The commercial characterization of ornamental stones through the use of multivariate statistics: the roman travertine case study

The present work regards the commercial characterisation of the roman travertine, an ornamental stone of great value and ancient tradition, with the aim to define the existing correlations between the different commercial types and the physical and mineralogical characteristics of the stone; all this in order to establish an objective criteria for the definition of slab quality and also to conceive a common methodology for the technical and scientific characterization of the commercial aspects of ornamental stones. As a matter of fact there is no technical or esthetical method in this field that can objectively define the quality of the slabs of a certain stone from a commercial point of view. The slabs of each travertine variety were analysed from a mineralogical point of view and then aesthetically characterised through image analysis procedures, showing their homogeneity, brightness, colour index, band pattern and porosity. All parameters were then processed through the aid of multivariate statistical techniques, in order to define which of them are more significant in the stone’s esthetical characterisation. By projecting the system’s variables on the factorial planes of the statistical analysis (MCA), factor F1 seems to be an index of the mineral composition and of the number of colours, being also able to discriminate among the various commercial types of roman travertine; F2 represents the average brightness and bands homogeneity, discriminating at the same time among the commercial quality of the slabs; F3 is an index for homogeneity, surface porosity, number of colours and it allows an easy discrimination among the qualities of the investigated typologies. Variations in the travertine’s commercial types are strongly related to mineral composition (contents in Sr and Mg), band pattern, brightness and colouring of the slabs. The Classico travertine is characterised by low tenors of the above mentioned elements, by average brightness and scarce colouring; the Bianco and the Alabastrino by intermediate tenors and high brightness; Noce and Striato by high tenors, low brightness and evident colouring. Discriminations within quality classes in the same commercial typology are instead related to variations in unhomogeneity, porosity and colouring of the slabs: specifically, an increase in quality for the different types is directly related to a decrease in these parameters.