially throughout the United States at many locations.  The well-known Berea Sandstone quarries in Ohio (shown at right) have been a major source of building stone in the eastern U.S. since the mid-1800′s and some of these quarries are still in operation.  The Berea is somewhat unique because it has very consistent grain size and can be worked in all directions due to its relative lack of bedding planes.  In the quarry business this is called a freestone.

Brownstone
Another sandstone that has been widely used in the U.S. is Brownstone, which was quarried extensively in the 18th and 19th centuries around Portland, Connecticut.  These quarries provided stone for countless buildings in New York city, many of which are now called “brownstones.”  Brownstone quarrying was a huge industry, particularly in the mid- and late-19th century, but dwindled in the early 20th century.  In the 1930′s a hurricane swept up the east coast of the U.S. and flooded the quarries, most of which never reopened.  One brownstone quarry (Portland Brownstone Quarries) started operations again and now provides brownstone mostly for restoration work.

Flagstones
Flagstones are flat stones that split into thin layers and are commonly used for paving.  Many of the flagstones quarried around the world are sandstones.  Since sandstones are deposited in layers, they can sometimes be split down into sheets ranging from 1″-3″ thick.  At flagstone quarries, thick slabs of sandstone are extracted and then split down with chisels to the desired thickness.  In the U.S., flagstones are produced in Pennsylvania, Tennessee, Oklahoma, Colorado, Arizona, Montana as well as other states.  The flagstone shown at right is Frontier Sandstone, quarried in Montana.

Sandstone Physical Properties
The physical properties of sandstones vary more than any other common building stone and this affects their resistance to weathering.  The key factors controlling the weather resistance of a sandstone are the amount of open pore space and the type of cement that binds the sandstone together.  The density of the commonly quarried sandstones (130-165 lbs/cf) is less than most igneous and metamorphic rocks because of the open pore space between sand grains.  The open pore space in a stone is measured by an ASTM method called absorption (click ASTM for an explanation of stone testing).  Absorption values for sandstones are preferably less than 6%, and the lower the better, with the best sandstones having absorption values around 1%.  Water penetration into sandstone can lead to freeze-thaw deterioration, along with other weathering problems.  Many sandstones that were used historically in the U.S. don’t stand up to weathering very well due to water penetration.

Sandstones are some of the most beautiful and interesting building stones; however, they were not all created equally.  A little testing and little common sense will go a long way in choosing an appropriate sandstone for a project.

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Half Dome, a granite feature in Yosemite National Park

Granite has been quarried for millennia on every continent except Antarctica.  It is processed into everything from wall stones and cobblestones to countertops and gravestones.  The streets in many cities in the eastern U.S. were originally paved with granite cobblestones (commonly now covered with asphalt).  These cobblestones came from local quarries as well from overseas quarries as ballast on sailing ships.  Granite is probably the most widely quarried type of stone worldwide.

To a geologist, the term granite refers to an intrusive igneous rock (one that crystallized below the earth’s surface) that contains a very specific suite of minerals – primarily quartz, feldspar, amphibole and biotite. These minerals give true granites their characteristic pinkish color as shown on the right.

In the stone industry, we dispense with this narrow definition and use the term granite for almost all igneous rocks, with the exception of volcanic ashes, and nearly all metamorphic rocks, with the exception of marbles and quartzites.  This is a useful convention because most crystalline igneous and metamorphic rocks have similar physical properties – tight interlocking crystals, very low porosity, and high density (relative to most sedimentary rocks).  Because of these physical characteristics, granites are generally the most durable of building stones.

Quarried granites range in color from very light gray to black and almost every color in between.  The color is determined by the minerals that comprise the granite.  Light colored minerals like quartz and feldspar create a light-colored granite. Dark iron- and magnesium-rich minerals like pyroxene and amphibole make for dark gray and black granites.  Since the dark-colored minerals are denser than the light-colored minerals, dark granites are denser that the light-colored granites.  While the density of a light gray granite may be as low as 165 pounds per cubic foot, the density of the darkest granites can be over 200 pounds per cubic foot.

In the 19th and early 20th centuries, granite was quarried commercially throughout New England and the Mid-Atlantic states as well as in many other parts of the United States.  Since shipping was exorbitantly expensive in the early days, each quarry serviced a relatively small area.  Nowadays, with modern transportation, most of the smaller granite quarries have closed, with only a few active granite quarries remaining in Vermont, New York, Massachusetts, South Carolina, Georgia, Minnesota and South Dakota.  These remaining quarries face brutal competition from quarriers in countries like China and India where labor costs are much lower.

Granites are usually quarried in large rectangular blocks.  In the quarry, the blocks are delineated by drilling closely-spaced holes.  Either wedges or expanding materials are put into the holes to break the block loose. This process has certainly been improved with modern technology, but in essence hasn’t changed for hundreds of years.  The blocks are then sawn into slabs or otherwise broken down into sizes and shapes useful as building or paving stones.  The majority of granite today is sold as sawn slabs for panels and counter tops and as custom pieces for memorials (gravestones); however, there are still a few granite quarries that produce building stones as their primary products.

Granites usually have the highest compressive strengths and lowest water absorptions of any of the common building stones (click ASTM for an explanation of stone testing).  They are typically the most resistant stones to staining and deterioration by acid rain and, due to their hardness, are also difficult to vandalize.  Granite colors rarely change from exposure to the elements.  If you are looking for the most resistant building material ever produced, granite is the answer.

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The ancient stonemasons that built Machu Picchu and other Inca structures lacked a written language, iron and steel tools and the wheel. This didn’t stop them from quarrying and transporting enormous blocks of granite and building structures in places and in ways that are difficult to imagine. The attached article by Kenneth Wright describes years of research into how the Inca accomplished these phenomenal tasks.

The Stonemasons of Machu Picchu by Kenneth Wright
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