“The Stone of Heaven,” is one of the most revered natural substances in the world, such has been the case for centuries, particularly in Asia. What other mineral would make a Chinese emperor offer fifteen cities for a jade carving that he could hold in the palm of his hand or make Montezuma smile when he heard that the Spaniard Cortez was interested only in gold, since Montezuma’s most precious possession was jade.
Since Neolithic times, no other mineral has been so venerated, nor so often intertwined with the dead to accompany them into the afterlife. In this life, to the Chinese, jade embodies the five cardinal virtues of life: charity, modesty, courage, justice, and wisdom. But China and Asia have not had a monopoly on jade; jade art and tools have been found in the Maori, Olmec, Aztec, and Mayan cultures, and among the NW American Coast Indians and Eskimos.
The English word jade has a circuitous derivation. It started with the Spanish expression “piedra de hijada”, meaning the “stone of the loins”, because it was claimed that this stone could cure diseases of the kidneys. This gave rise to the word nephrite, from the Greek word for kidneys: nephros. The French equivalent l’ejade eventually evolved into le jade, and its English translation, jade.
Physical Traits
Jade is actually two minerals: nephrite and jadeite. Nephrite is the ancient stone, a calcium magnesium silicate: Ca2 Mg5 Si8 O22 (OH)2. Jadeite is the upstart of the two, having been discovered only 200 years ago, it is a sodium aluminum silicate: Na A1 Si2 O8. Both are insoluble in acids.
Nephrite is a tough, compact variety of tremolite and actinolite (amphiboles) with a specific gravity of 3.0 to 3.3, and a hardness ranging from 5 to 6 on the Mohs Hardness Scale. It is considered to be the world’s toughest stone. Although it is not as hard as jadeite, it is much harder to break than its cousin. The reason for this is the filament-like crystalline structure of the mineral. When polished, nephrite has a soapy or oily luster. It is commonly referred to as “mutton-fat” jade, because of the marbled appearance that resembles animal fat. Nephrite is normally associated with serpentine, hornblende gneiss, and schist.
Jadeite is a hard, brittle mineral with a 3.3 to 3.5 specific gravity and a hardness of 6.5 to 7. It is pyroxenes, whose crystals are shorter and granular, and are more closely interlocked, as in a mosaic. Although it is harder than nephrite it fractures more easily. When polished, jadeite is vitreous or glassy. It is commonly found in association with albite feldspar, nepheline, and quartzite and is surrounded by serpentine.
Pure jade is white or transparent; the transparent variety is very rare. All other colorations of the stone owe to the addition of other elements. Although the five traditional colors of Chinese jade are red, black, white, yellow and green, jade can be found in all colors of the rainbow. Both nephrite and jadeite may be white or transparent or other colors. However, the vitreous apple and emerald green versions are almost assuredly jadeite. The milky, silky, or “mutton-fat” appearance is a clue that nephrite is the mineral inside. In general, the bold colors are never seen in nephrite, they are reserved for jadeite.
Geology
Jadeite and nephrite are minerals formed in a metamorphic environment associated with high pressure and a low temperature. While there are as many as seven hypotheses to explain the formation of the jade minerals, the most plausible is that they form under anomalously high water pressures on the lower plate of a low-angle thrust fault. In these circumstances, the fluid pressures may exceed the pressure of the rock overburden.
Such conditions may occur at the margins of the earth’s plates, such as in the Alps, California, the northwest North American coast, and the Asian part of the Pacific Rim.
For twenty centuries, the main source of jade was the nephrite from China, found primarily as cobbles and boulders in the rivers and creeks. The river stones have a tough oxidized rind, making them not easily distinguishable from other stones. The Khotan and Yarkand regions, once the prime providers, are reportedly mined out of nephrite now, and nephrite boulders are only occasionally encountered. In 1784, jadeite was discovered in northern Burma. Originally, it was also in the form of river cobbles and boulders. However, eventually the mother lode was found and mined. Although Burmese jade is not a large part of the international jade market because of political conditions there, these mines are still in operation. Jadeite was also located in China’s Yunnan Province. Pre-Columbian implements and works of art in jadeite are thought to have originated in Guatemala.
Around the world, other sources of nephrite are: Lake Baikal, Russia; New Zealand; Switzerland; Zaire; Jordanow Slaski, Poland; British Columbia, Canada; Kotzebue, Alaska; Lander, Wyoming; and San Benito, California. Northern British Columbia now has the world’s largest active nephrite mine near Dease Lake. About 300 tons are exported to Asia annually, and about 7 tons are used in North America for carvings and jewelry. Jadeite comes from Japan, western California, the Celebes, New Guinea, and Guatemala.
