Slate Industry in Spain
Encyclopedia
90% of Europes's natural slate
used for roofing originates from the slate industry in Spain , with the region of Galicia being the primary source of production.
Slate is a fine-grained metamorphic rock
that splits into thin, smooth-surfaced layers. Metamorphic rock is rock which has changed form one form of matter into another and is not as was originally laid down by nature. It is used extensively in the building industry, particularly for roofing tiles. The production of slate is estimated at 4 million tons per year .
, just above Portugal
, and is subdivided into 4 provinces: La Coruña, Pontevedra
, Lugo
and Ourense
.
The main areas of slate production are:-
4500 – 1500BC The Megalithic culture was the first major influence in the region. They were noted for their skills in construction and architecture, and the remains of thousands of tombs (or dolmens) can be found in the region.
1500 – 700BC There was a substantial increase in trade with the rest of Europe during the Bronze Age
, prompted largely by developments in metallurgy.
800 – 500 BC During this period the region was heavily influenced by Celtic culture, and caestros (circular fortified areas atop a hillside, surrounded by a moat) became a common sight.
137BC The first Roman
to enter the area was Decimus Junius Brutus
.
410AD Galicia fell to the Suevi ( or Suebi
) German tribe, following the fall of the Roman Empire
, and they formed the first post Roman medieval kingdom in Europe.
584AD Galicia was invaded by the Visigothic King Leovigild
711 – 718AD The Moors
attempted to gain control over Galicia, as they had done the rest of Spain, but with little success.
739AD The Moors were defeated by Alfonso I of Asturias
and Galicia became integrated into the kingdom of Asturias
835AD Relics said to originate from the Apostle St James (James, son of Zebedee) were discovered in Santiago de Compostela
, and the area became one of the most important pilgrimage sites in the Christian world. This remains one of the heritage sites for which the region is renowned even today.
, and this has led to the region’s unique geological development in respect of slate.
Slate deposits are over 500 million years old, having been formed during the Cambrian
, Ordovician
and Silurian
periods of the Palaeozoic era.
Essentially, slate forms when the clay
minerals in shale
are out under increased pressure with high temperatures. The clay starts to revert to the mica minerals which cause the rock to grow hard with pronounced cleavage (see Cleavage (geology)
) directions. It is this cleavage which enables slate to be broken along flat, even planes.
A large variety of colours and textures of slate can be produced, and the final product depends largely on the tectonic environment, the source of the sedimentary material, and the physical/chemical conditions prevalent during the sedimentation
process.
As the sedimentary deposits were compacted, water was squeezed out, hence turning clay minerals into mica, and then solid mudstone. The subsequent mountain building compaction which occurred over the next few million years resulted in an even stronger rearrangement and planar orientation of the minerals. In addition, the earth’s gradual cooling and shrinking caused immense lateral pressure – particles which were horizontal altered their direction and finished up at 90 degrees to the direction of the lateral pressure. This means that in true slate the cleavage plane is very different from the original bedding plane and led to the well developed cleavage for which the region is famous .
An important use of Spanish Slate is as a roofing material, often being specified by building professionals as a result of its aesthetic appearance and durability characteristics. Slate is particularly suitable as a roofing material as it has an extremely low water absorption index of less than 0.4%. Its low tendency to absorb water also makes it very resistant to frost damage and breakage due to freezing.
The CE mark certifies that a product has met the EU requirements for consumer safety, health and environmental issues .All Spanish Slate bears this mark.
The recent independent study from Fundación Centro Tecnológico de la Pizarra’s into the ’Technical properties of Bambui Slate from the State of Minas Gerais (Brazil) to ascertain its compliance with the Standard EN12326’ has been summmarised by Consultant Geologist J A Walsh and describes how certain products originating from Brazil on sale in the UK, do not bear the CE mark. Such products are sedimentary rocks that have split along their original bedding plane, whereas true slate has been subjected to metamorphism and does not split along bedding, but rather along planes associated with the realignment of minerals during metamorphism. This realignment, known as ‘schistosity’, bears no relationship to the original horizontal bedding planes .
There are further national and international standards that can be applicable to slate, and give information as to the quality levels reached. Some of these are:-
In addition, the quarrying of Spanish Slate does not involve any chemical processes, and usually only local water is used to keep cutting machinery cool.
After quarrying regional authorities often dictate that the landscape is returned as much as possible to its original state, and wasted rock left as part of the extraction process is returned adjacent to the quarry
Slate
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. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
used for roofing originates from the slate industry in Spain , with the region of Galicia being the primary source of production.
Slate is a fine-grained metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
that splits into thin, smooth-surfaced layers. Metamorphic rock is rock which has changed form one form of matter into another and is not as was originally laid down by nature. It is used extensively in the building industry, particularly for roofing tiles. The production of slate is estimated at 4 million tons per year .
