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The Stone Corner

Stone Queries: Stone Tapping - Sept/Oct 2007

Ed: Ron Geitgey (pronounce it GET chee) is a long-time member of NWSSA, was a professional geologist for 40 years and is a talented sculptor and photographer. In 2002 and 2003 the Journal ran an informative series entitled “Stone Queries” in which Ron answered questions that the membership had about stone. We hope that you who remember the series appreciate the review and that you newcomers will find his breadth of knowledge and practical advice helpful. Ron tells us that he is once again willing to answer any questions relating to stone that you want to ask him. Remember that the only foolish question is the one you don’t ask, so take advantage of this fount of information and send your questions to Ron Geitgey at This email address is being protected from spambots. You need JavaScript enabled to view it.

 

I have noticed people choosing stone to carve by tapping the stone and listening.  What are they doing?

 

Tapping the stone is an attempt to determine if the stone has any fractures. Some stones go thud, some ring, and some buzz or sound “odd.” Supposedly, a stone that rings has no fractures. Based on many years of experience as a stone carver and even more as a geologist collecting samples in the field, I consider the method unreliable, or at least one to use with caution. The presence or absence of fractures is only one of many factors that may affect the tonal qualities of stone.

 

Ringing is not necessarily dependent on stone type. Alabaster, some Soapstones, limestones, travertines, and sandstones may ring, or not, and be completely fracture free. Other stone, including marble, may ring nicely and still fracture during carving. In general, more brittle stones ring better; black marble, flint and obsidian are extreme examples.

 

The shape and means of support strongly affect ringing. A shapeless lump of stone is unlikely to ring while a thin bar or slab of the same stone may ring clearly. A stone lying on the ground or held in hand may not ring because its vibration is damped or muffled by the ground or the grip. A stone suspended by wire or rope, or one resting on narrow supports can vibrate much more freely. (So can one struck while being tossed in the air but that approach has obvious limits and hazards.) Suspended and supported stones have been used as musical instruments for millennia as gongs and xylophones, or more correctly, “lithophones.”

 

Finally, ringing can depend on the striking tool. A loosely held point or chisel can generate a clear tone but it’s not the stone that’s ringing.

 

So where does that leave us? Use a hammer, tap lightly, and be aware of shape and support. Test many pieces of stone to learn its typical sound, and listen for an abnormal sound or buzz. Of course you have learned something if the stone splits when you tap it. And accept the fact that stone is a natural material with natural variations. It is not a medium for those who don’t deal well with surprises.

Stone Queries - Jan/Feb 2004

Q: I have seen the term “freestone” used in describing Gothic sculpture.  What is freestone?

A: Freestone means stone that is uniform in all directions, that is, stone which carves uniformly in all directions, exhibits no tendency to break preferentially, and has uniform structural properties. Originally the name was used for limestones quarried in England and France. These stones’ working properties made possible the detailed sculpture and delicate traceries of Gothic cathedrals. Later the term was also applied to fine-grained, massive sandstones that had similar carving characteristics. Medieval stoneworkers skilled in working these materials became known as freestone masons, a term which many scholars believe was the basis for the name Freemasons.


Sedimentary rocks such as sandstone and limestone typically are layered or bedded, indicating slight interruptions or variations in deposition. The thickness of individual beds may be from only fractions of an inch up to many feet.  Freestone is a very thick bed, indicating continuous deposition over an extended period of time for limestone, or a single episode of very rapid accumulation for sandstone. A freestone bed often has thinner beds above and below it.


The carving benefits of freestone are obvious. You don’t have to worry about directional breakage. In architectural uses (as well as sculpture) it is customary to place stone with the bedding horizontal. This minimizes water infiltration along the bedding and utilizes the stone’s strongest direction for load bearing. There are exceptions. Bedding may be placed vertically when there are strong compressional forces from the sides, such as a keystone in an arch. Using freestone simplified addressing all of these concerns, demanded less skill of the quarrier (although perhaps not of the mason), and wasted less stone.


