Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original
shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional
metamorphism. It is the finest grained foliated metamorphic rock. Foliation may not correspond
to the original sedimentary layering, but instead is in planes perpendicular to the
direction of metamorphic compression. A very strong foliation is called "slaty cleavage".
It is caused by strong compression causing fine grained clay flakes to regrow in planes
perpendicular to the compression. When expertly "cut" by striking parallel to the foliation,
with a specialized tool in the quarry, many slates will form smooth flat sheets of stone
which have long been used for roofing and floor tiles and other purposes. Slate is frequently
grey in color, especially when seen, en masse, covering roofs. However, slate occurs in a
variety of colors even from a single locality; for example, slate from North Wales can be
found in many shades of grey, from pale to dark, and may also be purple, green or cyan.
Slate is not to be confused with shale, from which it may be formed, or schist.
The word "slate" is also used for certain types of object made from slate rock. It may
mean a single roofing tile made of slate, or a writing slate. This was traditionally
a small smooth piece of the rock, often framed in wood, used with chalk as a notepad or noticeboard,
and especially for recording charges in pubs and inns. The phrases "clean slate" and "blank
slate" come from this usage.
Historical mining terminology Before the mid-19th century, the terms slate,
shale and schist were not sharply distinguished. In the context of underground coal mining
in the United States, the term slate was commonly used to refer to shale well into the 20th
century. For example, roof slate referred to shale above a coal seam, and draw slate
referred to shale that fell from the mine roof as the coal was removed.
Mineral composition Slate is mainly composed of the minerals quartz
and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite and,
less frequently apatite, graphite, kaolinite, magnetite, tourmaline, or zircon as well as
feldspar. Occasionally, as in the purple slates of North Wales, ferrous reduction spheres
form around iron nuclei, leaving a light green spotted texture. These spheres are sometimes
deformed by a subsequent applied stress field to ovoids, which appear as ellipses when viewed
on a cleavage plane of the specimen. Uses
Slate in buildings
Slate can be made into roofing slates, which are installed by a slater. And, are a type
of roof shingle, or more specifically a type of roof tile. Slate has two lines of breakability
– cleavage and grain – which make it possible to split the stone into thin sheets. When
broken, slate retains a natural appearance while remaining relatively flat and easy to
stack. A "slate boom" occurred in Europe from the 1870s until the first world war allowed
by the use of the steam engine in manufacturing slate tiles and improvements in the road and
waterway transportation systems. Slate is particularly suitable as a roofing
material as it has an extremely low water absorption index of less than 0.4%, making
the material waterproof. In fact, this natural slate, which requires only minimal processing,
has the lowest embodied energy of all the roofing materials. Natural slate is used by
building professionals as a result of its beauty and durability. Slate is incredibly
durable and can last several hundred years, often with little or no maintenance. Its low
water absorption makes it very resistant to frost damage and breakage due to freezing.
Natural slate is also fire resistant and energy efficient.
Slate roof tiles are usually fixed either with nails, or with hooks as is common with
Spanish slate. In the UK, fixing is typically with double nails onto timber battens or nailed
directly onto timber sarking boards. Nails were traditionally of copper, although there
are modern alloy and stainless steel alternatives. Both these methods, if used properly, provide
a long-lasting weathertight roof with a lifespan of around 80–100 years.
