New version of roofing slates of the World clasiffication

Hello to everyone. I’ve been working hard in this new version of the classfication for roofing slates, I think now looks more comprenhensive.

Regards.Roofing slates of the World v_2

Publications on “roofing slate” topic

Sin título-1

This is a graphic on the number of scientific-technical publications/reports/articles/patents published under the roofing slate topic. It is not 100% accurate, since there is much literature not easily reachable, or even lost and forgotten forever. However, some interesting general conclusions can be taken from this graphic. The first point is the role of the US on roofing slate research. During the first half of the XX century, several high-quality scientific reports were published, mainly by the US Geological Survey and the Journal of the Franklin Institute. The most prominent authors were Nelson T. Dale, Edwin C. Eckel and Oliver Bowles. Personally, I have greatly enjoyed reading these papers. The conclusions of these papers are completely valid today. The US scientific production decayed in the second half on the XX century, and for the first years of the XXI century, again the number of works increased, but this time focusing on roofing techniques.

In Europe the tendency is opposed. Most of the publications are from the end of the XX century-beginning of the XXI century. Here can be deduced two tendencies: publications from the former producing countries, like France, UK, Germany and Belgium have a clear bias to heritage-related issues, while for the producing country (Spain) the publications have a strong technical component. The first tendency is especially perceptible for the UK, in which the Heritage Associations (Historic Scotland and English Stone Forum) have played an important role in preservation and documentation of ancient slate roofs.

For the new producing countries (China, Brazil and India), the number of international publications is still low. In the case of Brazil, several reports were issued during the first years of the 2000’s, related to the “Brazilian Slate – CE mark” conflict.

Some interesting publications available on the internet:


Eckel, E.C., 1904. On the chemical composition of American shales and roofing slates. The Journal of Geology 12, 25-29.

Dale, N.T., Eckel, E.C., Hilldebrand, W.F., Coons, T., 1906. Slate deposits and slate industry of the United States. USGS Bulletin #275. United States Geological Survey.

Bowles, O., 1926. Recent progress in slate technology. Journal of the Franklin Institute 202, 668-669.


Davies, D.C., 1880. A treatise on slate and slate quarrying, London.

Hughes, T., 2005. Vernacular slate and stone roofs in England, England’s Heritage in Stone. English Stone Forum, Tempest Anderson Hall, York, pp. 28-42.


Filho, C.C., 2011. As ardósias Bambuí na Marcação CE. ABIROCHAS, p. 19.


Cárdenes, V., Cnudde, V., Cnudde, J.P., 2015. Iberian roofing slate as a global heritage stone province resource. Episodes 38, 97-105.

Roofing Slates of the World, part IV

I’ve been working on a poster summarizing the main roofing slate outcrops in the World. It compiles the location of the outcrops together with a new classification of roofing slates taking into account the color and the petrology. For example, the lithotype B1 corresponds to the black-grey metamorphic slates, the most common type, while R1 would correspond to also metamorphic slates but with purple-red colors.

Roofing slates of the World

Roofing slate lithotypes


First of all: lithotype, what’s this?. A lithotype, as used in Geology, is a stone which represents the characteristics of one group. So when I talk about lithotypes I’m just referring to a set of characteristics and properties. Something like a stereotype but without negative implications.

Roofing slates are classified by the sector according to commercial terms, frequently referring to a specific brand. The geographical names are very frequent (i.e. Mosselschieffer, Brazilian Slate, Spanish slate). The market distinguish three main qualities or choices (first, second and third), but there are many other like Cofina, Historical Monuments, Scottish, etc. The characteristics of these qualities are more or less recognized by the market. This makes that the purchaser rather looks for a specific commercial brand than for a type of slate. However, it is important to define lithotypes for roofing slates, that is, to group and name the slates depending on the characteristics that clearly make this distinction. In other words, if I tell you “there is a bird” you will think in a bird, obviously, an animal with wings and two legs. But if I tell you “there is a sparrow” then you will have in mind an specific kind of bird. You might know few or a lot about the sparrow, but you know which kind of bird I’m talking about. For the slate is the same. Someone that has been working for decades in the slate world knows the slate by the name of the quarry, is like when you know a group of people just by their names, you don’t need more. But other people might do not know the names, and then needs the surnames to get the picture. This is my aim, to give general names and surnames to roofing slates, combining both geological and commercial information, and then proposing a common classification useful for everyone working with roofing slates. How?.

Dale, in 1906 proposed a classification in which the first distinguishing element was the geological origin (Sedimentary or Igneous), and then the matrix arrangement seen at the petrological microscope (Clay slates or Mica Slates). There was a third subdivision regarding the potential change in color (Fading or Unfading), which in fact is still widely used in the U.S.A. This changes in color are due to the occurrence of carbonates.

The proposed classification uses in fact the same parameters, but actualized. Two universal distinguishing features are color and petrology. Color is the reflect of the mineralogy of the roofing slate. The main minerals of slates are quartz, mica and chlorites, plus some other minerals in different proportions and occurrences. Then, there are three main families of color for roofing slates: black-grey, red-purple and green. These general colors are a reflect of the average mineralogy. Black and green slates are the result of reducing conditions, so they can contain iron sulphides and carbonates, two groups of minerals very important from a quality point of view. On the other hand, red slates never have iron sulphides, since they come from oxidizing conditions. Instead they contain abundant crystals of hematite of small size, being this one the coloring mineral. The amount of organic matter, usually present as carbon, is also a determining component for color. The organic matter gives dark tones, and is, as the iron sulphides, related with reducing conditions. Generally speaking, the contents of iron oxides versus iron sulphides and organic are inversely proportional in roofing slates. This is the first level for classification, the color, which gives then rough information about the mineralogy and geological conditions. However, there are some exceptions. In some areas, there is a especial occurrence of roofing slates known as “multicolor” or just “colored” These slates are characterized by presenting non-homogeneous red-orange-black surfaces, product of infiltration of superficial water in the cleavage planes and deposit of iron oxides. This effect is just an alteration, so the color of the roofing slate should be defined in the non-altered planes. Something different are the “variegated” slates, which are red-purple lithotypes showing areas of green lithotypes. This is typical in slates from the UK and North America, and it is due to mineralogical processes developed in the slate matrix. These type of slates are included as a sub-group of the red lithotype, which is their original lithotype.