More recently, “freestone” has been used for any uniform rock, regardless of its origin or composition. But often freestone is a matter of perception. Few stones whether igneous, sedimentary, or metamorphic are truly structureless. I have a friend who has done stone restoration work on centuries-old buildings in Europe. He has noted many instances where, after several hundred years of weathering, supposed freestone blocks now show subtle bedding, sometimes oriented to the detriment of the structure. On the other hand if I am carving what now behaves as freestone I’m not worrying that posterity and a few centuries may prove otherwise.

Stone Corner - Pipestone Nov/Dec 2003

 

Q: What is pipestone and does it come only from Minnesota?


A: “Pipestone” is a much generalized term meaning exactly what it looks like, stone used to make smoking pipes. Throughout the millennia and among numerous cultures worldwide, many types of stone have been carved into pipes, probably at least as many as the kinds of things smoked in them. The requirements seem to be ease of working and some inherent esthetic appeal, often color.


Commonly carved materials include 1. soapstone (talc, steatite), 2. chlorite, 3. pyrophyllite, 4. specific clay minerals such as kaolin or sepiolite (meerschaum), and 5. argillite, deposits of very fine-grained mixtures of various minerals hardened to varying degrees by metamorphism, heat, and pressure. Argillites have many different colors and all can occur as massive deposits showing little or no bedding or layering so they carve uniformly in all directions. Material found in the Queen Charlotte Islands and carved by Haida tribal members is a well-known black argillite. Terminology can get a bit fuzzy here because we have geologists, archeologists, ceramicists, museum curators, art dealers, and stone dealers all using these same names but not always with the same meanings.


In North America “pipestone” is often taken to mean red material, previously (but erroneously) thought to come only from a small area in southwestern Minnesota. Red pipestone artifacts have been found at archeological sites throughout the continent and the material is still of unique importance in many Native American cultures. The Minnesota pipestone is also called catlinite. The entire catlinite deposit lies within the Pipestone National Monument and can be quarried only by Native American tribal members. It is not the only occurrence of red pipestone but properly it is the only catlinite.


Continental glaciers carried red argillite from other occurrences and deposited it throughout the upper Midwest and many of the original source locations have been found. Detailed scientific studies have shown that most red pipestones contain varying proportions of kaolin, pyrophyllite, diaspore, muscovite, and quartz. The red color is from hematite, iron oxide, which coats the mineral grains and fills the spaces between them. Some deposits, including catlinite, have their own unique proportions of these minerals — a chemical and mineralogical fingerprint. Catlinite, for example, contains pyrophyllite, diaspore, muscovite, and a little kaolin, but no quartz. With appropriate analysis it is possible to determine if a sample or artifact is catlinite from the Minnesota locality or red argillite from some other area.

 

Stone Corner - Travertine July/Aug 2003

 

Q: What causes the holes in travertine and do they affect carving?


A: The kind of travertine with holes in it is a form of limestone, calcium carbonate, or calcite, deposited from hot springs and hot lakes. When thermal water saturated with carbon dioxide and calcium reaches the earth’s surface, some of the carbon dioxide bubbles off or is removed by biologic activity. This changes the water chemistry causing calcite to crystallize as thin sedimentary layers and as coatings on the cell walls of bacteria and algae and on the stems and leaves of more complex plants growing in the water.


Eventually the organisms die and decay leaving cavities within and between cells and plant fragments. The cavities range from microscopic to an inch or more. After initial deposition continued circulation of saturated water may add more calcium carbonate, filling many of the holes. Evidence of infilling can often be seen on polished surfaces as a rim or layer lining the larger cavities. The smaller ones are already filled. This lack of porosity, in spite of appearances, is why travertine is useful as building stone.


Travertine has been quarried in the Siena and Tivoli areas of Italy for over 2000 years and material from those quarries is still exported around the world. Turkey, Spain, and Mexico are also major modern producers and many countries export smaller amounts. Although there are many occurrences in the United States, only three are currently in production: southwestern Montana near Yellowstone Park, southeastern Idaho, and central New Mexico south of Albuquerque.