Jade does not have any other use in the modern world, except as a precious mineral and carving medium. In ancient times, it was also used to make household implements and weapons.
Quarrying
In the old days in Asia, jade was mined by building a charcoal fire near the mineral vein. At night, when the temperature dropped, the stone in the vein would crack. Workers would then drive stakes into the cracks to keep it open. Holes were drilled into the cracks, which were then filled with water so the freezing water would further expand the crack, eventually breaking a block free.
Modern methods are not entirely different. Drilling and blasting, along with heavy machinery, are used to remove the overburden of soil and rock. In British Columbia, large blocks and slabs are then removed from the mountain by drilling (2-inch-diam. holes) and hydraulic splitters. Hydraulic splitters are the mechanized versions of feathers and wedges. The low-quality nephrite is then separated from the high-quality with a 72-inch diameter saw. In general, the larger blocks are 5 to 10 tons, but the largest for a special carving was 25 tons. Cobbles and boulders are still used when found, but they are a minute proportion of the yield compared to years past
Working Jade
Jade is too hard to carve with conventional stone carving tools such as hammer and chisels. It is worked by grinding or abrading the stone away. From ancient times to only a few decades ago, jade was shaped by using hand-dipped quartz abrasives, along with hand tools, foot treadle machines, and bow drills. Presently, cutting and shaping of the stone is accomplished with diamond saws, drills, and grinding wheels.
Although tungsten carbide tools can be used with jade, the most cost effective are diamond tools. Diamonds can be used on all of the tools described, including the tiny bits that fit on the Foredom machine. The diamond sintered points with seven different grits can get into the smallest nooks and crannies. Sanding is normally accomplished with diamond compound that can be obtained in a lapidary shop. Deborah Wilson’s secret polishing compound recipe contains diamond powder, vaseline, and lipstick (for tracking where you have polished). A water bath is used during grinding and polishing. The diamond polishing compound however, is only used dry.Flaws
When selecting jade, it is best to have two sawn sides to view. Early October is the best time of the year to obtain good pieces of the stone, because the best pieces of jade are still available soon after the summer quarrying season. White streaks in the otherwise green stone can be softer zones that may not work consistently with the rest of the stone. Some of the inclusions that may be harder than the rest of the stone are blue-gray streaks, garnets, and flecks of chromite. Chromite inclusions in the finished piece have appeal to North Americans, but not the Chinese, who prefer pure colors.
Safety
Because jade contains silica in both minerals and nephrite is comprised of actinolite and tremolite, it is important to maintain a constant stream of water on the stone during grinding and to wear a respirator with very fine dust filters. Don’t forget about eye protection.
Appreciations
Thanks to artist, Deborah Wilson of Vernon, British Columbia for sharing her jade carving experiences and to quarry operator and jade merchant, Kirk Makepeace of Jade West in White Rock, British Columbia for information on quarrying and availability of jade.
Sandstone has been a reliable utility stone throughout the centuries. Many cities, both ancient and modern, take advantage of its fine qualities. It is easy to quarry, found throughout the world, is relatively easy to shape and carve, and resists erosion in most climates. While it is limited in colors, it is commonly uniform, compared to other stones, and therefore provides the architect with a useful building stone.
Geology
In very basic terms, sandstone is a sedimentary rock that is nothing more than cemented sand grains. If grain size gets too large, the rock is a conglomerate; if the grains are too fine, the rock is a claystone, shale, slate or argillite. There are several types of sandstone: arkose, arenite, graywacke, orthoquartzite and protoquartzite. The classification depends on the kind of cementing agent and the percentage of quartz and feldspar sand particles. The chief types of cementation include silica (quartz. opal and chalcedony), calcite, dolomite, clay and limonite. The cementing agent can either be deposited at the same time as the sand particles or at a later date.
The grains themselves can be derived from other existing rocks that are nearby, and they may be any shape, although they are commonly rounded to subrounded owing to collisions with other particles during transportation. They may have accumulated in ancient sand dunes, on river bottoms, in the shallow portion of deltas of fresh water lakes or in a shallow marine environment. Very slowly over millions of years, the deposits of sand are compacted by overlying rock strata. The pressure and the cement cause the rock to gain strength. As a general rule, the older the rock the higher the strength.
Because sandstone is deposited in environments where organisms live, fossils are common. For similar reasons, concretions - hard nodes with interesting shapes - are also found in sandstone. The concretions themselves are sometimes mined and their interesting shapes are objets d’art. Because sandstone is deposited in environments where organisms live, fossils are common. For similar reasons, concretions - hard nodes with interesting shapes - are also found in sandstone. The concretions themselves are sometimes mined and their interesting shapes are objets d’art.
