The Use of Classical and New Digital Imaging-based Spectrophotometric Approaches in the Evaluation and Control of Ornamental Stone Finishing Surface Status

Ornamental stone products are commercially classified on the market according to their aesthetical characteristics, that is on the base of their overall visible pictorial attributes. Such attributes results from the interactions of two different groups of factors: the first, intrinsically linked to the material characteristics (i.e, mineralogical composition, textural characteristics, presence of decays and/porosity, etc.); the second, strongly dependent on the production processes utilised to realise the final product. This latter aspect is quite critical because, affecting stone's surface finishing, can contribute to enhance or not certain aesthetic features of the stone itself. When stones are manually worked, that is when a big added value is given to this natural product thanks to human expertise, it is very difficult to define standardized procedures able to perform an objective evaluation of their surface polishing. Such a difficulty also negatively influence the possibility to define “objective” procedures allowing to perform an aesthetic characterization of the hand manufactured stone products in order to define pre-defined classes of surface finishing or to achieve, for a certain stone, a desired level of appearance. These aspects are of primary importance to certify, in pictorial terms, the aesthetic attributes of slabs, tiles and, in general, stone-based products. The study was addressed to evaluate the possibility to develop an innovative methodologies and techniques able to quantify the level of polishing of stone products, evaluating, at the same time, as polishing can affect the effect the overall pictorial evaluation of a stone. To achieve this goal, two strategies have been adopted: i) one based on a classic spectrophotometric approach and ii) the other based on an innovative digital imaging-based spectrophotometric technique. In the first case, a specially-designed probe holder together with a computer-generated spectrum analysis technique was used. In the second case, imagingbased logic, according to intrinsic spectra handling, was applied. In both cases, measurements were performed on various rock samples to quantify their degree of polishing independent of stone surface appearance characteristics, such as colour and texture. The results deriving from both the approaches have been compared and analyzed in order to establish, in respect of the potentialities of each investigated approach, the correlation existing between stone surface polishing status and their quantified aesthetic attributes.