Etched in Stone
Experts provide guidelines to specifying natural stone.
By Gordon H. Bock
An average architect might spend an entire career on conventional buildings without ever having to deal with natural stone, but not so anyone working in historic or traditional architecture. For centuries stone reigned as the fundamental building material for structural as well as decorative purposes, and it is still in demand for historic repair work and new construction on the best public, institutional and ecclesiastical structures. The trouble is that a lot of the once-common working knowledge of what natural stone is and how it should be used – though still everyday practice at quarries and fabricators – has been lost in the information overload on manmade building materials. To help fill that gap, consider the following insights from stone-source experts before diving into your next stone project.
The source of Connecticut brownstone through much of the 19th century, the recently reopened Portland Brownstone Quarries now use 21st-century equipment to extract the chocolatey rock for the restoration market. Photo: Jon Crispin|
The Match Game
The biggest challenge architects face in a stone restoration or addition project is trying to match existing work. "There are many more technologies than in the past for cutting, splitting and finishing stone efficiently," says Ken Jackman of New World Stoneworks in Massachusetts, "but the real tough part is matching color." Even if the quarry is still open, there's no guarantee that the stone produced today will match what came out 120 years ago. As Jackman explains, depending upon when and how the quarry was first opened, the owners may have been pulling stone near the top. "Then, over the decades, as the quarry was worked down through the layers of stone, the composition can change," he says. "The elements that tend to give it color can vary, and the stone can be very different than what was taken out previously."
Then there's the influence of weathering. "Stone outdoors is subject to surface erosion, pollution, staining, and so forth," says Laurie Wells, of Old World Stone in Ontario, Canada, "so old stone looks very different from new stone simply because of the surface changes." She adds that sandstones that contain iron can change color slightly due to oxidation. "We always tell people they should match old and new stone when they are wet because, compared to an old weathered stone, new stone has a fresh, new surface with dust particles in the pores." The ideal situation, says Wells, is to wet down a clean break in an old stone and compare that with a wet sample of new stone. The goal is to find new stone that matches as closely as possible the original appearance of the old stone – not its weathered appearance.
Even with a perfect match, weathering still leaves many old and new stones looking dissimilar, so the stone trade has evolved methods to downplay the differences. "If you have the luxury of not putting new stone right next to old stone," says Jackman, "that gives you a little bit of a break." Wells says that a very light sandblasting that erodes the surface slightly can help new stone blend in. "That way you don't have a perfectly smooth-sanded stone against a 100-year old pitted stone, and the roughened surface attracts dirt faster." Jackman adds that some masons will paint on their own mix of rusted nails and vinegar to try to change the color a little or even apply buttermilk to encourage mildew growth. "There are a million tricks," he says.
High-density, low water-absorbency limestone from North Shore Architectural Stone was installed in Greenwich CT. The self-supporting, 3-in. thick veneer stone is tied back to the structure with stainless steel anchors.||Old World Stone provided the cut and carved St. Bee's sandstone for the entry portico of the Cathedral of the Immaculate Conception in Albany, NY.
The Lowdown on Bedding Height
In the case of sandstones especially, the availability of the stone desired is closely tied to the bedding planes (see sidebar). "The usable stone is the vertical height, or the bedding height, between flawed bedding planes," says Wells. "With many stones, the bedding height may be very shallow – only eight or ten inches." As she explains, there are many instances where builders wanted to use stone as ashlar (block-like) facing, with dimensions of 24 or 30 inches, so they would turn the stone up on its side or face. "The problem with that scenario is you get delamination – a peeling off of the layers like the pages of a book. When you're looking for the right stone to put into a building, you want to make sure that the naturally bedded planes are sufficient for the course height or other dimension."
Alas, with stone being a natural material, this is not always as easy as it sounds. As Wells notes, ashlar installations, string courses and balustrades, as well as carved details like pinnacles and finials typically require very tall bedding. In the case of brownstone, this is why she imports stock from Cumbria, England, that has an extraordinary bedding height of up to 48 inches. Even stones that are not sedimentary can have limitations.
According to Hugh Tanchuck at North Shore Architectural Stone in New York, "You generally can cut polished granite (as in countertops) with or without the grain. However, if you are doing hand finishes, such as rock-pitch finish, you have to work with the grain or you will have cracks and spalling." Wells adds that, "A lot of people assume everything is available in any height at any time, but you have to be aware of what's available and be prepared to change your material or your course height. "
Fabricators typically use machine and power tools to create traditional stone finishes, but some, such as the controlled spalling that produces the pitching or rock-face seen here, may still be done by hand. Photo: Jon Crispin|
Richard Rhodes of Rhodes Architectural Stone in Seattle, WA, notes that the bedding plane problem is really due to lack of education and is nothing new. "Guild-trained stone craftsmen have always known about correct grain orientation," he says, "but in boom times, there weren't enough craftsmen to go around."
