( Fr- ardoisé; Ger- Ton-Schiefer; Nor- skifer; Rus- )
SLATE (See also ARGILLITE entry)
A. Slate plaque (width - ca. 12.5 cm). Vermont slate with photograph of a pet parrot decoupaged on its surface. (© photo by Cherie Vergos, Personalized Pet Slates, www.petslates.com)
B. Slate plaque "Kokopelli - The Red Rock Minstrel" (height - ca. 25 cm). Engraving and oil painting on slate from Purcell Mountains, British Columbia, Canada. (© photo by Elaine Sell Prefontaine, Slate Stone Art web site - www.slatestoneart.ca)
C. Slate clock (diameter - 16 cm). Reclaimed Scottish slate exhibiting original sedimentary bedding -- manifest by the irregular, roughly horizontal lines -- at an angle to the slaty cleavage that constitutes the surface of the clock. (© photo The Just Slate Company, www.butterchurn.co.uk)
D. Slate carving (size unknown) fashioned from Welsh slate. (© photo permission of The Slatesite Project, Caernarfon, Wales, www.llechicymru.info)
E. Slate tiles (individual tiles are ca. 30 x 30 cm) made from Africa slate. These tiles are ndicative of the diverse colors and patterns available to anyone interested in fashioning decorative items using this rock. (© photo courtesy www.switch-trade.com)
DESCRIPTION: Slates are microcrystalline
metamorphic rocks. When thin sections of slates are viewed using a
petrographic (polarizing) microscope, they can be seen to consist
largely of mica (variety sericite), quartz, and/or one of the
chlorites plus or minus some carbonaceous material, commonly
graphite; a comprehensive list of mineral constituents is given
by Mitchell (1985). Slates differ from argillites in that the
mineral grains of slates have a high degree of preferred orientation
(i.e., foliation). In fact, the foliation is such that
-- by definition -- splits readily along relatively closely
planes widely referred to as slaty cleavage. The cleavage may be
to or at an angle to the bedding of the precursor sedimentary or
rock (cf. specimen shown as Fig.C with other illustrated
Colors - diverse -- the following are widely accepted correlations of colors with common mineral "pigments": blue gray - sericite; red - hematite; green - chlorite; black - carbonaceous material; brown to yellowish brown - "limonite". Also, combinations of the pigments give additional hues and some slates are mottled, streaked, spotted, etc. -- indeed, slates provide a plethora of colors and combinations of colors. (see www.findstone.com)
H. > 5½ -- i.e., if used as the scratcher, most slate will scratch glass, steel knife blades, and hammers; however, a knife point (etc.) may disaggregate the grains of a slate, thus scratching it and making it appear to have a hardness of less than 5½.
Light transmission - opaque
Luster - "cleavage" surfaces range from shiny to dull; cross fractures are typically dull.
Miscellany - as mentioned above, slate exhibits slaty cleavage.
OTHER NAMES: From a geological standpoint, many slate units comprise mappable units and have been given formal stratigraphic unit names (see Appendix B, Glossary). Three examples are the Poultney Slate of western Vermont and eastern New York, the Martinsburg Slate of eastern Pennsylvania, and the Arvonia Slate of central Virginia.
Several slates have been referred to on the basis of their colors or patterns in the marketplace -- e.g., red slate and spotted slate. In addition two rocks -- one an impure slate that probably should not even be called a slate; the other that, which on the basis of descriptions seems to be a true slate -- have been given the following special names:
USES: Slate's uses as a gemrock include
following: Liu (1995, p.156) illustrates spacer beads from
Peru, which he labels slate (on the basis of his
illustrations, I suspect the
identification is incorrect).
More recently, slate, the identity of which is not questioned, has
a surface upon which several artists engrave
or paint their works -- some of these
slates have been chosen for their colors and/or special surface
for such use; many of these works are used as plaques or as parts of
decorative clocks, plaques, etc.
(see Figs. A, B, C, F & G).
has also been fashioned into such diverse things as designer switch
plates; frames for pictures, mirrors, etc.; coasters and
sconces and candle holders; sundials; table and wall-mounted fountains;
for oil lamps (i.e., pieces of slate of diverse sizes and shapes
drilled so a wick holding hollow cyclinder can be passed through them
into an oil reservoir); and even into "stones," most of which are
or painted, in jewelry -- e.g., earrings and pins.
OCCURRENCES: In regions of folded and/or faulted, chiefly sedimentary or volcano-sedimentary rock sequences that have undergone what most geologists usually refer to as regional metamorphism.
