The compressive stress, which transforms a mudstone into slate results in the development of cleavage perpendicular to the direction of the maximum applied stress. It is along these cleavage planes that the slate can be split to produce thin sheets suitable as a roofing material.
In addition to the maximum applied stress, which results in the development of cleavage, the stresses in the other directions may vary which can affect the shape of individual mineral present ; This is due to the minerals being stretched in the direction of lower stress and at the same time compressed in the direction where it is greater, giving the slate a grain analogous to that of wood. The resulting slates are weaker in the direction parallel to the grain and stronger at right angles to it. Not all slate have a grain -if the stress is same in all directions (perpendicular to the cleavage) no grain results.
The grain of a slate is an important property of a slate yet it is the most misunderstood. probably in areas where the true grain is poorly developed, For example, in Spain it is generally used to describe the intersection of bedding with the cleavage surface while in Scotland it was used to describe the orientation of the crenulation cleavage. In Welsh slate, which has a pronounced grain, it is referred to as the ‘pillaring line’. Here it gives the rock a secondary line of weakness which can be exploited in extraction from the quarry or mine to the the final splitting into roofing slabs. Although the grain is not readily visible in fine-grained slate, it can be recognised in coarse-grained slates by the elongation of individual minerals. It can also be inferred from the alignment of minerals and the shape of reduction spots.