Just such a boom time was the heyday of the East Coast brownstone row house from the 1850s to the 1890s, when immigrant workers arrived and filled the jobs for building blocks of brick row houses, then facing them with veneers of brownstone in step with the Italianate style. "While the new stone-cutters could successfully imitate traditional work," explains Rhodes, "they really didn't have the underlying knowledge of the material that the guild-trained craftsmen did – and those craftsmen were not about to tell them. The result is work that is ultimately inferior."
Rhodes adds that in Boston, for example, a building by H.H. Richardson, an architect who understood stone grain and worked with experienced people, may be in fine shape, but across the street a building from the same timeframe – and perhaps even the same quarry – is completely falling apart. "It's all about grain orientation."
This Florentine arch (left) was created using antique and new limestone from Rhodes Architectural Stone. A textbook example of broken ashlar stonework (center) from Rhodes may look unsophisticated because there is no intent to have the stone run in horizontal courses more than four feet long. However, it takes considerable time and skill to cut each block for its location, as well as tool them in a hammer- or rock-face, the typical dressings. Rhodes supplied the new pearl veneer, quoins, corbel and cornices for the new Ralph Lauren store in Greenwich, CT (right).
However You Slice It
Lest there be any confusion, when it comes to processing, stone is not like wood. "Stone is sawn to size," says Jackman, "so unlike lumber, there are no standardized dimensions – except, perhaps, cubic feet or meters that are used to size monuments. Aside from a few stock items like stair treads, you're not buying out of a yard."
What's more, today's technology of super-sharp blades and CNC (computer numerical control) machines makes it possible to deliver very precise stone. "With the technology we use in our plant," says Wells, "the tolerances are extremely close – down to 16ths of an inch – so the dimensional stone you get is perfectly straight and flat, especially when set next to, say, precast concrete, which comes out of a mold and tends to be a little bowed here, a little warped there."
Wells notes that the precision of well-sawn stone can actually be an issue when repairing older buildings that are usually off a bit one way or another. For example, setting a new piece of stone into an old coping, where the thickness and straightness varies, requires finessing to make each piece line up with the next one. "Turned balusters were done by hand in the old days," she says. "Each one was slightly different but, up on a building, you get used to seeing that slight irregularity. However, when you replace them with new, machine-made balusters that are all perfect and identical, suddenly you can lose some of that hand-crafted quality."
Modern precision tools can influence how stone is expected to perform too. "We've gotten really adept technically at making very thin stone, to the point there are now stone micro-veneers used in boats and airplanes," says Rhodes, "but most stone needs depth and mass for strength - a minimum of 1½ to 2 in. in thickness, and many times 3½ in. As an example, he cites seeing flooring installations that are only 1/2-in. thick stone on a plywood base, where 1½ in. on a concrete base would be recommended.
Tanchuk agrees. "Stone can be cut 160 to 200 pounds a cubic foot, so it does not take much of a change in dimensions to increase weight by tons. This is why we see thinner and thinner veneers, but thinness is not necessarily good for standing up to freeze-thaw on the East Coast and in the Midwest. We generally like our stone veneers to be 1¾ to 3-in. thick; 2-in. for pavers."
Why the Pain Over Bedding Planes?
Sedimentary stones are basically formed in layers through deposits of mineral or organic particles. Occasionally, a layer of sediment may contain clay or other contaminants that create a weak bedding plane, leaving the stone with a tendency to shear along this plane. If the stone is naturally bedded – that is, used architecturally so the bedding plane is oriented the same way as in nature, as in say patio paving – this presents little potential for problems. However, if the stone is face-bedded – that is, turned so that the bedding planes are at right angles to the way they were oriented in nature – the weak bedding plane risks exposure to the weather and the stone can delaminate – an infamous condition afflicting thousands of brownstone-veneered rowhouses and peeling away since the 1890s.
Timeless Finishes in New Ways
Modern technology also now serves the ancient craft of stone finishing. "Thermally removed surfaces are more common now than in the past," says Jackman, "but for the most part you can match any prior work and more with today's surfacing, machining, profiling, and cutting technologies."
"Typically we still do tooling traditionally, by hand, but instead of mallets we use air tools that speed up the process by reducing fatigue," says Wells. Her company has a repertoire of scores of finishes and, in the case of restoration projects, modern tooling can do patterns once made by hand with surprising depth of detail. Several traditional surface treatments – edge margining and broaching among them – require working the stone with rows of fine, parallel grooves similar to corduroy. Wells adds, "My usual questions to a customer are ‘How fine is the spacing? How many lines to an inch – three, four, or five?'"