Carving travertine does require some extra attention. A cluster of holes can behave as if the stone were softer and it’s easy to over-carve or over-grind. Carving from more dense stone up to a hole can fracture the edges deeper than you intended, especially if you are using a point. Your choice then is to leave the tool mark or more carefully carve the entire surface deeper to remove the mark. As I approach the final surface I use a wide claw, an inch or more with 5 or 6 teeth, which spans the entire hole reducing the risk of over-breaking. Grinding or sanding with a rigid disk is another solution. Using your fingers or some other flexible backing in the final stages of sanding can round the lip of a cavity creating a surface that reflects a narrow rim of light around each hole. You may or may not want that.


I made the qualification in the first sentence because dense, banded, translucent limestone deposited from cold springs and as various cave formations is also called travertine. Other names used include onyx marble, Mexican onyx, and (incorrectly) alabaster. To add to the confusion the term “onyx” with no modifiers means a banded form of quartz, which is considerably harder than calcite.

 

Stone Corner - Anhydrite May/June 2003

 

Anhydrite has not been a widely popular stone, but its availability on the west coast of North America and the beauty of its finish are likely to result in more sittings in sculpture exhibits in the future. It has the pearly luster of alabaster and yet carves and finishes more like harder stone.


Geology

Anhydrite is both a mineral and a sedimentary rock. Its name comes from the Greek for “without water:” essentially it is the anhydrous form of calcium sulfate (CaSO4). Anhydrite is the sister or brother of alabaster (gypsum), which is the hydrated form of calcium sulfate (CaSO4: 2H2O). Two molecules of water are attached to each molecule of calcium sulfate in gypsum.


Anhydrite commonly is found in white, gray, brown, and light red, or pink. It is heavier than alabaster, weighing in at about 187 pounds per cubic foot with a specific gravity of 3.0. One of the key differences between alabaster and anhydrite is the hardness: alabaster is 2 and anhydrite ranges from 3 to 3.5. It is insoluble in hydrochloric acid and sulfuric acid; however, it is easily attacked by water. In the earth, anhydrite converts to gypsum when it absorbs water, and in your yard it will break down with facility, if exposed to the elements.


Anhydrite is one of a group of rock-forming minerals called evaporites, so called because they form by the precipitation of the minerals from evaporating brines or salt water. The other evaporites are gypsum and halite (salt). They form in shallow salt water seas that are alternately submerged and dewatered. Beds of the rock can be found in thicknesses of a few feet to hundreds of feet. Which mineral is precipitated at any time depends on the temperature, pressure, and salinity of the water. At higher temperatures, anhydrite is the first one to precipitate, followed by gypsum; however, at lower temperatures, the opposite is the case. Halite is normally the last to form. In the laboratory, and presumably in nature, gypsum and anhydrite can be made to convert to one another by the changing of pressure and temperature. It is also known that anhydrite can be changed to gypsum by the addition of water, such as during the weathering process. Conversely, the reverse can be achieved by the baking or drying of gypsum.


In North America, anhydrite is found in the salt domes of Louisiana and Texas and in stratified deposits in Nova Scotia, New York, New Mexico, and British Columbia. It is also in large layered salt deposits of Poland, Germany, Austria, France, and India. Anhydrite is not considered an important industrial mineral and is treated as a contaminant in large deposits of gypsum. It is used as a source of sulfuric acid, as a retardant for concrete curing, for a filler in paper, and as a soil conditioner.


Quarrying

Although anhydrite is relatively hard stone, it is somewhat easy to mine because it is naturally broken by numerous joints. This makes mining anhydrite, for sculpture, easily done without blasting, which would shatter and make unusable much of the stone. It is removed from the ground or rock face by hand tools or with a backhoe. The best method for determining the integrity of the stone is to tap the stone with a hammer and listen carefully for changes in the ring.


Working Anhydrite

Anhydrite is brittle and hand tool working is not recommended. Although a hardness of 3 to 3.5 is not extreme, sculptors who have worked the stone definitely prefer power tools. Hand chisels tend to create small chips in the surface, and sharp corners are commonly lost when unexpected breaks occur. The good part is that anhydrite is unlikely to bruise, in the manner of alabaster. It can be worked with a hand-held grinder as well as a stationary wheel, and the stone cuts very evenly and easily with such tools. Anhydrite takes well to a grinder and carbide disks will show little wear after working the stone.


