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.


About vcardenes

3 Responses to Oxidation of iron sulphides in roofing slates

  1. Haroun says:

    Great research Victor! Very useful for those that really want to understand what is going on with pyrites and finally a process to determine how to not get caught out….

    • Peter says:

      A Spanish slate (Villar del Ray) was used to roof my house in 2000. Some of the slates (5% approx) developed brown rust like marks and have become porous with holes 2mm in diameter. A minor leak has occurred recently (indicated by a water mark on a ceiling).The importing company rep.,on an initial cursory inspection,dismisses the posibility that the leak is caused by the porosity in the slates. A more thorough inspection of the roof is planned for next week. I am afraid he will be able to attribute the leak to something else when he proceeds to strip the slates from the affected area. Where can I get qualified independent advise

      • vcardenes says:

        Hello Peter. The slate from Villar del Rey was knonw for the rather big pyrite cubes. They went out of bussines some years ago. In this type slate it is possible that the oxidation of the pyrite could lead to the perforation of the tile, since the pyrite cubes can be bigger than the tile thickness, and when they dissapear the hole they leave just goes through the tile. Anyway, before blaming the slate it is important to look the position of the leak on the roof and how is the entire situation. A good slater should be able to determine the origin of the leak. My advise is to ask for a second opinion, but I don’t know anyone in Ireland.
        Good luck!

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