Wells says that one treatment unlikely to be replicated by modern tools is a shot-sawn finish. Traditionally, stones were slabbed by slowly cutting with a large motor-driven reciprocating blade – similar to a lumberjack's two-man crosscut saw. To enhance the blade, the saw operators would add shot (metal filings) to the saw kerf (channel). This shot would not only make the blade more abrasive, it would also leave characteristic, irregular indentations on the stone that were considered a finish in their own right, and sometimes seen on monuments.
Measuring Stone Muscle
By many reckonings, stone color may be the buyer's prime concern but strength is equally important. "While the market may be driven by color and price," says Wells, "there's no point in repeating the mistakes of the past by using inferior quality stone." Strength is measured by testing and can confirm that the stone meets the minimum industry standards, as well as that it is the right stone for the application. As Wells points out, the most common ASTM tests are for water absorption, density, abrasion resistance, flexural strength, compressive strength, and modulus of rupture (see sidebar). For example, the compressive strength test – applying an even load to a 2x2-in. cube or core until it crushes – is useful for determining not only if a stone is appropriate for load-bearing applications, but also simply good for use in floors.
"What's happened in the last 20 years is that anything that could get to the market got there," says Rhodes, "and this left the market with unqualified materials that don't classify as building stones. When you put them on buildings, or use them as paving, they exhibit a variety of problems that, I feel, ultimately damage all of us in the stone industry."
According to Rhodes, limestone and sandstone are where the big problems pop up. Limestone, for example, has three classifications: low-density, medium-density, and high-density. "Low-density limestone is not a building material;" he says, "it's like tufa – the easily carved limestone used in the original walls around ancient Rome." Rhodes adds, "If you're buying from people that can't give you the testing data, then they're marketing people, not stone people, and you should seek another vendor."
Wells adds another perspective. "My biggest concern is for durability in our climate, where we experience freeze-thaw cycles." She notes that there have been a lot of materials coming from Asia that look great, but after one winter they're falling apart. "My advice would be to obtain a sample of the material – not a select piece, but a representative sample – and have it tested by a professional to see if it meets North American requirements."
Blocking Out Lead Times
Natural stone that has been selected, cut, and trimmed is called dimension stone and, though it may not be obvious, lead time can be one of the critical dimensions, depending upon where the stone is coming from and at what time of year. "When ordering from Indiana," says Wells, "where there are multiple quarries relatively nearby, you can get stone most of the year. But if you order something exceptionally long or exceptionally wide – on the order of 12 to 14 feet – then you need to have a custom quarry block extracted and that will mean a longer lead time."
Some quarries close for winter months, however, and while fabricators such as Wells' company will plan to stock a few extra blocks in their yards through the winter, if the stone is out-of-the-ordinary, it may have to be ordered way in advance of the fall shut-down.
Quarry schedules aside, with some stones – especially sandstones – seasoning adds another time factor. "The idea of seasoning is to get the quarry sap – the natural ground moisture – to dissipate from the stone," says Wells. "Most sandstones that we have are a year out-of-the-ground old, and that is sufficient to remove most moisture." The concern here is to avoid freeze-thaw that can damage the stone. With this in mind, Wells cautions that stones quarried late in the season need to be protected from hard frosts – say by storing in dry sheds. (She notes that some yards will bank quarried stone with earth for protection if it is not sold by the end of the year.)
Ordering stone from overseas adds yet another layer of planning. "Places like Russia and China can close quarries in winter for up to six months," says Tanchuck. Then, even if you got your order out before closing, the freshly quarried stone might still contains a lot of water, and potentially freeze and split during the winter shipboard voyage. So Wells advises, "If you're going to import brownstone from,
say England, and you want it for next March, you'd better get it here before the end of October."
6293 N. Matthews Dr.
Ellettsville, IN 47429
Kopelov Cut Stone
P.O. Box 1628; 423-D Calle Industrial
Bernalillo, NM 87004
New World Stoneworks
57 Industrial Dr.
Uxbridge, MA 01569
North Carolina Granite,
151 Granite Quarry Trail, P.O. Box 151
Mount Airy, NC 27030
North Shore Architectural Stone
66 Glen Head Rd.
Glen Head, NY 11545
Old World Stone Limited
1151 Heritage Rd.
Burlington, ON, Canada L7L 4Y1
Rhodes Architectural Stone
2011 E. Olive St.
Seattle, WA 98122
2701 Washington Blvd. Ste. C
Bellwood, IL 60104
7953 Angleton Court
Lorton, VA 22079
2 Spectacle Pond Rd.
Littleton, MA 01460
Traditional Cut Stone
1860 Gage Court
Mississauga, ON, Canada L5S 1S1
Gordon H. Bock is a writer, architectural historian, technical consultant, lecturer, and co-author of the book The Vintage House (www.vintagehousebook.com).