Aesthetic characteristics of roofing slates – part I
02/17/2013 1 Comment
Color, brightness and texture
The aesthetic characteristics of roofing slates can be defined by the color, brightness and texture. These three parameters are to be taken into account when choosing a slate variety, but also are essential in case of replacement a slate tile in a roof due to repairing or restoration. Traditionally, both slate producers and customers have been referring to the color with somewhat vague terms, such as gray, gray-blue, black, etc. These terms can easily lead to confusion.
Today it is possible to measure color and brightness precisely on any object, including slates. One of the first jobs I did in slates was the measurement of these parameters in slates of the whole Iberian Peninsula. The results show a great uniformity in most of the slates.
Besides the color, the other two parameters that determine the aspect are brightness and texture. The brightness depends basically on the crystallization and orientation of the mica minerals, while the texture depends on the grain size and the traces of the deformation phases on the slate. The most characterisitc of these traces is the intersection between the slaty cleavage and the sedimentation and which forms the lineation. This structure is known among the miners as hebra (Spain) or longrain (UK), and has a decisive role in many of the properties of the slate tile.
In the Iberian Peninsula, and from a geological point of view, the Ordovician (ORDmid and ORDup) slates from Galicia and Leon present colors a bit lighter than the Devonian slates (DEV) of Villar del Rey in Extremadura, while Bernardos Precambrian (PRE) slates in Segovia are light gray, and finally Cambrian slates (CAM) from Lugo are light green.
Cárdenes, V., Prieto, B., Sanmartín, P., Ferrer, P., Rubio, A., Monterroso, C., 2012. The influence of chemical-mineralogical composition on the color and brightness of Iberian roofing slates. J. Mater. Civ. Eng. 24, 460-467.