Anhydrite can be taken to a 70 to 600 grit finish, depending on the color and pattern of the stone, and buffed with tin oxide using a wet rag or a mechanical buffer. Of all the colors, the translucent white produces the most lustrous finish although, the brown may have the most interesting patterns.


Flaws

Cracks and joints are not common flaws in anhydrite. In general, it is a solid stone. The white variety tends to be the most consistent in color and hardness, containing scattered soft spots. Sometimes, small white spots may be present in the stone.


Although not exactly a flaw of the stone, a major weakness of all anhydrite is its susceptibility to water. It must be kept indoors before and after carving. If left outdoors, it will absorb water and deteriorate.


Safety

No particular safety hazards are reported for anhydrite however, we all need to remember to protect our lungs, eyes, and ears at all times. Be a sage and safe sculptor.


Appreciations

Thanks to Randy Zieber of Vancouver, British Columbia for his quarrying and carving expertise and to Carol Way of Seattle, Washington and Vic Picou of Burbank, California for sharing their carving experiences with me.

 

Stone Corner - Jade Jan/Feb 2003

“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.

 

Stone Corner - Sandstone - July/Aug 2002

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

 

Studio Notes - Slate - May/June 2002

 

Slate is a rather mundane, common stone that we remember from our school days.  Recall those days when the teacher asked you to do your math problem on the blackboard (slate, in the olden days) and the dog had eaten your homework?  Perhaps you could have pulled out your carving tools, turning it into an art class, and rendered a spectacular art piece right there in front of the class for a bit of diversion. In the hands of an artist, this stone is indeed anything but mundane. Reliefs of beautiful and varied texture can be rendered from this stone.


Geology

Slate is a metamorphic rock, found in many places throughout the world.  Its main constituents are quartz, illite, sericite and calcite, but other minerals such as plagioclase feldspar, chlorite, dolomite, pyrite and graphite are also found.  Where the state is colored red, it probably contains a significant amount of hematite.  Its particles are very small, less than 0.001 millimeters in diameter.


The parent rock for slate is shale, which was formed from the deposition of mud (clay and silt). Slate is a very close cousin to argillite, also used for sculptural purposes, and associated with the Haida of the Northwest Coast. Slate is moderately hard, but it is very brittle.  Its durability is one of its chief attributes.  Its unconfined compressive strength ranges from 7,000 to 10,000 pounds per square inch.  Its very low porosity, less than 2 percent, is responsible for its impermeable nature and the reason that it has been used for roofing for centuries.  It is difficult to assign a hardness to this stone, because it depends on the direction in which it is worked; however, it normally falls between 1.5 and 2.5.


Slate is formed deep beneath the earth’s surface by the slow pressurizing of fine-grained sediments, such as shale and clystone. The pressure increases from the addition of overlaying sediment, slowly squeezing the water out of the pores. Then folding caused the individual mineral grains to realign and form parallel sheets. The resultant rock has a very pronounced directionality, known as cleavage; that is, the rock will split easily in one preferred plane.  The plane of cleavage is usually not the same as the plane of the bedding, and in fact, the original bedding of the stone may be very hard to discern.  In natural deposits of slate, veins of quartz and dikes of intrusive igneous rock are not uncommon, and the rock may be severely fractured along fault zones.


Although black is the most common color of slate, it is also gray, purple, green and red.  It is found in France, Finland, Scotland, Pennsylvania, Vermont and California. In Washington State, poor quality slate is present in the western part of the state, but commercial deposits are found in Stevens County in the northeastern corner.


The highest quality slate is used for blackboards and pool tables.  Because of its high electrical resistivity, it is used for switchboards and electrical panels. It is also used for mantles, flagstone (interior and exterior), baseboards and roofing. As there is very high wastage in the production of dimensional, the waste is crushed and used for roofing granules, insulation material known as rock wool, and for filler for paint, linoleum, acoustical tile and brick.


Quarrying

Because of the extreme directionality of the cleavage in slate, quarries sometimes continue at near-vertical angles to depths of 700 feet. Drilling and blasting are utilized to remove the weathered rock, but discontinued thereafter owing to the sensitivity of this brittle rock to shock.


