Soapstone, also known as steatite or soaprock, is a talc-schist, a type of metamorphic rock composed largely of the magnesium-rich mineral talc. The definitions of the terms "steatite" and "soapstone" vary with the field of study.
In geology, steatite is a rock that is, to a very large extent, composed of talc. The mining industry defines steatite as a high-purity talc rock that is suitable for manufacturing insulators. Lesser grades of the mineral can be called simply "talc rock".
It is produced by dynamothermal metamorphism and metasomatism, which occur in subduction zones, changing rocks by heat and pressure, with influx of fluids but without melting.
Here is an overview of the composition of soapstone:
- Predominantly talc
- Varying amounts of chlorite and amphiboles (typically tremolite, anthophyllite, and cummingtonite, hence its obsolete name, magnesiocummingtonite)
- Traces of minor iron-chromium oxides
It may be schistose or massive.
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Soapstone is formed by the metamorphism of ultramafic protoliths.
Varieties of Soapstone
When someone uses the colloquial term “soapstone,” they could be describing architectural or artistic soapstone. Architectural soapstone is used in the building industry. Typically, it has 15 to 50 percent talc and comes in varying shades of gray with wide variations in veining pattern-surface treatment makes it black. Sometimes, architectural soapstone features a hint of blue or green. Soapstone varieties closer to 15 percent talc will be more scratch-resistant, while varieties in the 50 percent range will show scratches more often.
Artistic soapstone is the variety you see in sculptures and handicrafts and is not suitable for countertop and other building use. It comes in a broader color variety, including browns, yellows, greens, blues, and whites.
Properties and Characteristics
Soapstone is relatively soft because of its high proportion of talc, which has a definitional value of 1 on the Mohs hardness scale. Softer grades may feel similar to soap when touched, hence the name. Soapstone is easy to carve; it is also durable and heat-resistant and has a high heat storage capacity.
Here are some key characteristics of soapstone:
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- Softness: Easy to carve due to high talc content.
- Durability: Resistant to wear and tear.
- Heat Resistance: High heat storage capacity.
- Chemical Inertness: Does not react with most chemicals.
- Non-Porous: Does not absorb liquids or harbor bacteria.
Historical and Cultural Uses
Throughout history, it has continued to prove very useful for human purposes, and as we can see, it has not ceased to be so even to this day. This is also contributed by the widespread presence of soapstone on the Earth’s surface, which is essentially found worldwide.
Ancient Egyptian scarab signets and amulets were most commonly made from glazed steatite. The Yoruba people of West Nigeria used soapstone for several statues, most notably at Esie, where archaeologists have uncovered hundreds of male and female statues about half of life size. Native Americans have used soapstone since the Late Archaic period.
Here are some examples of its historical use:
- Ancient Egypt: Used for scarab signets and amulets.
- West Nigeria: Used by the Yoruba people for statues.
- Native Americans: Used for bowls, cooking slabs, and other objects since the Late Archaic period.
- Arctic Indigenous Peoples: Used for carvings of practical objects and art.
- Myanmar: Used as a writing pencil as early as the 11th-century Pagan period.
- Minoan Civilization: Used on Crete; at the Palace of Knossos, a steatite libation table was found.
During the Archaic archaeological period (8000-1000 BC), bowls, cooking slabs, and other objects were made from soapstone. The use of soapstone cooking vessels during this period has been attributed to the rock's thermal qualities; compared to clay or metal containers, soapstone retains heat more effectively. Use of soapstone in native American cultures continues to the modern day.
Later, other cultures carved soapstone smoking pipes, a practice that continues today. Soapstone was used as a writing pencil in Myanmar as early as the 11th-century Pagan period. The Minoan civilization on Crete used soapstone. At the Palace of Knossos, a steatite libation table was found.
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Modern Applications
"Mariana" Soapstone countertop installation Ocean Grove, NJ
Soapstone is sometimes used for construction of fireplace surrounds, cladding on wood-burning stoves, and as the preferred material for woodburning masonry heaters because it can absorb, store, and evenly radiate heat due to its high density and magnesite (MgCO3) content. It is also used for countertops and bathroom tiling because of the ease of working the material and its property as the "quiet stone".
Here is a list of modern applications:
- Countertops and Tiling: Easy to work with and provides a "quiet" surface.
- Fireplace Surrounds and Wood Stoves: Absorbs, stores, and radiates heat.
- Molds for Casting: Used for soft metals like pewter or silver.
- Welding Markers: Remains visible under heat.
- Carvings and Sculptures: Used by artists and indigenous peoples.
- Kitchenware: Used for pots, pans, and wine glasses, especially in Brazil.
- Beverage Chillers: Soapstones can be frozen and used as ice cubes.
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. Welders and fabricators use soapstone as a marker due to its resistance to heat; it remains visible when heat is applied.
Soapstone continues to be used for carvings and sculptures by artists and indigenous peoples. In Brazil, especially in the state of Minas Gerais, the abundance of soapstone mines allow local artisans to craft pots, pans, wine glasses, statues, jewel boxes, coasters, and vases from soapstone. These handicrafts are commonly sold in street markets found in cities across the state. Soapstones can be put in a freezer and later used in place of ice cubes to chill alcoholic beverages without diluting.
Potential Hazards
People can be exposed to soapstone dust in the workplace via inhalation and skin or eye contact. Exposure above safe limits can lead to symptoms including coughing, shortness of breath, cyanosis, crackles, and pulmonary heart disease. The Occupational Safety and Health Administration has set the legal limit (permissible exposure limit) for soapstone exposure in the workplace as 20 million particles per cubic foot over an 8-hour workday. The National Institute for Occupational Safety and Health has set a recommended exposure limit of 6 mg/m3 total exposure and 3 mg/m3 respiratory exposure over an 8-hour workday.
Here is a table summarizing the exposure limits:
| Organization | Limit Type | Exposure Limit |
|---|---|---|
| OSHA | PEL (Permissible Exposure Limit) | 20 million particles per cubic foot (8-hour workday) |
| NIOSH | REL (Recommended Exposure Limit) | 6 mg/m3 total exposure, 3 mg/m3 respiratory exposure (8-hour workday) |