- freely available
Sensors 2019, 19(19), 4335; https://doi.org/10.3390/s19194335
2. Sample Description
- lean: in case glues (animal or vegetal forms) or eggs are used as binders of the pigments.
- fat: in case an emulsion based on egg and oils or resins is used as a binder.
- The realization of the frame: a wooden frame (20 cm × 30 cm × 2 cm), as shown in Figure 2, was selected and the canvas was tensioned onto it.
- Preliminary preparation of the canvas: a rough linen was chosen, and it was cut larger than the wooden frame, frayed along the edges, washed in hot water, dried and ironed. The support was about 1 mm thick and had a regular weft-warp weave in a 1:1 ratio.
- Realization of Defect A: as first defect, a diagonal cut of 3 cm long was produced in the canvas and, subsequently, it was stitched to simulate the repair of a torn canvas (Figure 3a).
- Tensioning of the canvas: the canvas was tensioned on the frame by means of metal clips applied with a staple gun, fixing one side at a time. Firstly, one side was fixed with a central paper clip, then the same process has been repeated for the opposite side and, finally, this was done also for the remaining two sides; all the sides were pinned using twenty-one staples. Note that the canvas was tensioned with care as the remaining steps influence the final tension itself (Figure 3b).
- Dressing of the canvas: since the rough linen does not appear as a surface ready to receive the pictorial layer, a preparation step was required for this purpose. The first step followed the ancient technique, i.e., the canvas was waterproofed with a laying of rabbit glue dissolved in water at a ratio of 1:7 (Figure 4). The glue was left swelling for an entire night in cold water and then heated in a bain-marie to get it completely dissolved. The application of hot glue onto the canvas’s surfaces was carried out using a soft bristle brush. The so-obtained layer was left to dry for 48 h.
- Inclusion of Defect B: a Teflon insert with dimensions equal to 1.1 cm × 1.5 cm was placed onto the dried rabbit glue layer and folded up three times, so as to simulate a detachment between the canvas and the next layer of glue and plaster (Figure 5a,b). Defect B is located at about 2 mm depth from the final upper layer, i.e., the inspected surface.
- Spreading of the first preparation layer: a layer of about 1 mm of thickness made of Bologna plaster mixed with rabbit glue was applied (Figure 6a,b). The plaster was added to the glue until saturation, i.e., once the desired density was obtained. The laying was done using a soft bristle brush. This layer was left to dry for 48 h.
- Realization of Defect C: a network of cracks was crafted onto the previously realized layer with a maximum depth of about 1 mm. This was realized on the fresh plaster layer by means of engravings produced using small plastic chisels (Figure 7).
- Realization of Defect D: as for Defect C, a second net of cracks was fabricated on the still fresh plaster layer, this time within a different area of the mock-up (Figure 7).
- Sanding of the surface: after a complete drying, the surface was smoothed with a fine-grained abrasive paper.
- Isolation of Defect D: an amount of animal glue was injected into the crack network, so as to physically separate the cracks from the next preparation layer (Figure 8).
- Insertion of Defect E: as for Defect B, once both the plaster and glue layers were completely dried, a second Teflon insert (dimensions: 1.1 cm × 2.5 cm) was applied on the new surface—it was folded only once on itself (Figure 9a,b).
- Spreading of the second preparation layer: above the first preparation layer, a second layer (1 mm thick) of Bologna plaster mixed with rabbit glue was applied in the same way that the first layer was realized (Figure 10a,b), thus completely covering Defect E and all the previously-realized steps.
- Sanding of the surface: as for the first layer, the second layer was worked with fine-grained abrasive paper once dried properly; in this way, a surface finishing suitable for receiving the pictorial layer was obtained (Figure 11).
- Defect F: a crack with a length of about 15.5 cm was accidentally produced near Defect E; it was deliberately decided to preserve such natural defect and then cover it with the pictorial layer (Figure 12).
- Insertion of defect G: the last Teflon insert (dimensions: 1.1 cm × 3.5 cm) was added onto the surface, this time without being folded. It is located between the last preparation layer and the primer for pictorial drawing (see Figure 13a).
- Realization of the drawing: once the sample preparation was finalized (i.e., the surface suitable for the pictorial layer was realized), the representation of the selected character was reproduced by drawing the main lines. This was done using charcoal (Figure 13a). As mentioned, the selected detail of Botticelli’s painting is the face of Venus (Figure 1). In addition, a signature of the restorer who fabricated the sample (Figure 13a,b), along with two pentimenti (Figure 13a—see the part around the chin to understand the position of the first pentimento) were added to the sample using charcoal. The second pentimento is in the form of a wrongly positioned eyebrow over the right eye.
- Priming: first, a basic colored pattern was obtained for the pictorial layers, which was lighter for the sky and darker for the areas relative to the locks of hair of Venus (Figure 14).
- Pictorial drawing: for this step, a greasy tempera for painting was used. In addition, powder pigments were added as a binder, together with an emulsion consisting of an egg yolk, a teaspoon of linseed oil and two drops of vinegar. It should be noted that a greasy tempera based on egg and oil was commonly employed in the fifteenth century, especially in a transitional phase from painting on panel to oil painting on canvas. However, it was chosen to add vinegar to guarantee the preservation of the tempera. The mock-up was finally completed by a series of overlapping pictorial backgrounds realized by means of brushes of marten hair (Figure 15), and then by mixing the right amount of pigment diluted in water along with the binder each time.
- Finishing: a coat of a natural resin-based paint was spread on the paint layer using a brush (Figure 16).
3. Hyper-Spectral Imaging
4. Pulse-Compression Thermography
5. Integration Among PuCT, HSI and Post-Processing Analyses
6. Results and Discussion
Conflicts of Interest
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