The second level is the petrology, or the type of rock. Following the classical classification for rocks, there are sedimentary rocks (sandstones, siltstones and shales), metamorphic rocks (slates, phyllites and schists) and volcanic rocks (cinerites). Each of these groups have distinguishing features that influence their performance as a construction material. Sedimentary rocks usually have high Water Absorption (WA) and low Bending Strength (BS), while metamorphic rocks have low WA and high BS and volcanic rocks.

The proposed classification is an attempt to establish an understandable and easy way to name the different types of roofing slate used in the World. This classification has some advantages compared to the commercial terminology used nowadays. Every roofing slate in the World only matches in one category, and these categories describe the aspect and petrology of the roofing slate on a general way. From this description, some technical characteristics can be inferred, regarding to weatherable minerals, WA and BS. However, there are some minor exceptions linked to singular outcrops. For example, in the lithotype B1, which corresponds to slates s.s., the carbonate content usually is below 2%, but some outcrops from Canada and Italy have higher values of carbonate, in some cases over 20%. A complementary information to this classification might be then the geographic location. A technician from the slate market knows the regional characteristics of each slate, and a layman would know where to place it. The lithotype code would act then as the name of the roofing slate and the geographic location the surname.

Color changes in roofing slates exposed to high temperatures

According to EN 12326-1, roofing slate is a material which do not need additional tests regarding to fire performance, since it is obvious that it doesn’t burn. However, different types of roofing slates submitted to high temperatures showed important changes in color and also in water absorption. This might be important for some cases like when reusing a material coming from a house burn, or if one wants to estimate the temperature of a fire affecting roofing slates. For that, I submitted 6 different types of roofing slates to a thermal ramp ranging from 100 to 900 °C. The changes in aspect and water absorption are related to the slate bulk compositon. The slate samples used were:

BRA: Shale from Minas Gerais, Brazil

ITL: Carbonate slate (>20% CaCO3) from Liguria, Italy

ECA: Slate from Valdeorras, Spain

VXE: Phyllite from Lugo, Spain

ALT: Schist from Alta, Norway

BUR: Burlington slate, United Kingdom

Fire resistance

Color changes for the selected slates (Y axis) along the increasing temperature (X axis, in °C)

In the figure is clearly seen how the color is changing to red tones in all the slates with the increasing temperature. This is a normal effect in all the rocks, high temperatures favor the iron oxidation. However, for the carbonate slate ITL, color tends to white tones, due to the carbonate alteration. Respect to water absorption, all the slates increases their values due to the development of cracks and detachments because of the thermal stress. Again, the slate ITL results are different, reaching close to 20% of water absorption due to the disappearing of carbonate at 600 °C. Anyway, the conditions of this experiment are exceptional, and will never be reached under normal conditions of use of the slate, so these results are merely illustrative.

Water absorption evolution with increasing temperature

Water absorption evolution with increasing temperature


Oxidation of iron sulphides in roofing slates

In previous posts I have discussed one of the main pathologies of the slate, the oxidation of iron sulfides. According to EN 12326, to determine the oxidizability of the slate is used the thermal cycle test, which consists on submerging in water for 7 hours the slates, and then put them in an oven at 110 ° c for 16 hours. These steps form one cycle. The essay consists of 20 cycles, which in practice means that the total development time is 4 weeks, at a rate of 5 cycles per week. It also requires facilities and dedicated staff during that time. At the end of the test, depending on the alteration of iron sulphides, a code is given to the slate, T1/T2/T3 code, being the most favorable case T1, and T3 less favorable.

There is a much faster, simpler and cheaper method of determining the oxidizability of the slate. For several years I have been working with pre-oxidations induced by Hydrogen Peroxide, H2O2. With this method, within 24 hours it is possible to know how oxidizable is a slate. The concentration of H2O2 used is similar to what is sold in pharmacies, 3% concentration. Slate tiles are immersed in H2O2 during 24 hours at room temperature (20-25 ° C). After that time, tiles are washed with water (distilled is better, to avoid leaving any traces when dry) and examined. In the picture you can see a plate of slate without attack (0a), attacked 24 h (1a), 48 (2a) and 72 (3a) h. Most of the oxidation occurs within the first 24 h.


This technique has some elements to consider. The first is the analysis of the slate. Personally I always use image analysis, for which first I scan the slates with a normal scanner, at a resolution of 300 dpi. As my scanner has a maximum scan area of ​​DIN A4 size, what I do is cut tiles to a size that fits well, eg 15 x 15 cm.

Once I have the images, I use a free program that works great, ImageJ. With this program I calculate the surface of the slate occupied by oxidations (pictures 0b, 1b, 2b and 3b), so that I have is a real numerical data, not the subjective assessment by an operator.

However, this method does not make much sense if it does not correlate with the levels of oxidation of EN 12326, T1, T2 and T3, which today are recognized and adopted by the slate sector. Oxidizability depends largely on the type of iron sulfide, so that neither can relate the oxidized area of the standard grades … nowadays.

This is one of the things I’m working on, the relationship between H2O2 attack and thermal cycling according to EN 12326.’s Solution shortly.

Roofing slates of the world part III

Images of hand specimens and thin sections of slates from several world´s locations. Real color of the specimens may vary with respect of shown in the images.

Pizarras del Mundo03

13. Slate from Penrhyn, Wales, UK. This slate is extracted at the historic quarry of Penrhyn, and is very popular in historical buildings all over the UK. The green spots correspond to zones with reduced iron and high contents of Ca and Mg (Borradaile et al. 1990). This color change can be seen in the microphotograph of 200 microns.

14. Carbonate slate from Liguria, Italy. The Liguria slates have carbonate content (see microphotograph of 500 microns) of about 20%. However, this fact does not mean that these slates are more susceptible to weathering than other slates with carbonate contents much lower. The key factor is the specific mineralogy of the carbonate. This slate complies with the EN 12326 requirements, and constitutes a perfect material for roofing when used properly. Sample provided by Euroslate.

15. Slate from Benuza, Castilla y León, Spain. An Ordovician slate, fine-grained with some cubes of pyrite, with smooth surface and dark color. This is a classic roofing slate, i.e., a slate from the green schists facies made of quartz, chlorites and mica. Sample provided by Cupa Pizarras S.A.