Primary cuts are made by channelling or wire saw.  Individual blocks are then separated from the quarry floor by splitting parallel with the cleavage. Large blocks are then subdivided in a similar manner. Due to the brittle nature of the stone, the percentage of waste in a dimensional stone quarry may be 60 to 90 percent.


Working Slate

The most common manner of carving slate is in low relief;  however, it can be used to create a truly three-dimensional sculpture. Isamu Noguchi produced some large-scale pieces with thick slabs of slate, and Barbara Hepworth fashioned apiece called  “Two Figures” in which she pierced two large pieces of slate in a stunning slate sculpture.


Slate can be obtained from a salvage yard or thrift store where it may have been part of a blackboard or pool table. These are the highest quality slates available.  Alternately, flagstone slate can be bought from a stone yard or landscaping materials supplier.


In relief, slate can be worked in a similar manner to wood.  A drawing is copied onto the flat slate surface either by freehand or with carbon paper.   The outline of the drawing is scored deeply with a very sharp instrument.  Although a diamond tool may be the most efficient, any sharp steel tool will do. It is best to pull the instrument toward you to maintain control of the tool and to avoid chipping. The subject is then highlighted in relief by carving from the outside into the incision. Great care must be taken to avoid advancing past the incision because the layered slate will chip easily.   Standard soft stone carving tools can be used for this stage, such as flat chisels, ronelles and toothed chisels.


After completing the outline, the interior body of the sculpture can be modeled and curved with rasps and rifflers to create effective shadows and a sense of three-dimensionality. Polishing is started with 220 grit and can proceed to 1200 grit to obtain a high sheen. If desired, the surface can be waxed and buffed. Outlines should be retraced with a sharp tool to remove the wax in the grooves.


Texturing can be very effective on slate. This can be accomplished with a rasp, a toothed chisel or even a frosting tool, provided that the sculptor is very light-handed. A wooden mallet would be advisable for such work. Slate can be cut with a hack or coping saw, but care should be taken that the brittle nature does not cause the stone to break in a place not desired.


Flaws

The chief flaw in slate is the separation between the individual layers along the cleavage.  Look very carefully at the slate to see if there are any weak planes. As with other stones, tapping the stone with a small hammer or the butt-end of a chisel will tell the tale, but tap gently with slate.


Pyrite and quartz crystals are much harder inclusions that will ruin the integrity of the carving surface, so if some are showing on the surface, there may very well be some more on the interior.


Safety

Because no hammering and power tools are required, no eye protection is needed (unless you should choose to work with a Foredom and carbide bits).  However, the stone is fine-grained, so a mask would be prudent if you are making dust.



 

Stone Corner - Firing Soapstone Mar/Apr 2002

 

Many years ago I wrote an article about firing soapstone to make the stone harder. It is possible to harden the stone to the point that you cannot scratch it with anything short of steel. This will also close the structure of the stone enough to allow it to be used in outdoor applications. I have had several people approach me about this technique so I thought that I would reprise it with some of the things I have learned since then.


I have been working soapstone for the last 12 years and have talked to many of you about your experiences and here are some of the things I have found out. First, soapstone is soft. Now, that may seem obvious but there is more to it than that. Soapstone is chemically close to clay and as such, can be handled in a similar way. It can be “fired” to artificially do what takes nature much longer. I have found out from Ron Gietgey that this technique of heating soapstone is not new or strange. Insulators on the electric poles were at one time made of fired soapstone.


This is not a technique to be used with any other type of stone, especially alabaster. I have not taken the opportunity to try this on other types of stone in the same family, such as talc, pipestone or chlorite. Many of these stone will get harder over time or as they are worked so I didn’t feel this extra step necessary.


Some problems with this method occur because soapstone can have organic intrusions that will burn out during firing affecting the outcome of the piece. Be careful picking the stone that you are considering firing.  Be aware that these intrusions may not be noticeable prior to firing. If you are feeling very attached to a piece, it may be better to simply accept the limitations of the stone. So, keep intrusions in mind when choosing and working soapstone you are planning to fire. In the worst of cases they can leave voids that may have devastating results, even causing the pieces to fall apart. In addition, soapstone, which is available in a wide variety of colors, may alter in color when exposed to high temperatures.


