The use of multivariate statistics in the commercial characterization of ornamental stones: the “perlato” Coreno case study

This work regards the quality characterization of the Miocenic limestone of the Coreno Ausonio mining basin (Latium region, central Italy). The aim is to verify the existence of correlations between the various commercial classes produced in the basin and the physical, chemical and petrographical characteristics of the ornamental stone. This work aims furthermore to identify a general methodology for the technical and scientific characterization of the stone’s commercial quality and it is a part of a more ample research work, on part of the author, regarding some types of Italian ornamental stones. The investigation on the material was carried out on two different scales: macroscopic and microscopic. It has concerned those characteristics that define the stone’s state of quality such as the chemical composition and the slabs’ external aspect: the latter was investigated through the use of image analysis techniques. This has enabled to define, for each slab, a colour index (both a general and a background one) and an index of granulometric homogeneity. The presence and frequency of microfractures and the different fossil types were also studied. All the measured variables were elaborated with the aid of multivariate analysis techniques in order to define which of them is more relevant in the definition of the material’s esthetical qualities. By projecting the variables on the factorial planes of the MCA, it is evident how the factors are strongly representative of some of the stone’s characteristics. In particular the factorial variable F1 is an index of color and of the presence of microfractures on the slabs; F2 seems correlated to the quality and quantity of fossils present and must be also considered as an index of the slab’s grain unhomogeneity and high luminosity; F3 can be also considered as an index of high grain unhomogeneity and elevated presence of fractures and algal fossils. The different commercial classes set themselves on the factorial planes showing a quite strong correlation with color, quality and quantity of fossils present and with the index of grain homogeneity, which therefore represent the most important variables for the commercial classification of this ornamental stone.