used to monitor the surface of some natural stones, such as Carrara marble, the Noto Limestone and the Serena Stone subjected to mechanical cleaning subjected to mechanical cleaning by micro-sandblasting.
Therefore, we focus on the evaluation of the effects of cleaning on new natural stones taken from the quarry, through 3D mapping and the roughness profile. The selected samples were analysed with the Alicona IF-Portable optical profilometer before and after cleaning to monitor any morphological changes induced on the marble substrate.
Introduction
In this paper we will analyse the accuracy of 3D optical microscopy to evaluate the impact of a common cleaning technique, called micro-sandblasting.
The cleaning of the stones with a view to preserving cultural heritage is a crucial phase that shows some risks, it is an irreversible process and sometimes can alters the surface and original finishing layer of the building work. There are no real procedures that quantify the degree of cleaning but only a guideline (Mecchi et al. 2008), (Revez, M. J.; Delgado Rodrigues, J., 2016), (Demoulin, T.; et al).
For years, the efforts of research in this sector, have been aimed at finding increasingly respectful solutions towards the materials treated and which, of course, are always in more or less precarious conditions.
To evaluate the effectiveness of the process of removing undesired materials (soiling, surface deposits, alterations of the surface due to degradation, etc.) (ICOMOS, 2008) laboratory tests are frequently carried out, which provide the removal of small portions of material to be subjected to laboratory analysis. (BS8221-1:2012, ASTM C1515-14, ASTM E1857-97).
The possibility of evaluating the removal of soiling, effectiveness in situ in a totally non-destructive way and at the same time quantifying any alterations of the original surface due to degradation is an objective that every conservator should set.
270The Alicona IF-Portable, used for this research, is a portable instrument, therefore it has the possibility of being mounted on a frame or on a trailer in order to carry out on-site acquisitions, thus able to meet one of the minimum requirements that allow the operator to verify the effectiveness of the intervention by monitoring the degree of cleaning and the harmfullness of the treatment, with the possibility of interrupting the operation at any time.
The Alicona IF-Portable microscope proves to be a suitable instrument for this purpose as there is no interaction with the surface, allowing a rapid acquisition.
An attempt was made to evaluate, by means of the Optical Alicona IF-Portable Profilometer acquisition, the efficacy and impact (harmfulness) of the cleaning following micro-sandblasting on stones of different hardness, as such is known to increase surface roughness in some cases (Carvalhão M., Dionísio A., 2015), (Pozo-Antonio J. S., et al. 2016).
The Alicona IF-Portable Optical Profilometer is generally used to perform high resolution 3D surface measurements for the evaluation of production quality in research and development in the laboratory. The key applications for this tool are used for surface analysis and characterization of materials in mechanical engineering, for example for the construction of tools and moulds, for precision mechanics, in the aerospace sector, in the automotive industry, in the field of science materials of all kinds, for the evaluation of corrosion, in electronics and for the development of medical devices. Thanks to its technical specifications, the Focus-Variation technique is used for both shape and roughness measurements (Avagliano R. et al. 2013).
Cleaning methods
There are different cleaning methods: the choice of the same must be from time to time, through laboratory tests, based on the nature of the substance to be removed, the type of surface and object to be cleaned, the nature of the stone material, the type and of the degree of alteration. It is therefore clear that cleaning involves both extremely delicate aesthetic and technical problems that only a specialized technician can solve.
The cleaning techniques most frequently used in the stone conservation sector can be distinguished in chemical, mechanical or laser methods.
In general terms, the cleaning of a historical monument should meet minimum requirements such as: 1) the absence of harmful products which, remaining on the stone, could compromise its future conservation; 2) a cleansing action that does not produce surface irregularities, micro-fractures, increase in porosity, dissolutions, mineral transformations or colour changes in the original material; 3) a sufficiently slow action over time to allow the operator to continuously check the degree of cleaning and stop the operation at the desired time; 4) moreover, they must be gradual and selective processes with affordable costs. (Pozo-Antonio J. S., et al. 2016), (Turk, J., et al. 2019), (Gulotta, D., Toniolo, L., 2019), (Perez-Monserrat, E. M. et al.), (Doehne, E. F.; Price, C. A., 2011).
In this research, only micro-sandblasting was evaluated as it is a recommended technique for cleaning all types of natural stone. Unlike water-based cleaning techniques that are usually applied in the conservation of buildings without historical and artistic interest, dry systems are suitable for the cleaning of stone monuments, in particular those covered with atmospheric particles and other crusts. The use of regular sandblasting is accepted only in certain circumstances, for example on large stone surfaces and on areas without any artistic or historical interest, due to its high aggressiveness caused by high pressure and high hardness of the projected sand (Pozo-Antonio J. S., et al. 2016).
Material and methods
The possible harmfulness of the cleaning method with regard to the stone material is generally assessed only with laboratory tests, water absorption and water absorption coefficient by capillarity. The method in question was applied, with techniques 271as close as possible to those that will be adopted on site, on perfectly shaped specimens, perfectly polished 50 × 50 × 20 mm plates.
Table 1: Porosity properties of the three materials tested.
| Type of materials | Water absorption As (%) | Porosity in Vol (%) | Apparent density g/cm3 | abrasion measure [mm] |
| Carrara marble | 11.05 | 0.25 | 2.69 | 22.0 |
| Serena Stone | 4.05 | 3.15 | 2.65 | 18.0 |
| Noto Limestone | 18.05 | 33.04 | 1.86 | 42.00 |
In this research, three types of stones of different hardness and abrasion resistance were chosen:
Carrara marble: a white fine-grained crystalline marble widely used for decorations, it is a compact stone with low porosity and very low resistance to chemical attack, high resistance to weathering, an apparent density of 2.69 g/cm3, and a total abrasion measure of 22.0 mm (UNI EN 14157:2017).
Stone Serena: a gray sandstone,
