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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, is a sedimentary stone with a fine to medium-large grain size, compacted with a clay matrix. With an almost uniform light cerulean color, it shows shiny dots marked by the presence of mica flakes. The blue-gray color due to a chemical reaction in the stone, it turns to into a red color. This is due to oxidization in the clay matrix of the stone. The Pietra Serena used widely for architectural details in Italy during the Renaissance is commonly used for outdoor flooring in Northern Italy, it is a hard stone with low porosity. It has a high resistance to chemical attacks and erosion, and low resistance to atmospheric agents due to pollution that leads to the formation of black crusts. Its bulk density is 2,65 g/cm³, and the measured abrasion is 18,0 mm.

Noto Limestone: a yellow sedimentary sandstone mainly composed of organogenic limestone (formed by sediments deriving, in different ways, from living organisms) with fossil inclusions and polygenic gravel, formed in the Miocene period. The Noto Limestone is commonly used for historical constructions in Southern Italy, it is a soft stone with high porosity (33 %) and low resistance to chemical attacks. Due to its easy workability, it is also used as a decorative element typical of the Sicilian Baroque. Very low resistance to weathering due to its low effectiveness in withstanding damage, an apparent density 18,56 g/cm³, abrasion measure of 42.00 mm.

In table 1 the porosity characteristics of the three materials used for the tests.

Before carrying out the cleaning by micro-sandblasting, the 50 × 50 × 50 stone samples were superficially polished to obtain a surface as homogeneous as possible, in order to better quantify any alterations related to cleaning. The polishing was carried out through wet petrographic polishing using abravise papers. The Noto Limestone and the Stone Serena, we were unable to polish them perfectly.

The precision micro-sandblasting was carried out with a precision micro-gun (Colibrì 8) with a 2 mm nozzle that shoots siliceous river sand (no powder < 0,063 mm) with grains of about 0.25 mm diameter an with an outlet pressure of 2 bar and an operating time of 1 and 2 minutes.

For the assessment of damage due to mechanical cleaning, an Alicona IF-Portable, a three-dimensional optical device based on non-contact Focus-Variation was used.

Position volume (X, Y, Z): 50 mm × 50 mm × 26 mm, lens magnification used: 5X, lateral measuring range (X): 3.52 mm, lateral measuring range (Y): 2.64 mm, and vertical resolution: 7.58 µm.