As a sedimentary rock, it is deposited in layers, and is commonly interbedded with shale, limestone and coal. Where the four repeat in sequences, they are called cyclothems. Where higher depositional energy is involved, the gravel or cobbles change the rock designation to conglomerate.
As a rock (as compared to a mineral), sandstone does not have a designated hardness. Its hardness ranges considerably because of the wide range of cementing agents and degrees of weathering. Caveats aside, sandstone probably ranges from 3 to 6 in hardness.
There are, of course, softer sandstones; however, they would probably not hold any sculptural detail and would spall badly in the elements. The hardness of sandstone is normally expressed in terms of its unconfined compressive strength, the strength obtained by compressing a cylindrical piece of the stone to its breaking point. Unconfined compressive strength of stone suitable for buildings and sculpture is between 5,000 and 30,000 pounds per square inch (psi). Wilkeson sandstone is reported to be one of the strongest sandstones in the world. Sandstone weighs between 136 and 166 pounds per cubic foot.
Sandstone is most commonly brown or gray, however, white, yellow, green and red exist. Some sandstone has layers or streaks of red iron oxide, which may make it attractive for sculpture, but not desirable as a building stone.
Its most common use worldwide is in buildings. Can you guess what rock type are the “brownstone” buildings of eastern U.S. cities? Sandstone is also used for curbstones, bridge abutments and retaining walls, because it can be quarried relatively easily to very close tolerances. It can be pulverized into sand particles to use as sandblasting material or foundry sand. The hardest sandstones have been used as grindstones and sharpening stones.
Quarrying
Sandstone is extracted in large open pit surface mines, generally from large faces of exposed rock. Because of its stratification, it naturally divides in one dimension. The somewhat horizontal bedding provides a convenient plane on which the rock breaks. Planes of weakness in the other two planes commonly develop during compression or tension of the earth’s crust. However, these planes of weakness may not be the correct size for building or sculpture stone. Therefore, cutting, drilling and light blasting are sometimes required to remove the stone.
In Washington State, Wilkeson, Tenino and Chuckanut sandstones have all been removed by cutting channels or slots in the rock and then drilling a row of holes along the bottom of the channeled rock. A row of drill holes across the back of the channeled section also separates the stone from the mountain. The width of the channels can be varied to render different thickness of slabs or blocks. Feathers and wedges can be used to bring the stones closer to the desired size. The need for carbide or diamonds for the drills is governed by the strength of the cementing mineral.
The rectangular blocks of stone are then milled by gang saws and planers to a predetermined size that closely fits the final carved stone.
Working Sandstone
Sandstone is chosen commonly for its uniformity of color and grain size. A uniform block is essential for a good sculptural stone. Some of the most common flaws in sandstone that are unique to that stone are coal seams, fossils and concretions. In addition to being unsightly, the coal seams are weak and can cause the piece to split. Fossils can either be harder than the stone or much softer; if softer, they can fall or spall out leaving a hole in the finished piece. Concretions are hard nodes within a softer rock that form around a small nuclear particle and are bonded with a harder cement, commonly iron oxide. As with many rocks, small seemingly undetected fractures in the stone can result in the splitting of the stone after it has been worked for some time.
Sandstone can be roughed out with a diamond or carbide skill saw. A point can be used to take the shape within about 1/2-inch of the final shape. Shaping can be accomplished with a toothed chisel, flat chisel and a cape chisel, a narrower version of a flat chisel. A “bull-nose” chisel is used to create concave surfaces Sandstone carvers have traditionally used wooden mallets instead of iron or steel. The wooden mallets are strong enough to work sandstone and yet they reduce the noise and seem to absorb much of the energy before it reaches the hand and arm.
Smoothing of sandstone can be performed with different implements, depending on the desired final effect. Initially, a carbide scraper or rasp is used, and then final polishing can be done with another piece of sandstone or with a diamond file. The latter is definitely recommended when working with Wilkeson sandstone.
Sandstone can be either an indoor or outdoor stone, the main differentiating factor being porosity. If the stone has a high porosity, such that water can fill the voids, it is likely better indoors. Left outdoors, moisture and freeze-thaw would soon soften the surface of the sandstone and ruin any details. Look at a piece of sandstone on a building sometime and note the occasional scallop out of the stone facade. Many of the older building of Europe require restoration after a century or two. The most effective deterrent is periodic cleaning of the stone.
And remember !! Keep your goggles and mask on. The dust is very fine grained and more than likely contains silica. Silicosis can be debilitating and even fatal.
Appreciations
Thanks to stone carver Keith Phillips of Tenino, WA, for sharing his knowledge and expertise of sandstone with us. Keith’s enthusiasm for the history of quarrying and carving of sandstone are an inspiration to all who meet him