16. Slate from Hubei province, China. Fine-grained slate, light colored with a marked tendency to acquire a reddish aspect which makes it very interesting for special cases, since this reddish does not seem to generate rust trails. Sample provided by the Laboratorio del Centro Tecnológico de la Pizarra.

17. Green phyllite from Lugo, Spain. This Cambrian phyllite is also a very special roofing slate, being used for some singular buildings such as the Shizuoka Convention Arts Center in Japan. It is quarried in several colors ranging from grey to green. This is the Verde Xemil variety. Sample provided by Pizarras Ipisa.

18. Slate from Villar del Rey, Badajoz, Spain. A very fine-grained slate with some pyrite cubes and a dark color, in fact this is the darkest slate quarried in Spain due to its content in graphite, up to 2%.  Sample provided by Pizarras Villar del Rey, S.A.

And please remember: There are no bad slates but bad uses. The slate should be used in accordance with the building and environment requirements, so it is critical to know and understand the rock we are dealing with.

Quality factors in slates – Part II

Grain size

The grain size of roofing slates is very small, similar to the clays. It is possible to distinguish two types of components depending on the grain size, the matrix (mica and chlorites) and the skeletal components (quartz and feldspar). The key factor is the components of the skeleton, not just the size of these grains, but their selection or uniformity in size (Figure 1). A roofing slate will have good fissility if their skeletal components have all similar size, whereas with diverse range of sizes the fissility is reduced.

Grafico ITGEeng

Figure 1. Relationship between slate components and grain size

Grain size also affects the external appearance, coarser slates have a more rough and irregular aspect, while the fine-grained slates have a more smooth and uniform aspect, and therefore brighter (Figure 2).

Figure 2. Comparision between a coarse grain slate (left) and a fine grain slate (rigth).

Figure 2. Comparision between a coarse grain slate (left) and a fine grain slate (rigth).

Textural homogeneity

By definition, a roofing slate should have a lepidoblastic texture (Figure 3). This term refers to the microscopic arrangement of the elements of the rock, which are strongly oriented along the direction of slaty cleavage or fissility. This texture must be uniform and consistent along the slate, otherwise the split process will be greatly hindered. In certain types of roofing slate, other textures can be found, but must always be homogeneous and continuous.

Figure 3. Classical lepidoblastic texture in a roofng slate (left). On the rigth, a slate with a coarser texture, which is called porphyro-lepidoblastic

Figure 3. Classical lepidoblastic texture in a roofng slate (left). On the rigth, a slate with a coarser texture, which is called porphyro-lepidoblastic

Presence of sedimentary layers

These sedimentary layers are mainly sandy levels, of thicker grain size, which were deposited when the sedimentary rock which subsequently result in the slate was formed (Figure 4), after metamorphic processes.

Figure 4. Deposition of sandy layers on the slate bulk during sedimentation.

Figure 4. Deposition of sandy layers on the slate bulk during sedimentation.

These layers can be recognized as bands of lighter colors. Since they have a grain size and texture different from the rest of the slate, they modify the homogeneity of the slate (Figure 5), so that their presence is undesirable in a good quality slate.

Figure 5. Sandy layers on a roofing slate bulk.

Figure 5. Sandy layers on a roofing slate bulk.

Quality factors in slates – Part I

Traditionally, the slate market has offered a wide variety of different qualities of slate. Each manufacturer has their own commercial references depending on the characteristics of its outcrops, so the market is full of specific commercial references, generating to a general confusion. The first class slate from a company may be very different from the first class of other company. In general, the quality criteria are similar for the entire sector (no alterable minerals, adequate thickness, uniform exfoliation, etc.), although it is the final use of the slate tiles which really define the specific requirements. For example, slate tiles used in Pyrenees, where the roof has to support the weight of the snow many days per year, have high thickness (8-12 mm), regardless of the presence of weatherable minerals. On the other hand, slate tiles used in Brittany, France, must be much more thinner (3-7 mm), without weathering minerals and smooth, uniform appearance. Broadly speaking, the different commercial varieties can be grouped into first, second and third quality, although there are plenty of references intermediate (rustic, first/second/third special quality, first/second economic, second selection, historical monuments selection, etc…).

The factors that determine the quality of a slate tile can be divided into three groups: petrological, tectonic and productive.

Petrological factors

These factors are referred to the mineral components of the slate and the spatial relationships among them.

Mineralogical composition

Slate is composed mainly of quartz, chlorites and mica, together with some other minerals present in variable amounts, like feldspars, chloritoid, tourmaline, carbonates, iron sulphides, etc. However, specific mineralogy depends on the petrological variety of the roofing slate (slate s.s., shale, schist, etc).

Sin título-1For slate s.s., the most typical variety of roofing slate, the average mineral proportions determined by different authors can be found at Table 1. Generally speaking, a good slate should have between 10 and 50 % quartz, 15 – 60 % chlorite and 20 – 70 % mica. Minor minerals like tourmaline, zircon, rutile, leucoxene and chloritoid are not important. Only carbonates and iron sulphides could affect the quality of the slate. Graphite fragments may also have some effect on slate quality by favoring oxidation processes, but only if there are iron sulphides in the slate. Further explanation on weathering of these two minerals can be found at their correspondent posts (oxidation and gypsification). Also, further explanation on slate mineralogy can also be found here.

Other petrological factors related with roofing slates quality are grain size, textural homogeneity and presence of sedimentation beds. These factors will be explained in following posts.

CE marking in roofing slates

PlantillamarcadoCECE marking is mandatory for all the products sold in the European Union, regardless of the country of origin of the products. In the case of roofing slate, this marking is done using the data obtained from the tests of EN 12326 (parts 1 and 2) Slate and natural stone for discontinuous roofing and cladding. The CE mark does not establish qualities, just gives information about product features. The different qualities of the slate are established by the manufacturer taking into account the market requirements and its own standards.

The results of the tests of EN have to be stated on a label attached to each pallet or slate cage. In paragraph ZA of EN 12326-1 is an example of CE label, although there are other solutions.




CE label as in EN 12326-1


CE Pizarra01

CE Pizarra00Two examples of CE marking

Together with this label must be included another document, the declaration of conformity, in which it is specified in detail the characteristics of the slate. This document must accompany each sold batch of slate; there is no need to include it to slate pallet or cage. As for the label, there is a model in Part 1 of the standard.

