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

 

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.

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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.

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.

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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.

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.

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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).

Imagen01

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.

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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

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