The fissility of slate

According to the knowledge so far, the fissility of a slate depends on several criteria. It is mainly influenced by the development or morphology, respectively, of the fracture cleavage or mica layers which again depend on the grain size spectrum and the degree of deformation. The degree of deformation can be estimated by the finale elliptically R_f and thus can be theoretically a criteria for the fissility (Baum, 1994, Vulpius 1993). Another not yet exactly defined influence is suppose to have the amount of carbon because it supports the splitting due to its very good translation properties.

Baum (1994) describes in his investigation that values of the density of mica layers, the strain and the fluctuation between the Thuringian and the Moravian slate are nearly the same whereas the Moravian slates show a coarse grain texture with a lower carbon amount. This differences result in a less good fissility and in a more rustic appearance of the Moravian slates. He also mentioned that, based on the experience with Thuringian slates, the increasing amount of carbon and its concentration within the mica layers lead to a higher fissility .
Slates with the highest carbon amount are called 'sooty slates'. They are very vulnerable to weathering and are therefore not used for roofing.