Siegfried Siegesmund - Monument Future

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.

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.

Figure 4: 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 red line shows the measurement line.

Figure 5: 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 red line shows the measurement line.

The analysis with 3D Optical microscopy profilometer has highlighted quantitatively how a routine mild cleaned can alter the surface of a stone material.

The analyses have highlighted that in an abrasion-resistant material such as marble the effect of the impact of the grains of sand on the surface varies in relation to the angle of impact of the single grain, which in turn affects the roughness of the material by altering its surface polishing.

In this research, three materials of different hardness and abrasion resistance were analysed, and in fact it was observed that according to the morphology of the samples, the alterations generated by the treatment increase the roughness and in the case of the samples that were not very resistant to abrasion, such as the Noto Calcarenite Lime stone, also involved significant loss of material compromising its macroscopic appearance.

In an abrasion-resistant material, such as Serena stone, which apparently does not show any alteration of the surface visible to the naked eye, an increase in roughness is observed microscopically which increases the surface absorption.

This new technique is applicable as an alternative and/or in addition to optical color measurements during laboratory tests and complementary to investigations such as SEM (Schroettner et al. 2006).

274As a future perspective of the research, the possibility of creating an algorithm for the evaluation of surface porosity variations is being evaluated by comparing the data obtained with Alicona, mercury porosiment and micro-CT.

ASTM C1515 – 2014 Standard Guide for Cleaning of Exterior Dimension Stone, Vertical and Horizontal Surfaces, New or Existing.

ASTM E1857−97 (Reapproved 2013) Standard Guide forSelection of Cleaning Techniques for Masonry, Concrete, and Stucco Surfaces.

Avagliano R., Guidetto V., Martini G., Randone E., Riccardi M. P. 2013. Artwork analisis by optical surface profilometry. Conference 8790: Optics for Arts, Architecture, and Archaeology IV 15–16 May 2013 Proc. of SPIE Vol. 8790, pp. 1–4.