Monument Future - Siegfried Siegesmund

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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.

       Experimental Results and Discussion

      The surface of each specimen (50 × 50 mm) was divided into four areas of 25 × 25 mm, thus dividing the surface into 4 quadrants.

      Cleaning was not carried out on one of the 4 quadrants, because it was left as the control field surface of the untreated material, while the second quadrant was treated by cleaning with water.

      Before starting the cleaning of the three quadrants, a Plexiglas mask was placed on the surface 272of the sample to avoid damaging the part not to be cleaned.

      Figure 1: Three types of stone of different hardness and abrasion resistance: Carrara Marble, Serena Stone and Noto Limestone.

      Figure 2: The Alicona IF-Portable Optical Profilometer used to perform high resolution 3D surface measurements.

      The two remaining dials were treated with a micro-sandblaster, the first at a constant pressure of 2 bar for 1 minute, while the other dial was treated for 2 minutes. The two remaining quadrants were treated with a micro-sandblaster, the first at a constant pressure of 2 bar pressure for 1 minute, while the other quadrant was treated for 2 minutes. The sandblaster was held at a distance of 40–50. The 3D scans with profilometer and the optical observation with Alicona and static contact angle test were carried out before and after the treatment.

      In order to assess the impact of micro-sandblasting on the samples, all areas were scanned. The 3D scans were acquired with 5x magnification from which the profiles that allowed to compare the differences in surface roughness.

      Carrara marble: the untreated surface appears as a homogeneous and has a roughness in the range of 10 µm. The treated surface has an increase in roughness of 20–30 µm and the creation of some porosities with a maximum diameter of 300 µm, and a depth of 80 µm. a table with the results would be important.

      The hardness of the material preserves it from the loss of mass, the extreme delicacy of the marble is also highlighted, a small variation in the surface roughness is clearly reflected on the macroscopic aspect of the object by altering its brightness. In fact it is possible to see from Figure 1, how the specimen appears damaged on the surface, but it is only a change of opacity without loss of material, opacity that is also obtained by water cleaning. Serena Stone: The 3D acquisition highlights a non-homogeneous granular material with a surface roughness on the range of 30 µm and the presence of numerous natural pores the roughness depends very much on the production or preparation process.

      The cleaned surface does not show an increase in roughness and porosities with a depth of 50–60 µm 273are highlighted. Cleaning did not have a great impact on the total roughness and therefore also a limited macroscopic effect, but observing the profile data, we find some variations in the pore diameter and therefore in the surface porosimetric distribution.

      Noto Limestone: From the 3D acquisition, the material is immediately very rough and uneven, with a roughness ranging from a few µm to 40–50 µm. The acquisition after treatment highlights a conspicuous loss of material and an increase in roughness this is typical for this type of limestone. Its very soft nature makes it the most threatened sample by micro-sandblasting.

      Figure 3: 3D images representing the Carrara marble sample before and after cleaning by micro-sandblasting and its corresponding graph of surface roughness before and after cleaning. The blue line shows the measurement line.

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