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Amazing World of Slate

What is Slate?

Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. It is popular for a wide variety of uses such as roofing, flooring, and flagging because of its durability and attractive appearance.

Composition of Slate

Slate is composed mainly of clay minerals or micas, depending upon the degree of metamorphism to which it has been subjected. The original clay minerals in shale alter to micas with increasing levels of heat and pressure. Slate can also contain abundant quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals.

Color of Slate

slate-floors 02

Most slates are gray in color and range in a continuum of shades from light to dark gray. Slate also occurs in shades of green, red, black, purple, and brown. The color of slate is often determined by the amount and type of iron and organic material that are present in the rock.

slate-floors

How Does Slate Form?

How Does Slate Form

The tectonic environment for producing slate is usually a former sedimentary basin that becomes involved in a convergent plate boundary. Shales and mudstones in that basin are compressed by horizontal forces with minor heating. These forces and heat modify the clay minerals in the shale and mudstone. Foliation develops at right angles to the compressive forces of the convergent plate boundary to yield a vertical foliation that usually crosses the bedding planes that existed in the shale.

Uses of the Word “Slate”

The word “slate” has not been used consistently over time and in some industries. Today most geologists are careful not to use the word “slate” when talking about “shale.” However, in the past the word slate was often used freely in reference for shale.

This confusion of terms partially arises from the fact that shale is progressively converted into slate. Imagine driving your car eastwards in Pennsylvania through areas of increasing metamorphism, starting where the rock is definitely “shale” and stopping to examine rock at each outcrop. You will have a difficult time deciding where on that route “shale” has been converted into “slate.” It can be difficult to pick up a rock and apply the proper name where the rocks have been lightly metamorphosed.

In the coal mining industry of the Appalachian Basin, the word “slate” is still used by many miners in reference to the shale that forms the roof and floor of a mine, and for fragments of shale that are separated from the coal in preparation plants. Experienced miners train newer miners, and archaic language is passed along.

In the 1800s, elementary school students used a small piece of slate mounted in a wooden frame for writing practice and arithmetic problems. Writing was done with a small pencil made of slate, soapstone, or clay. The slate could be wiped clean with a soft cloth. Small slates were also used in schools and businesses to list daily events, schedules, menus, prices, and other notices. Today, over 150 years after writing slates started to disappear from schools, the word “slate” is still used in phrases such as “clean slate,” “wipe the slate clean,” “slated for today,” “put it on the slate” and more.

Slaty Cleavage

Slaty Cleavage

Foliation in slate is caused by the parallel orientation of platy minerals in the rock, such as microscopic grains of clay minerals and mica. These parallel mineral grain alignments give the rock an ability to break smoothly along planes of foliation. People exploit this property of slate to produce thin sheets of slate that are used in construction projects and manufacturing.

Uses of Slate:

Slate in buildings

painted-desert-multi-color-slate-roof-2

Slate can be made into roofing slates, a type of roof shingle, or more specifically a type of roof tile, which are installed by a slater. 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 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 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 01

Slate roof tiles are usually fixed (fastened) either with nails, or with hooks as is common with Spanish slate. In the UK, fixing is typically with double nails onto timber battens (England and Wales) or nailed directly onto timber sarking boards (Scotland and Northern Ireland). 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 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.

SlateTremeddaSlate 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 (apart from roofs) and in slate quarrying areas such as Blaenau Ffestiniog and 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.

vermont-royal-purple-slate-roof

Other uses

slate swtich board

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.

whetstone to hone knives

Fine slate can also be used as a whetstone to hone knives.

cool-pool-tables

Due to its thermal stability and chemical inertness, slate has been used for laboratory bench tops and for billiard table tops.

slate - slate

In 18th- and 19th-century schools, slate was extensively used for blackboards and individual writing slates, for which slate or chalk pencils were used.

Stele51CalakmulMuseum

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.

go game set

Slate was traditional material of choice for black Go stones in Japan. It is now considered to be a luxury.

 

 

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