NOTEWORTHY LOCALITIES: Perhaps the best known slates in the world are those from Scotland and northern Wales. It has been said that early Celtic structures in the British Isles had slate roofs, and it is well documented that slates from Scotland have been used for roofing and paving (tiles) in castles, cathedrals and townhouses of the affluent since early in the 16th century. Indeed, it is noted on the Internet (www. bluestone25. freeserve.co.uk /slat) that from the 17th century onwards blue, green, grey and purple slates "from . . . the highland border belt that traverses the geological fault between Highlands and Lowlands [have] been shipped to every continent." And, it seems that most -- perhaps all -- of the slate used in America during pre-Revolutionary War times was transported -- in some cases as ballast -- on ships from the British Isles. Since then, however, relatively large quantities of slate have been produced in North America from quarries in central Maine, western Vermont and adjacent New York, and also in eastern Pennsylvania, northern Maryland and central Virginia; and, what appear to be good deposits also occur in Arizona, Arkansas, California, Colorado, Georgia, Minnesota, Nevada, New Jersey, Tennessee, and Texas as well as in the Canadian provinces of New Brunswick, Nova Scotia and Ontario. In South America, "Brazilian rusty slate," the colors of which are described as "subtle greys [with] splashes of orange, yellow, green, red and brown" is used for, among other things, fashioning clock faces. Etc., etc. -- Indeed, I suspect that a comprehensive list of localities where slate could be collected and used for fashioning ornamental items would include localities in several countries throughout the world.
REMARKS: The word slate is derived from Middle English and Scottish sclate, which is apparently related to the Old French verb escalater (= to break or splinter), which was possibly derived from the Old German skleizan (= to break).
Some slate is honed and sometimes polished. More frequently, however, natural cleavage surfaces are used. Either may be sealed and coated to give high gloss or satin sheen effects. Some items fashioned from slate have been waxed or wiped with, for example, linseed oil, or varnished or lacquered.
As already noted, slate has been quarried for use as roofing and tiles for at least the last four centuries. Slate slabs also found use for many years, usually near working quarries, as flag stones; today, this general use has expanded both as to preparation and distribution -- e.g., slate stepping stones and patio tiles, commonly detailed with art and/or bearing one's name or initials, are marketed widely. Until the last few decades, slate blackboards were an almost essential part of school classrooms, and, before chalk was used, "soft slate" or talc "slate pencils" were used to write on those wall-mounted slates and also on small, hand-held, commonly framed, slates that pupils often had before pads and pencils or pens became the norm. Two other general uses that have persisted through untold decades are for hearth and mantle stones and for counter and table tops, especially for decorative end tables and coffee tables, but also for high-quality billiard tables. In addition, the use of slate in turkey calls has a long history, which may well predate the Pilgrims' first Thanksgiving.
A noteworthy oil painting "Men Working in Slate Quarry," by Martha Levy, was created under the auspices of the Works Progress Administration (Federal Art) in 1939. It is in the Slate Valley Museum, Granville, New York, and can be seen on the museum's web site www.slatevalleymuseum.org.
In the 1960s, I made a xylophone – range-1
½ octave – from pieces of slate collected from the scrap heap
a quarry in the Arvonia Slate district of central Virginia, and we have
a roof tile from the same quarry that serves as a dinner bell that
a fine relatively long resonating tone when struck with a felt
mallet. Mobiles have also be created from such pieces.
I have seen none, I feel sure that slate dinner bells and pieces used
mobiles with their appearances modified by, for
adding airbrush paintings are available on the market; I suspect,
however, that applications
of paint, engraving or decoupage might reduce the tone quality of the
Slate is one
of three rocks that are considered state rocks of Vermont.
SIMULANTS: A number of manufactured materials that resemble slate are used as both exterior and interior building materials. Although I know of no use of these simulants so far as their being parts of objets d'art (etc.) represented as slate, it seems likely that such use has occurred or will be made in the future. This is true because of the widespread availability of so many slate simulants produced primarily for use in the building industry. These simulants -- marketed under such names as "African slate," "Authentic Roof," "Dura Slate," "Eternit", and "Majestic Slate Tile" -- are composed of such things as recycled rubber, diverse plastics, ceramic materials, "fiber-cement," fiberglass, intermixture of slate and resin, and combinations of two or more of these materials, plus or minus pigments. - [All these materials, upon close examination, can be distinguished macroscopically from natural slates because, for example, their colors tend to be highly uniform and a "bit off" the typical colors of natural slates and most are noticeably lighter in weight than slate.].
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Last update: 27 July 2010
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