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.
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.
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.
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.
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. And don't forget about eye protection.
I gave this talk at Camp Brotherhood in July 2010.
The Geology of Sculpting Stone: Indiana Limestone
This link ties to this presentation. It is a narrated powerpoint version for download. It's big (19 Meg) so after you click on "The Geology of Sculpting Stone: Indiana Limestone" link above please you be patience as it is downloaded. Once downloaded, I suggest you open the presentation and click on the "Slide Show" tab then "View From the Beginning" to hear my associated narration.
The presentation covers:
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 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 of our workshop shelves.
Alabaster is one of several forms of gypsum, and it is both a mineral and a sedimentary rock. It is a hydrated calcium sulfate, Ca S04' 2 H20 that is found in many places throughout the world. It is a sedimentary evaporite deposit that precipitates from the evaporation of saline water. The ideal conditions for its formation are (I) a restricted arm of the sea, (2) intense evaporation, (3) replenishment by normal sea water and (4) gradual sinking of the basin.
It is unknown whether gypsum is deposited directly in its hydrated form or if it evolves from another mineral. Gypsum may be transformed from anhydrite (Ca S04), as this anhydrous version takes on water as it is exposed to the elements near the earth's surface. In support of this hypothesis, gypsum is only found in the upper 100 to 300 feet of the crust, where the weathering process has affected the rock. Alabaster is the massive, finegrained crystalline variety of gypsum. Other rocks that are commonly found in assocIatIon with gypsum deposits are halite (salt), calcite, dolomite, clay and limonite.
Alabaster has a hardness of 2. It comes in a wide range of colors--white, translucent, gray, yellow, brown, orange, pink, green, raspberry, strawberry and variegated shades. The colors are the results of impurities such as organics, clay and iron oxide (rust), among others. Green alabaster may take its coloring from smectite clay that is commonly found in the same depositional environment. Commercially mined alabaster is bedded, with strata ranging from 3 to more than 100 feet thick. Products for which gypsum is used are fertilizer, concrete additive to retard setting time, a yeast growing nutrient, a flux for pottery, plaster of Paris, patching compounds and stucco.
Some of the locales where alabaster is found in the world are Utah, Colorado, New Mexico, Florida, Tuscany (Italy), Iran and Pakistan. Much of the alabaster that fmds its way to the Pacific Northwest is from Utah and Colorado.
Most of the alabaster quarried in southern Utah is very close to the ground surface. Only a few inches to a few feet of overburden are removed by a dozer to expose the stone. Because of the high elevation and rough winters, the work is carried out during long work days for about five months in the summer. The quarry areas are normally inaccessible during the winter.
Deposits of alabaster in southern Utah are layered in strata ranging from a few inches to four feet thick. In some quarries the stone breaks out in rounded to oval boulders that average two to three feet in diameter. The most common methods of removing the stone are with a dozer and the use of a hand-held drill. The drill holes are closely spaced and then shims and wedges are used to break the stone into desired sizes. At some quarries, light blasting is used to loosen the stone. Experienced quarry operators check the stone for inclusions and fractures before breaking it into smaller pieces and displaying it for sale.
Alabaster is well known as a stone for teaching, because it is soft, carves easily with hand tools, and if you are fortunate, does not contain big surprises. The hardness (or softness) of 2 is conducive to easy removal of stock and yet alabaster has the ability to hold detail in the manner of many harder stones. Unlike another soft stone, soapstone, that commonly changes hardness, has hidden fractures and spalls unexpectedly, alabaster is generally uniform in hardness and contains fewer veins or fractures.
Some of the flaws that do arise are veins or voids filled with clay. Fortunately these mud veins or pockets are not laterally continuous, so that although the design. of a sculpture piece may have to be altered, it is uncommon that the stone is a loss. Unfortunately, the mud veins or pockets are not normally evident on the outside of the stone. While cracks or fractures in alabaster are not common, there has been some experience that Colorado pink may contain more hidden fractures than other varieties of this stone.
Alabaster can be worked easily with hand or power tools. Roughing of the form is accomplished with points, followed by toothed and flat chisels. Chisels bruises this stone easily, so inspect the surface very carefully as you start to refme the piece with rasps and sandpaper. Rasps of different roughness or die grinders (with water jet to keep the dust down) are then used to impart the details of the piece. The degree to which sanding is taken is a personal decision of the artist. Some prefer to stop at 600 grit, which gives a "soft" fmish, whereas others go to 1200 or 1800 grit, which imparts a bright polish. Akemi polishing fluid or floor wax can also be used to bring out an even brighter sheen. At least three rounds of wax-and-polish are necessary to bring out a good shine.
Because of its softness and susceptibility to the natural elements, alabaster is an indoor stone. Left outdoors, moisture and freeze- thaw would soon soften the surface of the stone and ruin any details in a few years. This may be one of the reasons that the price of an alabaster art piece is limited, in comparison with the harder stones.
And remember!! Keep your goggles and mask on. The dust is very fme grained and keep in mind that it is the same mineral that is used to make plaster of Paris, spackling compound and other quick-setting fillers. Don't let it set up in your body.
Many thanks to artists Meredith Earls of Seattle and Neil Gemmill of Kirkland for sharing their knowledge and expertise of alabaster with us. Thank you also to Evelyn Dettamanti of Cedar Memorials and Southwest Stone of Cedar City, Utah for information regarding the mining of alabaster in southern Utah.