Violin Varnishes: Microstructure and Nanomechanical Analysis

Abstract

The aim of the current work is twofold: to demonstrate the application of in situ non-invasive imaging by portable atomic force microscopy (AFM) on the surfaces of a violin and to integrate compositional and mechanical analysis at the nano scale level on model samples of varnished wood. These samples were prepared according to traditional recipes by an Italian lute-maker family well practised in the art. Samples of oil and spirit-based varnishes on maple wood, naturally and accelerated light aged, were studied. AFM was used to measure the nanomechanical properties of the model samples and established that the spirit-based varnish was stiffer than the oil-based. Synchrotron radiation micro- Fourier Transform Infra-red analysis of the layer structure revealed that stiffer spirit-based varnish showed less penetration into the wood than the oil-based. Further PeakForce Quantitative Nanomechanical Mapping (QNM) demonstrated a difference in adhesion values between the oil- and spirit-based samples.

Keywords: AFM; SR micro-FTIR; in situ imaging; nanomechanics; violin.

Figure 1

(a) Nanosurf Easyscan 2 AFM head on a violin placed on an anti-vibration table (b) closer view of AFM head on the violin placed on plastic spacers to protect the surface of the violin and showing location of the AFM tip (black arrow) (c,d) AFM images (20 μm × 20 μm) recorded directly on the violin surface in two different locations [location (c) z-scale image = 1.28 μm/Roughness Ra 0.176 μm; location (d) z-scale image = 2.03 μm/Roughness Ra 0.291 μm].

Figure 2

Model systems optical images together with AFM images of samples (1 to 6) before ageing: oil-based (1 and 5); AFM image 1, z scale image = 159.4 nm/Roughness 19.1 nm; AFM image 5, z scale image = 164.5 nm/Roughness 18.2 nm; and spirit-based varnish layers (2, 3, 4, 6); AFM image 2 z scale image = 235.6 nm/Roughness 10.5 nm; AFM image 3, z scale image = 522.6 nm/Roughness 33.6 nm; AFM image 4, z scale image = 823.9 nm/Roughness 18.6 nm; AFM image 6, z scale image = 260.0 nm/Roughness 14.5 nm on top of maple wood over preparation layers of various compositions (Table 1, described in Section 4.1.2).

Figure 3

(a–d) shows SR-micro-FTIR mapping for the naturally aged oil-based varnish (Figure 2 sample 1): chemical image mapping of the absorbance at (a) 1000 cm⁻¹ and (b) 1735 cm⁻¹, respectively, and (c) photomicrograph of the microtomed cross-section of width 10 μm. (the squared area outlined in black has a depth of 250 μm and indicates depth of penetration of the oil into the wood); and (d) FTIR spectra of layers (1, 2, 3) as shown in (b) and corresponding to varnish, penetration areas of oil in wood, and oil, indicating high penetration into the wood porous structure (around 200 µm). (e,f) shows mechanical data and distribution of Young’s modulus in the unaged which, at time of analysis, had undergone four years of natural ageing in the dark (e) and the artificially accelerated aged sample (f) where median values of Young’s modulus are (7.04 +/− 1.75) GPa and (57.2 +/− 5.1) GPa, respectively.

Figure 4

(a–c) shows SR-micro-FTIR mapping of the naturally aged spirit-based varnish (Figure 2 sample 6): chemical image mapping the absorbance at 1000 cm⁻¹ (a) and 1700 cm⁻¹ (b) respectively. FTIR spectra (c) representative of the varnish layer (1-blue line) and the underlying wood (2-red line) in (b). The spirit-based varnish remains on the surface with a clear separation from the wood as shown by the difference in spectra for layers 1 and 2. (d,e) shows mechanical data and distribution of Young’s modulus in naturally aged (4 years) (d) and artificially accelerated aged (e) samples where median values of Young’s modulus are 29.7 +/− 3.1 GPa and 75.7 +/− 9.1 GPa, respectively.

Figure 5

Mechanical data and distribution of Young’s modulus obtained for sample 2, 3, 4 and 5.

Figure 6

The PeakForce adhesion image of the naturally aged spirit-based sample (sample 6) is shown in (a) and the image for naturally aged oil-based varnish sample is shown in (b). Bearing analysis gave large differences in the distribution of adhesion values for the two samples (c): the oil-based varnish has a narrow range of adhesion values while the spirit-based has a broad distribution and can be fitted to 2 curves, indicating the possibility of greater heterogeneity in the mechanical properties of the surface of the spirit-based varnish.