The CE marking is mandatory for roofing slate since 2004, so it is sufficiently well established between producers and consumers. However, there are still companies that refuse to incorporate it to their products, either by ignorance or negligence law, thereby risking a sanction by the competent authority.

Roofing slates of the world, part II

Images of hand specimens and thin sections of slates from several world´s locations. Real color of the specimens may vary with respect of shown in the images.

Pizarras del Mundo02

7 – Slate Valentia, Ireland. This is a coarse-textured gray slate, with no or few iron sulphides, quarried at the region of Valentia, S of Ireland. Stratigraphic level: Middle Devonian. Sample provided by the company Valentia Slate Ltd.

8 – Mica-schist from Finnmark, Norway. This is type of rock is not usually used for roofing. However, at N of Norway  are several quarries of different varieties of mica-schists thin enough to be used for roofing. These rocks have higher metamorphic degree and mineralogy clearly different to those of the slates s.s. Stratigraphic level: Lower Cambrian. Sample provided by Minera Skifer.

9 – Slate Valongo, Portugal. Dark slate, fine-textured, with some cubes of pyrite. It is quarried in the Valongo area, near Porto, in Portugal. It is similar to some levels of Galician slate, in Spain. Stratigraphic level: Middle Ordovician. Sample provided by Pereira Gomes & Carballo.

10 – Slate Green Lugo. This type of slate is extracted at the Pol area in the province of Lugo. It is characteristic its intense green color, result of the predominance of the magnesic term of the chlorite group, clinochlore. Stratigraphic level: Lower Cambrian. Sample provided by the company Pizarras Ipisa.

11 – Filita from Bernardos. Gray slate, coarse-textured, with no organic matter nor iron sulfides. It is extracted in Segovia, N of Madrid, and is the slate with which was built the Escorial Monastery roof. It has a slightly higher metamorphic degree compared with slates s.s., as evidenced by the presence of biotite. Stratigraphic level: Lower Cambrian. Sample provided by the company Pizarras J Bernardos.

12 – Ballachulish slate. This slate is from an historical quarry no longer in operation. It is a coarse-grained rock with abundant quartz grains and little or no iron sulfide. Sample collected in quarry.

And please remember: There are no bad slates but bad uses. The slate should be used in accordance with the building and environment requirements, so it is critical to know and understand the rock we are dealing with.

Summer course at Oviedo University

Ornamental rocks in construction: granite and roofing slate

This year I have organized a summer course on various technical aspects of slate and granite. The objectives and current programming are summarized as follows:

Spain is the largest producer of roofing slate and the second largest producer of granite in the world. Currently, both sectors are suffering the effects of the economic crisis, which is forcing companies to restructure looking for R & D developments that open new markets, and at the same time, incorporate the latest technologies in production processes in order to optimize operating costs.

This summer school will have a special focus on new technologies and products that have emerged in recent years, and also in the practical application of EN norms in both materials, with the aim of improving the training of technicians specialized laboratory tests. Special attention to the section of petrographic analysis and its practical applications as a qualitative indicator will be given.

The course is aimed primarily at university students of engineering and geology, architects, extractive companies and laboratories of accreditation of ornamental rocks.


  • Overview of the sectors of granite and slate roofing: history, economic, productive areas.
  • Technological advances in the sector in R & D and industrial development.
  • Geological and technical features granite and slate roofing.
  • Laboratory testing and regulations.
  • Practical aspects of implementing petrographic examination techniques in both materials.


  • Lope Calleja Escudero, PhD in Geological Sciences and Professor in the Department of Geology at the University of Oviedo.
  • Victor Cárdenes Van den Eynde, PhD in Geological Sciences and Master in Geological and Geotechnical Resources.
  • Nuria Sánchez Delgado, BA in Geological Sciences and head of the Laboratory of the Technological Center of Granite, Porrino.
  • Alvaro Ordoñez Rubio, PhD in Geological Sciences and assistant professor in the Department of Geology at the University of Oviedo.
  • Victor Pais Diz, Degree in Geology, senior geologist at Cupa Slates.

Dates and price:

15 to 19 July 2013. Enrollment period April 18 to July 8, 2013.

Price: 98,51€ for students of the Oviedo University, 140,73 € for the rest.

The course will be given in Spanish and English.

Pathologies in slates, part IV


Gypsification is the phenomenon by which the carbonates that may be present in the slate is transformed into gypsum by contact with the sulfur (S) coming from the atmosphere or from the iron sulfides, following the reaction:

H2SO4 + CaCO3 –> CaSO4 · H2O + CO2

Fig02The transformation from carbonate to gypsum is potentially harmful, because the gypsum has a mineral size substantially larger than the carbonate, so a swelling occurs inside the slate (figure 1), affecting seriously the integrity of the tile. As oxidation, gypsification is very evident when occurs, since it develops a characteristic whitening along the surface of the slate tile (figures 2 and 3). The gypsification is closely linked to acidic environments, especially urban environments where sulfur concentrations are usually high.

Sin título-1

Figure 2 (left): Cover affected by gypsification
Figure 3 (right): Slate severely affected by gypsification after exposure to SO2 test

Gypsification prevention

The best way to know if a slate may suffer gypsification are the normative tests of exposure to SO2, as expressed in EN 12326, or to the test of weather resistance of ASTM C-217. Both tests submit the slate to acid conditions, and then quantify the alteration suffered by giving three degrees. EN 12326 provides three visual alteration levels (S1, S2 and S3), while ASTM performs a scraping of the slate surface after the acid exposure, and then makes three estimates of the service life depending on the depth of the scratch (S1:> 75 years, S2: 40-75 years, S3: 20-40 years).

The carbonate content test of EN also gives an idea of how susceptible to gypsification can be a slate. In theory, higher carbonate content will lead to a high susceptibility. However, this fact has to be taken with caution, as the carbonate may be present as well crystallized calcite, which resists very well against yesificación. Again, petrographic examination can help in this case, since it will determine the form in which is present the carbonate.

Carbonate crystal in a schist roofing slate

Carbonate crystal in a schist roofing slate, transmitted light microscopy, zoom 250, crossed polarizers

Further reading: Standard tests for the characterization of roofing slate pathologies

Roofing slates of the world, part I

Images of hand specimens and thin sections of slates from several world´s locations. Real color of the specimens may vary with respect of shown in the images.

