soapstone

Soapstone (also known as steatite or soaprock) is a metamorphic rock, a talc-schist. It is largely composed of the mineral talc and is rich in magnesium. It is produced by dynamothermal metamorphism and metasomatism, which occurs in the areas where tectonic plates are subducted, changing rocks by heat and pressure, with influx of fluids, but without melting. It has been a medium for carving for thousands of years.

Petrology

A block of talc

Petrologically, soapstone is composed dominantly of talc, with varying amounts of chlorite and amphiboles (typically tremolite, anthophyllite, and magnesiocummingtonite), and trace to minor FeCr-oxides. It may be schistose or massive. Soapstone is formed by the metamorphism of ultramafic protoliths (e.g. dunite or serpentinite) and the metasomatism of siliceous dolostones.

Pyrophyllite, a mineral very similar to talc is sometimes called soapstone in the generic sense since its physical characteristics and industrial uses are similar,^{[citation needed]} and because it is also commonly used as a carving material. However this mineral typically does not have such a soapy feel as that from which soapstone derives its name.

Physical characteristics and uses

Soapstone sculpture on the Hoysala temple at Belur, India [1]

Steatite is relatively soft (because of the high talc content, talc being one on Mohs hardness scale), and may feel soapy when touched, hence the name. Soapstone is used for inlaid designs, sculpture, coasters, and kitchen countertops and sinks. Traditional Inuit carvings often use soapstone, and some Native American groups made bowls, cooking slabs, and other objects from soapstone, particularly during the Late Archaic archaeological period.^[1] Soapstone is sometimes used for fireplace surrounds and woodstoves because it can absorb and evenly distribute heat while being easy to manufacture. This is found in many upscale Alaskan and Scandinavian homes. It is also used for griddles and other cookware. Generally, suppliers of soapstone counter tops recommend the use of mineral oil to enhance the color of soapstone. Mineral oil, also known as liquid petrolatum, is a by product in the distillation of petroleum when producing gasoline, will not produce the desired darker appearance of black coloration with one application.^{[citation needed]} It takes many applications over a long period of time to get to an acceptable difference in the color. Soapstone naturally forms a patina over time, similar to the oxidization process that occurs with other materials like copper.^{[citation needed]} A weathered or aged appearance will occur naturally over time as the patina is enhanced. Applying mineral oil will simply darken the appearance of the stone; it will not and does not protect it in any way.

The outer layers of the Christ the Redeemer sculpture are made of soapstone

Tepe Yahya, an ancient trading city in southeastern Iran, was a centre for the production and distribution of soapstone in the 5th–3rd millennia BC^[2] Another instance of use in the ancient world is found in Minoan Crete at the Palace of Knossos, where archaeological recovery has included a magnificent libation table made of steatite.^[3]

Soapstone has been used in India for centuries as a soft medium for carving, but world wide demand for soapstone is threatening the habitat of India's tigers.^[4] The Hoysala Empire temples were made from soapstone.^[5]

Soapstone is used by welders and fabricators as a marker because, due to its resistance to heat, it remains visible when heat is applied. It has also been used for many years by seamstresses, carpenters, and other craftsmen as a marking tool because its marks are very visible and not permanent. For these purposes, it is often sold in 6 inch long square or round sticks.^{[citation needed]}

Soapstone can be used to create molds for casting objects from soft metals, such as pewter or silver. The soft stone is easily carved and is not degraded by heating. The slick surface of soapstone allows the finished object to be easily removed.

Soapstone smoking pipes are found, for example, among Native American Indian artifacts.^[6]

Locally quarried soapstone was used for gravemarkers in 19th century northeast Georgia around Dahlonega and Cleveland, as simple field stone and "slot and tab" tombs.

Soapstone is also a basic stone used to carve Chinese seals.

An Egyptian carved and glazed steatite scarab amulet.

The term steatite is sometimes used for soapstone. It may also denote a type of ceramic material made from soapstone with minor additives and heated to vitrify (to change or make into glass or a glassy substance, especially through heat fusion).^{[citation needed]} It is often used as an insulator or housing for electrical components, due to its durability and electrical characteristics and because it can be pressed into complex shapes before firing. Soapstone has been used for loudspeaker cases ^[7] . It was used for beads and seals in ancient civilizations. Steatite undergoes transformations when heated to temperatures of 1000-1200 °C into enstatite and cristobalite; in the Mohs scale, this corresponds to an increase in hardness from 1 to 5.5-6.5.^[8]

Other names

Kisii stone from Kenya is a type of pyrophyllite used by the Kisii people of the Tabaka Hills in Western Kenya. They use this material to make pots, used to carry fat for massaging into their skin to guard against the elements. They also produce carvings for tourists and for export.

Combarbalite stone, exclusively mined in Combarbala, Chile, is known for its many colors. While they are not visible during mining, they appear after refining.

Palewa and gorara stones are types of Indian soapstone.

A variety of other regional and marketing names for soapstone are used.^[9]

See also

Wikimedia Commons has media related to: soapstone or steatite

Steatite in its raw mineral form

• Ultramafic rock
• Talc carbonate
• Serpentinite
• List of minerals

References

External links

• Ancient soapstone bowl (The Central States Archaeological Journal)
• Soapstone Native American quarries, Maryland (Geological Society of America)
• Prehistoric soapstone use in northeastern Maryland (Antiquity Journal)
• The Blue Rock Soapstone Quarry, Yancey County, NC (North Carolina Office of State Archaeology)