Geography of Galicia
Galicia is situated on the North West coast of SpainSpain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
, just above Portugal
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
, and is subdivided into 4 provinces: La Coruña, Pontevedra
Pontevedra
Pontevedra is a city in the north-west of the Iberian Peninsula. It is the capital of both the comarca and province of Pontevedra, in Galicia . It is also the capital of its own municipality which is, in fact, often considered as an extension of the actual city...
, Lugo
Lugo
Lugo is a city in northwestern Spain, in the autonomous community of Galicia. It is the capital of the province of Lugo. The municipality had a population of 97,635 in 2010, which makes is the fourth most populated city in Galicia.-Population:...
and Ourense
Ourense
Ourense is a city in northwestern Spain, the capital of the province of the same name in Galicia. Its population of 108,674 accounts for 30% of the population of the province and makes it the third largest city of Galicia.-Population:...
.
The main areas of slate production are:-
- ValdeorrasValdeorrasValdeorras is a comarca in the Galician Province of Ourense. The overall population of this local region is 28,984 .-Municipalities:*O Barco de Valdeorras, capital of the comarca*O Bolo*Carballeda de Valdeorras*Larouco* Petín* A Rúa* Rubiá...
– the larger slate production companies are concentrated in this area of Orense - QuirogaQuirogaQuiroga may refer to:in geography,* Quiroga, Galicia, a municipality in Lugo province, Galicia, Spain* Quiroga Valley, a valley in Lugo province, Galicia, Spain* Quiroga, Michoacán, a municipality in the Mexican state of Michoacánfor people,...
– is the second largest production area, situated on the province of Lugo - OrtigueiraOrtigueiraOrtigueira is a seaport and municipality in the Province of A Coruña, Galicia, Spain; it is located on the northern slope of the Serra da Faladoira, the river Mera and on the eastern shore of the Ria de Santa Marta—a winding, rock-bound and much indented inlet of the Bay of Biscay, between Capes...
– in La Coruña - MondoñedoMondoñedoMondoñedo is a small town and municipality in the Galician province of Lugo, Spain. , the town has a population of 4,508. Mondoñedo occupies a sheltered valley among the northern outliers of the Cantabrian Mountains.-History:...
– including Mondoeñdo, Pastoriza and Lorenzana in Lugo
Early history of Galicia
The early history of the region has been shaped by a variety of key cultures and influences:-4500 – 1500BC The Megalithic culture was the first major influence in the region. They were noted for their skills in construction and architecture, and the remains of thousands of tombs (or dolmens) can be found in the region.
1500 – 700BC There was a substantial increase in trade with the rest of Europe during the Bronze Age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...
, prompted largely by developments in metallurgy.
800 – 500 BC During this period the region was heavily influenced by Celtic culture, and caestros (circular fortified areas atop a hillside, surrounded by a moat) became a common sight.
137BC The first Roman
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....
to enter the area was Decimus Junius Brutus
Decimus Junius Brutus
Decimus Junius Brutus may refer to:*Decimus Junius Brutus Scaeva was consul in 325 BC*Decimus Junius Brutus Scaeva was consul in 292 BC; he is best known for introducing gladiatorial games to Rome in 264 BC...
.
410AD Galicia fell to the Suevi ( or Suebi
Suebi
The Suebi or Suevi were a group of Germanic peoples who were first mentioned by Julius Caesar in connection with Ariovistus' campaign, c...
) German tribe, following the fall of the Roman Empire
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....
, and they formed the first post Roman medieval kingdom in Europe.
584AD Galicia was invaded by the Visigothic King Leovigild
711 – 718AD The Moors
Moors
The description Moors has referred to several historic and modern populations of the Maghreb region who are predominately of Berber and Arab descent. They came to conquer and rule the Iberian Peninsula for nearly 800 years. At that time they were Muslim, although earlier the people had followed...
attempted to gain control over Galicia, as they had done the rest of Spain, but with little success.
739AD The Moors were defeated by Alfonso I of Asturias
Alfonso I of Asturias
Alfonso I , called the Catholic , was the King of Asturias from 739 to his death in 757.He was son of Duke Peter of Cantabria and held many lands in that region. He may have been the hereditary chief of the Basques, but this is uncertain...
and Galicia became integrated into the kingdom of Asturias
Asturias
The Principality of Asturias is an autonomous community of the Kingdom of Spain, coextensive with the former Kingdom of Asturias in the Middle Ages...
835AD Relics said to originate from the Apostle St James (James, son of Zebedee) were discovered in Santiago de Compostela
Santiago de Compostela
Santiago de Compostela is the capital of the autonomous community of Galicia, Spain.The city's Cathedral is the destination today, as it has been throughout history, of the important 9th century medieval pilgrimage route, the Way of St. James...