Pizarras del Mundo01

1 – Slate from Labassere, Pyrenees, France. Dark and homogeneous slate quarried in the French Pyrenees. Nowadays, the quarry keeps a small production focused in the local market. Stratigraphy: Ordovician. Sample picked directly at the quarry.

2 – Shale from Minas Gerais, Brazil. Green rock, although other colors are quarried. It has a metamorphic grade slightly lower than slate. Also, it may have carbonate inclusions (red colored zones in the 200 zoom microphotograph) located in sandy levels. Stratigraphy: Bambui group, Ediacaran. Sample courtesy of Pizarras SAMACA.

3 – Red slate from Newfoundland, Canada (Trinity slate). Fine-grained and homogeneous slate with abundant iron oxides which gives it the red color. Stratigraphy: Bonavista Formation, Lower Cambrian. Sample courtesy of Laboratorio del Centro Tecnológico de la Pizarra.

4 –Himalaya slate. This rock is actually a layered volcanic rock, as it can be deduced due to the epidote crystals seen in the thin section. There are some studies about roofing slates in the Nepal and Himalaya zone Himalaya (Neupane 2007, Neupane 2012). The production potential for this area is still unknown. Stratigraphy: Nourpul and Benighat Formations, Neoproterozoic/Lower Cambrian.

5 – Shale from Jiangxi, China. Light grey rock, fine grained, with homogeneous texture and abundant opaque minerals. Roofing slates form China are very varied both from  petrological and commercial points of view. Thus, there are some exceptional god materials together with other with less quality. Stratigraphy: Shuidonggou Formation, Silurian. Sample courtesy of StoneV.

6 – Angers slate, France. Dark and fine-grained slate, with homogeneous texture, very typical in France. It has been quarried for centuries. Startigraphy: Grand-Auverné Formation, Middle Ordovician. Sample courtesy of Ardosieres d´Angers.

And please remember: There are no bad slates but bad uses. The slate should be used in accordance with the building and environment requirements, so it is critical to know and understand the rock we are dealing with.

Thermal behaviour of the slate

Temperatures reached by the slate on the roof

Once the slate cover is finished, each slate tile receives direct sunlight. Since this rock type has generally dark tone, the incidence of sunlight makes its temperature rises several degrees above the temperature of the air. When designing the roof, the effect of thermal expansion must be taken into account. The thermal expansion causes that each slate tile increases or decreases its volume depending on the temperature. Generally, the slater takes into account this effect, leaving enough space between the tiles. The variation in volume is measured by the coefficient of temperature variation, which for the slate  is estimated between 9.0×10-6 ·°C-1 and 6.5×10-6·°C-1.The linear increase in size for a slate tile can be calculated by using the formula R = X·L·t, ​​where R is the size increase in size, X the coefficient of temperature variation, L the length (in meters) and t the range of temperatures reached by the slate. For example, for a single slate tile with the following conditions:

L = 30 cm = 0.3 m

Minimum Temperature = -10 °C

Maximum temperature = 60 °C               R = 0.0000086 x 0.3 x 70 = 0.0001806 m

Temperature difference = 70 °C

X = 8.6×10-6·°C-1

Although this value is low, the sum of the total variations in size of all tiles is important for the whole cover.

This ratio should not be confused with the coefficient of thermal conductivity, defined as the heat transmitted through a body. For the slate, this thermal conductivity coefficient is estimated as 0.43 kcal/hour·°C·m-1 (1), lower than for the concrete, so in principle the slate should thermally insulate more efficiently than the concrete, considering two identical volumes of both materials.

Sunlight raises several °C the temperature of the slate. Since 2006 I have been measuring the temperature in a slate tile placed in a roof, together with the air temperature (Figure A).


During the winter months, the slate has lower temperatures than that of the air, but during the summer months the slate temperature is greater than that of the air. The measures show that when the slate does not receive sunlight (Figure B), the slate temperature is slightly below the temperature of the air, but when the slate receives direct sunlight (Figure C), its temperature raises, with a measured difference of 40 °C with the air temperature.


Installation of the thermal probe on the roof

Finally, the existence of discontinuities in the rock (microfolding, sandy levels, quartz veins) may cause tile rupture in some cases, so you have to be careful with this type of defects depending on the geographical area where the slate is going to be used.

(1) Menéndez Seigas JL. Architecture and techniques of slate roofing: Asociación Galega de Pizarristas; 2007. ISBN 84-920981-1-2

World´s roofing slate market in 2011

Producing countries versus consuming countries

A brief analysis of the global market for roofing slate in 2011 reveals a number of interesting conclusions. Spain still remains the largest exporter of slates in the world, followed by China and Brazil. Spanish exports in dollars (graphic 1) are well above those of China and Brazil, but not so for exports measured in tons (graphic 2), where China is close to the production volume of Spain. Regarding to consuming countries, in 2011 France was the largest consumer, followed by the UK, Germany and the United States.

Evolution 2011

Taking into account the selling prices for slate, measured in $/ton (graphic 3), Spain, the largest producer, sells its slate at an average price of $ 657/ton, down from the average of 855 $/ton. However, this price is higher than the sales of China (343 $/ton) and Brazil ($ 479/ton). In fact, the overall average price rises due to high sale prices of Central European countries (Germany, France, Belgium and Italy) that had a very limited production but sold at high prices their production into their own markets for restoration of historical monuments and singular buildings. On the other hand, the buying price (graphic 4) for all the countries is closer to the average (787 $/ton), except for the case of China (1,412 $/ton) and Brazil (1,071 $/ton). These two countries buy little roofing slate (graphics 1 and 2) but at very high prices. Thus, it is possible to draw a conclusion: China, and to a lesser extent Brazil, are potential consumers of roofing slate. The opening of these markets to European production companies can be a good solution for the economic crisis that many of these companies are experiencing.

Statistical data:, code 6803, category HS2002

Roofing slate mineralogy – Part II

Secondary and accessory minerals

Secondary minerals

Formed during the metamorphic processes that originated the roofing slates. The most common secondary minerals in slates s.s. are iron sulphides (pyrite and pyrrhotite), carbonates (calcite and ankerite) and chloritoid. The iron sulfides are formed during the post-metamorphic processes. Some authors point out their origin as the remains of organic matter that could be contained in the slate matrix. Depending on the geological conditions and the ratio of iron (Fe) and sulfur (S), these iron sulfides may end up being different minerals with different potentials for oxidation.

