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.

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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.