My first test of this technique was in a kiln. I created a number of small pieces of soapstone sculpture. They were then fired at different temperatures. Fortunately these pieces for the most part had very little intrusions of organic material. They became very hard and the color changed to a rich brown. The patterning remained but was much more subtle. However, later pieces I worked had intrusions that did leave voids.  In some cases I left these voids as they worked with the “look” I wanted. In other cases I filled the voids with clear or tinted epoxy. The different temperatures impacted the pieces in appearance and form. The higher the temperature, the harder and darker the stone becomes.


Getting the desired results may not require a high temperature firing. You can get reasonable results using your home oven, raising the temperature slowly over an extended period. However, if there are organic intrusions these will burn out even at the lower temperature.


Now for the specifics of how this is done. I found that it is best to finish the piece to the final sanding. Sign the piece at this point. Then you simply place the piece in a kiln and fire to cone 01 using a slow, stepped temperature increase that will probably take as much as 24 hours to achieve. Afterwards the cooling should take another 8 hours. When this has been done, the piece can be treated as you would any other stone with a sealant and wax. Patience is the most important ingredient. Although I have taken pieces up to cone 4, I do not think that it is necessary to achieve the desired results.


If you are going to use your home oven, put the piece into a cold oven on the middle rack and start at the lowest temperature available, slowly increasing the temperature as the stone, not the oven, reaches that temperature. This may take up to an hour or more with each step depending on the size of the stone. Repeat this as you raise the temperature in 100-degree steps, to a high of no more than 500 degrees. Cool the stone with the door closed for several hours, until the stone is cool to the touch. The center of the stone will probably still be hot so leave it alone for a day or so.


I was interested in investigating this because I wanted to make small pieces that could be handled and used as worry stones. I have had very good results with this technique. To use this on larger work it is important to go slowly. The larger the piece the more problematic it will be to heat it up evenly.

 

Stone Queries - Sept/Oct 2002

Q: Where can I go to collect my own stone for carving or for bases?

A: First, a word of warning.  Every bit of stone out there belongs to someone; individuals - companies, state governments, federal governments.  I strongly advise determining ownership and getting permission before collecting any material.  This can be more difficult than finding the stone itself.  With individual landowners and companies it may be as simple as just asking.  Most state and federal agencies have permit systems and nominal fees for noncommercial removal of small amounts of stone, although some will allow collecting only from existing or abandoned quarries rather just anywhere on their land holdings.  Check with district offices of the U.S Forest Service and the Bureau of Land Management.  In the U.S., staking a mining claim for decorative stone or building stone is not valid.

Every state and province has a geological survey (although it may go by a different name) that publishes geologic maps identifying the rock types in given areas, often specific topographic quadrangles.  Many maintain lists of active quarries or can guide you the appropriate regulatory agency that does.  Most also have an industrial minerals specialist on staff who will be knowledgeable about decorative or building stone.  An index of state geological surveys can be found at www.stategeologists.org Look for geologic maps in areas of your interest and check for reports on building stone.  There are few such quarries in operation any longer in either Canada or the U.S., but a century-old report can be useful in locating old quarry sites.  It will be helpful to learn basic geological terminology, how to read geologic and topographic maps, and how to use the section, township, and range system of locating land parcels.  You won’t find GPS coordinates in a 1914 report.

If such a systematic approach is not to your liking there is always the option of simply exploring beaches, riverbeds, and road cuts.

And now a final word of warning.  Most stone quarries produce crushed aggregate for concrete and asphalt.  Those uses require hard, tough stone - perhaps not the best choice for carving.  The first stage of crushing is usually blasting with explosives to produce maximum shattering.  Similarly, road cuts are drilled and blasted.  Any pieces left over big enough for carving may contain incipient fractures that will break open during carving, often as you are working on that final delicate bit.

Feel free to contact me: Ron Geitgey, (503) 235-3474,This email address is being protected from spambots. You need JavaScript enabled to view it.

Stone Corner - Basalt - 2001 Nov/Dec

Basalt is the predominant igneous rock on the surface of the earth, the moon and probably other bodies of the solar system.  It is hard and dark and its presence evokes feelings of strength and power.  In nature, basalt’s high vertical cliffs stand tall for millennia; in sculpture, it is a symbol of eternity and the simplicity and fundamentalism of nature.  We are blessed in the Pacific Northwest to be in the midst of one of the larger outpourings of basalt in the world.