Determination of Fe - S proportions of iron sulphides in several roofing slates. The two most common minerals are pyrite and pyrrhotite

Determination of Fe – S proportions of iron sulphides in several roofing slates. The two most common minerals are pyrite and pyrrhotite

On the other hand, the carbonates are normally deposited occupying the empty spaces and voids that could be in the rock matrix. The chloritoid is formed perpendicular to the slaty cleavage. This mineral appears only in some types of slates with a high content of magnesium (Mg) and a slightly higher metamorphic grade than the average of slates s.s.

From the point of view of quality, iron sulfides and carbonates play a decisive role, since they are alterable minerals. Their appearance is undesirable. The chloritoid may cause fissility problems, since it grows perpendicular to the slaty cleavage, hindering the correct elaboration of the tiles.

Accessory minerals

They are found in quantities below 5%. The most common are tourmaline, rutile-leucoxene, zircon, monazite and organic matter. They have no importance for the quality of the board, since they are not alterable, with exception of the organic matter. This can be found under the form of graphite, small fragments of between 5 and 30 microns, which are opaque seen to the microscope. It can become very abundant in localized areas of a quarry and is often linked to iron sulfides. Organic matter also undergoes oxidative processes, although do not causes color changes. Its alteration leads the pH to decrease, acidifying the medium and greatly accelerating the oxidation rate of the iron sulfides. Sometimes the accumulation of organic matter can be seen on the surface of the slate, forming what the miners call “burnt slate“. This slate is not usable to make plates, and it should be discarded.

1. Chloritoid crystal in a Galician slate, Spain<br />2. Small chloritoids in a slate from Arouca, Portugal<br />3. Turmaline fragment in a slate from Monte Rande, Galicia, Spain<br />4. Monazite in a slate from Puente de Domingo Flórez, León, Spain<br />5. Small black and rounded fragments of organic matter in a shale from Minas Gerais, Brazil<br />6. The upper half of the image corresponds to an exceptional accumulation of organic matter, known as “burn slate”

1. Chloritoid crystal in a Galician slate, Spain
2. Small chloritoids in a slate from Arouca, Portugal
3. Turmaline fragment in a slate from Monte Rande, Galicia, Spain
4. Monazite in a slate from Puente de Domingo Flórez, León, Spain
5. Small black and rounded fragments of organic matter in a shale from Minas Gerais, Brazil
6. The upper half of the image corresponds to an exceptional accumulation of organic matter, known as “burnt slate”

Different types of roofing slates

Definition of roofing slate after EN 12326

According to EN 12326-1:2005, from a commercial point of view, a roofing slate is a “rock which is easily split into thin sheets along a plane of cleavage resulting from a schistosity flux caused by very low or low grade metamorphism due to tectonic compression. It is distinguished from a sedimentary slate (shale, author´s note) which invariably splits along a bedding or sedimentation plane. Slate originates from clayey sedimentary rocks and belongs petrographically to a range which begins at the boundary between sedimentary and metamorphic formations and ends at the epizonal-metamorphic phyllite formations”.

This definition makes quite clear, from a petrological point of view, the range of rocks which can be considered slates. However, EN 12326-1:2005 continues defining roofing slate as a ”rock used for roofing and cladding, in which phyllosilicates are the predominant and most important components and exhibiting a prominent slaty cleavage”. Likewise, roofing carbonate slate is defined in the same way as above but with a minimum of 20% content of carbonate.

Metamorphic facies stability diagram. Modified from Spear, 1993.

Metamorphic facies stability diagram. Modified from Spear, 1993.

For the Subcomission on the Systematics of Metamorphic Rocks (SCMR), a part of the International Union of Geological Sciences (IUGS), a slate s.s. is “an ultrafine- or very fined-grained rock displaying slaty cleavage”. This slaty cleavage is also defined as “a type of continuos cleavage in which the individual grains are too small to be seen by the unaided eye”. The slaty cleavage is the most important characteristic of roofing slates, since it allows the rock to be split into large and thin tiles.

It is clear that there are two types of rocks, slates s.s., sometimes also called lutitic slates, which are low-grade metamorphic rocks (greenschist facies), and commercial slates or roofing slates, which are rocks composed mainly of phyllosilicates with an exfoliation which allows to produce tiles that may be used as roofing materials. This second group includes the slates s.s. together with other types of rocks, like shales, phyllites and schists.

A: Sedimentary slate (shale) with no develop of slaty cleavage. The planes correspond to sedimentation beds. Minas Gerais, Brazil.
B: Slate s.s., in which the planes correspond to slaty cleavage. Herbeumont, Belgium.
C: Phyllite, with a metamorphic grade slightly higher than the slate s.s., as the biotite crystals shows. Bernardos, Spain.
D: Schist, with a well developed schistosity. Finnmark, Norway.

Aesthetic characteristics of roofing slates – part I

Color, brightness and texture

The aesthetic characteristics of roofing slates can be defined by the color, brightness and texture. These three parameters are to be taken into account when choosing a slate variety, but also are essential in case of replacement a slate tile in a roof due to repairing or restoration. Traditionally, both slate producers and customers have been referring to the color with somewhat vague terms, such as gray, gray-blue, black, etc. These terms can easily lead to confusion.

Today it is possible to measure color and brightness precisely on any object, including slates. One of the first jobs I did in slates was the measurement of these parameters in slates of the whole Iberian Peninsula. The results show a great uniformity in most of the slates.

CIELAB color space for the roofing slates from the Iberian Peninsula

CIELAB color space for the roofing slates from the Iberian Peninsula

Besides the color, the other two parameters that determine the aspect are brightness and texture. The brightness depends basically on the crystallization and orientation of the mica minerals, while the texture depends on the grain size and the traces of the deformation phases on the slate. The most characterisitc of these traces is the intersection between the slaty cleavage and the sedimentation and which forms the lineation. This structure is known among the miners as hebra (Spain) or longrain (UK), and has a decisive role in many of the properties of the slate tile.