, and the area became one of the most important pilgrimage sites in the Christian world. This remains one of the heritage sites for which the region is renowned even today.
Spanish Slate Geology
The Northern region of Spain has been subjected to periods of magmatic activity with volcanismVolcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
, and this has led to the region’s unique geological development in respect of slate.
Slate deposits are over 500 million years old, having been formed during the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
, Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
periods of the Palaeozoic era.
Essentially, slate forms when the clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
minerals in shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
are out under increased pressure with high temperatures. The clay starts to revert to the mica minerals which cause the rock to grow hard with pronounced cleavage (see Cleavage (geology)
Cleavage (geology)
This article is about rock cleavage, for cleavage in minerals see Cleavage Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the...
) directions. It is this cleavage which enables slate to be broken along flat, even planes.
A large variety of colours and textures of slate can be produced, and the final product depends largely on the tectonic environment, the source of the sedimentary material, and the physical/chemical conditions prevalent during the sedimentation
Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
process.
As the sedimentary deposits were compacted, water was squeezed out, hence turning clay minerals into mica, and then solid mudstone. The subsequent mountain building compaction which occurred over the next few million years resulted in an even stronger rearrangement and planar orientation of the minerals. In addition, the earth’s gradual cooling and shrinking caused immense lateral pressure – particles which were horizontal altered their direction and finished up at 90 degrees to the direction of the lateral pressure. This means that in true slate the cleavage plane is very different from the original bedding plane and led to the well developed cleavage for which the region is famous .
Extraction of Spanish slate
The Galician slate industry is a major employer in the region and in the main slate is extracted from surface quarries. The process is as follows :- Geological surveys locate areas of potential slate extraction
- Samples are taken to allow the purity and quality of the slate to be verified.
- The quarry area is established, and any unusable material (“overburden”) is removed.
- Diamond beaded steel cutting cables are used to saw large blocks from the quarry.
- These blocks are transferred to the production centre to be sawn into regular smaller blocks, using their natural cleavage planes.
- Skilled craftsmen grade the blocks according to quality and thickness, after having split them to a predetermined size.
- After final quality control measures, slates are packed into crates, often with batch information so that future matches will be possible for longer term ongoing projects.
Properties of Spanish Slate
Since slate is a naturally occurring product, different kinds of slate are subject to varying chemical and mineral compositions, determined largely by the geological characteristics of the quarry from which it was mined .An important use of Spanish Slate is as a roofing material, often being specified by building professionals as a result of its aesthetic appearance and durability characteristics. Slate is particularly suitable as a roofing material as it has an extremely low water absorption index of less than 0.4%. Its low tendency to absorb water also makes it very resistant to frost damage and breakage due to freezing.
Slate certification
The nature of slate as a natural organic product results in a wide variety of qualities being available worldwide. An important way of determining the differences, and degree of quality, is through the use of country certification, or quality marks.The CE mark certifies that a product has met the EU requirements for consumer safety, health and environmental issues .All Spanish Slate bears this mark.
The recent independent study from Fundación Centro Tecnológico de la Pizarra’s into the ’Technical properties of Bambui Slate from the State of Minas Gerais (Brazil) to ascertain its compliance with the Standard EN12326’ has been summmarised by Consultant Geologist J A Walsh and describes how certain products originating from Brazil on sale in the UK, do not bear the CE mark. Such products are sedimentary rocks that have split along their original bedding plane, whereas true slate has been subjected to metamorphism and does not split along bedding, but rather along planes associated with the realignment of minerals during metamorphism. This realignment, known as ‘schistosity’, bears no relationship to the original horizontal bedding planes .
There are further national and international standards that can be applicable to slate, and give information as to the quality levels reached. Some of these are:-
- NF mark from France
- BS EN 12326-2:200
- IQNET and ISO 9001:2000
- AFNOR
- AFAQ
Using Spanish slate on roofs
Typically Spanish slate tiles are fixed using a hook fixing method, as opposed to the more predominant nail fixing used elsewhere in the world. The benefits of hook fixing are- There is no need to create a hole in the tile, which can sometimes weaken the slate.
- Narrower tiles can be used and hence this makes it easier for the roofer to create roof features such as valleys and domes.
- Hook fixing is particularly suitable in regions subject to severe weather conditions, such as Scotland and parts of Wales, since there is a greater resistance to wind uplift as the lower edge of the slate is secured.
Environmental implications
Dr Joan Walsh, a consultant geologist, writes in her article “Natural Slate: a Green Roofing Medium”, that natural slate offers strong benefits over other roofing materials in terms of carbon emissions and reduced environmental impact.In addition, the quarrying of Spanish Slate does not involve any chemical processes, and usually only local water is used to keep cutting machinery cool.
After quarrying regional authorities often dictate that the landscape is returned as much as possible to its original state, and wasted rock left as part of the extraction process is returned adjacent to the quarry