Some mainland European slate suppliers suggest that using hook fixing means that:
Areas of weakness on the tile are fewer since no holes have to be drilled
Roofing features such as valleys and domes are easier to create since narrow tiles can
be used Hook fixing is particularly suitable in regions
subject to severe weather conditions since there is a greater resistance to wind uplift
as the lower edge of the slate is secured. The metal hooks are, however, visible and
may be unsuitable for historic properties. Slate tiles are often used for interior and
exterior flooring, stairs, walkways and wall cladding. Tiles are installed and set on mortar
and grouted along the edges. Chemical sealants are often used on tiles to improve durability
and appearance, increase stain resistance, reduce efflorescence, and increase or reduce
surface smoothness. Tiles are often sold gauged, meaning that the back surface is ground for
ease of installation. Slate flooring can be slippery when used in external locations subject
to rain. Slate tiles were used in 19th century UK building construction and in slate quarrying
areas such as Blaenau Ffestiniog & Bethesda, Wales there are still many buildings wholly
constructed of slate. Slates can also be set into walls to provide a rudimentary damp-proof
membrane. Small offcuts are used as shims to level floor joists. In areas where slate
is plentiful it is also used in pieces of various sizes for building walls and hedges,
sometimes combined with other kinds of stone. In modern homes slate is often used as table
coasters.
Other uses
Because it is a good electrical insulator and fireproof, it was used to construct early-20th
century electric switchboards and relay controls for large electric motors. Fine slate can
also be used as a whetstone to hone knives. Due to its thermal stability and chemical
inertness, slate has been used for laboratory bench tops and for billiard table tops. In
18th- and 19th-century schools, slate was extensively used for blackboards and individual
writing slates for which slate or chalk pencils were used.
In areas where it is available, high-quality slate is used for tombstones and commemorative
tablets. In some cases slate was used by the ancient Maya civilization to fashion stelae.
Slate extraction
In Europe Most slate in Europe comes from Spain. Indeed,
it is the world's largest producer and exporter of natural slate and 90% of Europes's natural
slate used for roofing originates from the slate industry there.
Lesser slate-producing regions in Europe include Wales, Cornwall, Cumbria in the United Kingdom;
parts of France; Belgium; Liguria in northern Italy, especially between the town of Lavagna
and Fontanabuona valley; Portugal especially around Valongo in the north of the country.
Germany's Moselle River-region, Hunsrück, Eifel, Westerwald, Thuringia and north Bavaria;
and Alta, Norway. Some of the slate from Wales and Cumbria is colored slate: purple and formerly
green in Wales and green in Cumbria. In the Americas
Slate is abundant in Brazil, the world's second-biggest producer of slate, around Papagaios in Minas
Gerais, which extracts 95 percent of Brazil's slate. However, not all "slate" products from
Brazil are entitled to bear the CE mark. Slate is produced on the east coast of Newfoundland,
in Eastern Pennsylvania, Buckingham County, Virginia, and the Slate Valley of Vermont
and New York, where colored slate is mined in the Granville, New York area. Pennsylvania
slate is widely used in the manufacture of turkey calls used for hunting turkeys in the
U.S. The tones produced from the slate imitates almost exactly the calls of all 4 species
of turkey in North America: Eastern Wild Turkey, Rio Grande, Osceola and Merriam's.
A major slating operation existed in Monson, Maine during the late 19th and early 20th
centuries, where the slate is usually dark purple to blackish, and many local structures
are still roofed with slate tiles. The roof of St. Patrick's Cathedral was made of Monson
slate, as is the headstone of John F. Kennedy. Slate is found in the Arctic, and was used
by Inuit to make the blades for ulus. Asia
China has vast slate deposits; in recent years its export of finished and unfinished slate
has increased. It has slate in various colors. Fossils
Because slate was formed in low heat and pressure, compared to a number of other metamorphic
rocks, some fossils can be found in slate; sometimes even microscopic remains of delicate
organisms. References
Further reading Page, William. The Victoria History of the
County of Cornwall; vol. I. Westminster: Constable. Hudson, Kenneth. Building Materials; "Chapter
2: Stone and Slate". London: Longman, pp. 14–27. ISBN 0-582-12791-2.
External links AditNow—Photographic database of mines
John T F Turner - A Familiar Description of the Old Delabole Slate Quarries, 1864
Granville Slate Museum Hower’s Lightning Slate Reckoner, by F.
M. Hower, Cherryville, Penn., on Stone Quarries and Beyond
Stone Roofing Association website with detailed information about stone roofing