In the Iberian Peninsula, and from a geological point of view, the Ordovician (ORDmid and ORDup) slates from Galicia and Leon present colors a bit lighter than the Devonian slates (DEV) of Villar del Rey in Extremadura, while Bernardos Precambrian (PRE) slates in Segovia are light gray, and finally Cambrian slates (CAM) from Lugo are light green.

Aspect of the slates from the Iberian Peninsula

Aspect of the slates from the Iberian Peninsula

Further reading

Cárdenes, V., Prieto, B., Sanmartín, P., Ferrer, P., Rubio, A., Monterroso, C., 2012. The influence of chemical-mineralogical composition on the color and brightness of Iberian roofing slates. J. Mater. Civ. Eng. 24, 460-467.

Precise color communication – Konica Minolta

Mineralogy of roofing slates – part I

Main Minerals

From a petrological point of view, the minerals constituents of a rock can be divided into primary, secondary and accessories minerals. The primary minerals are the original components of the rock, and their abundance is higher than 5%, while accessory minerals  are found in abundances below 5%. Finally, secondary minerals are result of the geological processes subsequent to the slate formation.

In roofing slates, depending on the author, the percentages of different minerals vary, but the characteristic minerals are the same.


Therefore, the characteristics minerals in roofing slates are quartz, chlorite and muscovite.


Usually present as small rounded fragments formed by metamorphism which caused the slates.

Quarzt grains in a slate sample from Galicia, Spain.

Quarzt grains in a slate sample from Galicia, Spain.


There are two types of chlorite, chamosite, rich in iron, and clinochlore, rich in magnesium. Generally chlorites are secondary minerals formed during metamorphism, and it is usual to find them partially replaced by muscovite, which is a key factor to distinguish them from the quartz.

Chlorite crystals. Slate sample from Galicia, Spain.

Chlorite crystals. Slate sample from Galicia, Spain.


Always forms the matrix of the slate, since it has a very small grain size and at the petrological microscope is seen as a dark background. It is also often found as needle-like crystals formed during and after metamorphism.

Mica needles, slate sample from Galica, Spain.

Mica needles, slate sample from Galica, Spain.

Table captions:

[1] Pizarras ES 1975. Fraser-Española, 1975. Pizarras, in: IGME (Ed.), Monografías de Rocas Industriales Madrid, p. 46.
[2] Lombardero, M., Regueiro, M., 1992. Spanish natural stone: Cladding the World. Industrial Minerals, 81-97.
[3] García-Guinea, J., Lombardero, M., Roberts, B., Taboada, J., 1997. Spanish Roofing Slate Deposits. Transactions of the Institute of Mineral Metallurgy, Section B 106, 205-214.
[4] Lombardero, M., Garcia-Guinea, J., Cárdenes, V., 2002. The Geology of Roofing Slate, in: Bristow, C., Ganis, B. (Eds.), Industrial Minerals and the Extractive Industry Geology. Geological Society Publishing House, Bath, pp. 59-66.
[5] Ward, C., Gómez-Fernandez, F., 2003. Quantitative mineralogical analisis of spanish roofing slates using the Rielveld method and X-ray powder diffraction data. Eur. J. Mineral. 15, 1051-1062.
[6] Rodríguez-Sastre, M.A., Calleja, L., 2004. Caracterización del comportamiento elástico de materiales pizarrosos del Sinclinal de Truchas mediante ultrasonidos. Trabajos de Geología 24, 153-164.
[7] Cambronero, L.E.G., Ruiz-Román, J.M., Ruiz-Prieto, J.M., 2005. Obtención de espumas a partir de residuos de pizarra. Boletín de la Sociedad Española de Cerámica y Vidrio 44, 368-372.
[9] Cárdenes, V., Prieto, B., Sanmartín, P., Ferrer, P., Rubio, A., Monterroso, C., 2012. The influence of chemical-mineralogical composition on the color and brightness of Iberian roofing slates. J. Mater. Civ. Eng. 24, 460-467.
Abbreviations: Q: Cuarzo, Chm: Chamosita, Ms: Moscovita, Alb: Albita, Rt: Rutilo, Ill: Illmenita, Zr: Zircón, Trm: Turmalina, Ap: Apatito, Prg: Paragonita, Ana: Anatasa.

Roofing slate deposits in the world

There are several roofing slate deposits in the world. The biggest is located in the northwest of the Iberian Peninsula, although there are other large reserves which are not yet evaluated in China and Brazil. Thus, the main producers of roofing slate are Spain, China and Brazil, in that order.

Evolution of the roofing slate trade. Data: UNSTASTS,

Evolution of the roofing slate trade. Data: UNSTASTS,

The sector has been hardly hit by the global crisis of recent years, but the production is beginning to recover in Spain, although China is gaining ground especially in volume of production. However, Chinese slate is sold at a price significantly lower than the Spanish, hindering the takeoff of a strong slate production sector in this country. First consumers of Spanish slate are France, Germany and the UK:Grafico Imp paises_ENG

From a petrological point of view, the commercial denomination “roofing slate” includes various types of rocks, with the common characteristic that can exfoliate in large and thin tiles. The specific characteristic of each type of slate depends on its petrology. These specific characteristics control the performance of a slate depending on the conditions of use and the climate.

World´s main deposits of roofing slates.

World´s main deposits of roofing slates.

Pathologies – part III

Acting against oxidation

Above all, it must be remembered that oxidation is a purely aesthetic defect, which does not involve the loss of the roof waterproofing. Only in exceptional cases, where the size of the iron sulphide is greater than the plate thickness, the oxidation can break it.

Up: Lateral view of a slate tile, in which the thickness of the iron sulphides is lower than the thickness of the tile itselfDown: The thickness of the iron sulphides is now higher that the thickness of the tile, breaking it when the oxidation develops

Up: Lateral view of a slate tile, in which the thickness of the iron sulphides is lower than the thickness of the tile itself
Down: The thickness of the iron sulphides is now higher that the thickness of the tile, breaking it when the oxidation develops

It is also necessary to know the susceptibility of the slate to oxidation. An experienced technician will have no problem recognizing the existence, abundance and types of iron sulphides present, so it is possible to estimate the oxidizability of a slate variety quite rightly. Also, preoxidation treatments with H2O2 can be very illustrative, although the attack conditions must be carefully checked for no erroneous results.