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Stone Corner - Limestone 1999

Bill LapradeWhite, creamy limestone has been a staple of Pacific Northwest sculptors since 1992 when 6 tons of Utah oolitic found its way to the Northwest Stone Sculptors 5th annual carving symposium.

It owes its continued popularity (10 tons at the 1993 symposium and many more tons subsequently) to its ease of carving, low cost and wonderful finished demeanor. It is indeed a “poor man’s marble,” both

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Stone Corner - Rotten Granite - Nov/Dec 2001

 

Q: Someone told me they were carving rotten granite. What is that?

A: The term granite brings to mind concepts of hardness, permanence, and intractability of stone outcrops. But boulders can be weathered to a crumbly material, often brown stained that may be easily carved even without carbide or diamond tools.


“Rotten granite” is an informal term used rather accurately to describe this partially decomposed granite.


All rocks at or near the earth’s surface are subject to weathering. Changes may be brought about by physical, chemical, and biological processes. The end product is sediment and soil, although from a geological perspective it’s not the end, but simply a step in a great cycle toward sedimentary rock and beyond. Rotten granite is part way there.


By geologic definition, granite and granitic rocks contain visible grains of quartz, feldspar, and various iron-bearing minerals. These grains all formed from molten material, cooling and crystallizing into a tight, interlocking, three-dimensional puzzle. Subjected to rainwater, groundwater, organic acids, and biological activity the quartz changes very little, but the feldspar begins to alter to clay minerals. The iron-bearing minerals convert to brown, yellow, and red iron oxides: what artists know as ochre. The loosening of the puzzle pieces would hardly be noticeable in a human lifespan.


Weathering moves inward from the rock surface or outward through cracks and fractures, any feature that allows water to contact the rock. With an appropriate set of fractures, granite weathers much as an ice cube melts, from a cubic form to a rounded form—a boulder. A partially weathered boulder may still be coherent enough to carve. It won’t take a polish (it may not even tolerate grinding and sanding) but broad forms with rustic finishes could be carved.  However, rotten granite may not be uniformly rotten, a core or other areas of unaltered material may remain. But this offers an opportunity to contrast stained, altered, and unaltered surfaces with only a minimal investment of carving time. Of course there is also the risk of the stone’s simply falling apart with the first (or last) hammer blow.


As weathering progresses beyond any coherence, granite masses may disintegrate to sandy, granular debris called “grus” (from German for grit or gravel). This decomposed granite is used commercially for landscaping, road surfacing, and chicken grit.

 

Stone Corner - Alabaster - 2001 Mar/Apr

Author - Bill LapradaThe Stone Column

Bill Laprade of Seattle is a stone sculptor and geologist.  As a NWSSA Board Officer for four years, Bill helped shape the policies for many Association procedures and Symposia. This is the third publication of the Stone Column series, having begun  in January, 1993, with soapstone. This series continues to provide both veteran and neophytecarvers with information and useful insight into their treasured medium.

Alabaster

Waxy, multi-colored alabaster has been the choice of stone for artists and artisans for millennia.  It not only serves as the source of beautiful sculptures, but historically it has provided utilitarian objects such as jars and casks.  It was prized by the Assyrians and the Egyptians for its beauty.  One of its most handsome and unique characteristics is its ability to pass light; imagine, a stone that can transmit light through it.  While we sculptors use it for carving, alabaster’s cousin, gypsum, is around us most everywhere we go and is handy on all our workshop shelves.

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Stone Corner - Soapstone - 2001 May/Jun

Bill Laprade of Seattle is a stone sculptor and geologist.  As a NWSSA Board Officer for four years, Bill helped shape the policies for many Association procedures and Symposia. This is the third publication of the Stone Column series, having begun  in January, 1993, with soapstone. This series continues to provide both veteran and neophytecarvers with information and useful insight into their treasured medium.

During a one year hiatus from The Stone Column to read and write about the interesting sculpture books that we all know and love, I have been repeatedly accosted (in a friendly way, of course) to resume the articles about the stone that we carve.

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