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.

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.