In recent years there have been proposed two types of oxidation treatments, application of chemical products and passivation of the iron sulphides. The application of chemicals products is done in huge treatment stations located in the same slate producing factory. These products have several disadvantages to be considered, during the application stage and with the effective protection they can give to the slate. Still, there are already slate producing companies applying this type of products, albeit in a restricted way.

Experimental roof with tiles of slate treated with different products and treatments

Experimental roof with tiles of slate treated with different products and treatments

The other type of treatment involves selectively attacking the iron sulphides, weathering first their surface and then coating them with an inert mineral crust that protects against the environmental conditions. This method is effective in theory, and it has not been developed for practical use yet, so its real effectiveness can´t be known.

As a general recommendation, against oxidation on the roof, we should act calmly, first weighing the extent and type of damage, and then considering the possibility of changing the affected tiles. Each case is different, and not always the oxidized tiles are negative. In restoration of historic monuments it is common to search for oxidized tiles to replace the originals. Also in modern buildings, rusted slate offers new attractive textures and colors.

Portugal2007 (84)

Further reading:

Passivation techniques to prevent corrosion of iron sulphides in roofing slates

Oxidación de sulfuros en pizarra ornamental: tratamientos protectores con siloxanos

Protocolo de valoración de la efectividad de productos protectores de pizarra para cubiertas

Sealant composition for roofing slate

Pathologies – part II


Iron oxidation consists of the change of Fe2+ to Fe3+, by the gain of an electron. In roofing slates, most important iron minerals are iron sulphides, being the most abundant pyrite (FeS2), which is oxidized in the presence of oxygen according to the reaction:

FeS2 + O2 –> Fe2+ + 2SO2-4 + H+

The oxidation of these iron sulfides is favored by acid urban environments and coastal areas, where sea salts favor oxidation reactions.

However, not all the iron sulfides oxidize in the same way. There are several types of iron sulfides, such as pyrite, pyrrhotite, marcasite, arsenopyrite, etc, being the two firsts the most abundant by far. Each iron sulphide has a different structure. Thus, the oxidation susceptibility depends on the strength of this mineral structure. For example, pyrrhotite has a poorly ordered hexagonal structure, being more vulnerable to oxidation than the pyrite cubic structure. In real world, most oxidations developed in roofing slates are due to pyrrhotite, so it is very important to distinguish between these two minerals, since the oxidability of the slate depends on it.

Finally, the occurrence of organic matter in the slate favors the oxidation, due to the increase of acidity during its decomposition.

1 - Pyrrhotite, brown color, with not recognized shape. 2 - Pyrrhotite partially oxidized together with an inclusion of organic matter. 3 – Pyrrhotited fossil of a bivalve. 4 - Cubic pyrite crystal. 5 - Cubic pyrite crystals forming aggregates called framboids. 6 - Footprint of a disappeared cubic crystal of pyrite oxidized.

1 – Pyrrhotite, brown color, with not recognized shape. 2 – Pyrrhotite partially oxidized together with an inclusion of organic matter. 3 – Pyrrhotited fossil of a bivalve. 4 – Cubic pyrite crystal. 5 – Cubic pyrite crystals forming aggregates called framboids. 6 – Footprint of a disappeared cubic crystal of pyrite oxidized.

Further reading: Determination of iron sulphides in roofing slates from the north west of Spain

Pathologies – part I

Pathologies in roofing slates

The pathologies formed in slate roofs are mainly due to the presence of potentially unstable minerals (iron sulfides, carbonates and organic matter). These minerals may become altered by the effect of environmental agents, once the slate roof is finished. The pathologies are mainly associated with oxidation and gypsification processes of the cited mineral phases.

The oxidation is generated when the iron sulfides which may contain the slate became weathered, forming iron oxides. This forms reddish rust marks on the surface of slate tiles. This is mainly an aesthetic defect, as only rarely slate tiles disintegrate due to oxidation. However, it is the main fact in volume of complaints from slate customers (Figure 01). The presence of tiny fragments of organic matter may favor the oxidation processes.


Customers complaints by volume of monetary costs

The gypsification occurs when the carbonates react with the environmental SO2, forming gypsum. In this case, gypsum has larger size than carbonates, so a swelling may occur within the slate tile, causing it to disintegrate. Despite this, the incidence that this pathology in the customer complaints is significantly lower than oxidation, maybe since it is not as striking (Figure 01).

There are also other characteristic pathologies and minor defects but also must be taken into account.

Following the criteria dictated by ICOMOS, defects and pathologies found in roofing slates can be classified into 3 groups (Table 01).

Most common  pathologies in roofing slates

Most common pathologies in roofing slates

Further reading: Standard tests for the characterization of roofing slate pathologies


Introduction to this blog

Recumbent fold in the Truchas Syncline Domain, Galicia (N Spain). Pliegue acostado en el Dominio del Sinclinal Truchas, Galicia.

Recumbent fold in the Truchas Syncline Domain, Galicia (N Spain). Pliegue acostado en el Dominio del Sinclinal Truchas, Galicia.

Hello everybody. My name is Victor Cárdenes Van den Eynde, geologist with a PhD in roofing slate. I started working at the slate industry in Galicia at the beginning of 2000, soon after finishing my career at the University Complutense of Madrid. After 4 years working in the private sector, I changed the Slate Technology Center Foundation, a division of the Galician Association of Slate Producers. I worked there for two years, developing various projects related to the improvement of production and the inhibition of the oxidation of iron sulfides in roofing slates. After these two years, I moved to Oviedo, where I worked as a researcher for various projects about weatherability and petrophysics of different varieties of building rocks at the University of Oviedo. During this time, I finished a Master in Geological and Geotechnical Resources (2010) and a Ph.D. in roofing slate (2012), which had already started years ago at the University of Santiago de Compostela. Currently I work in research on petrophysics and weatherability of building rocks.

The purpose of this blog is to expose the results of my work to the public, since the scientific journals only reach the scientific community, but not the industry and customers of the building stones, which after all are the most interested in the applications of the research. In next posts I will write about different topics such as the color of the slates, the characteristics pathologies (oxidation and yesificación) and their possible solutions, the mechanical behavior during freeze-thaw cycles, the petrographic features of the different types of slates, the  methods for restoration of monuments and historic buildings, and many other topics.

I hope you find the information you are looking for.


Get every new post delivered to your Inbox.