IMEKO International Conference on Metrology for Archaeology and Cultural Heritage Torino, Italy, October 19-21, 2016 Measuring Stradivari violin “Cremonese” (1715) by 3D modeling Piercarlo Dondi 1,2*, Luca Lombardi 2, Marco Malagodi 1,3, Maurizio Licchelli 1,4 1University of Pavia, Arvedi Laboratory of non Invasive Diagnostics, via Bell’Aspa 3, 26100 Cremona, Italy, [email protected] 2University of Pavia, Department of Electrical, Computer and Biomedical Engineering, via Ferrata 5, 27100 Pavia, Italy, [email protected] 3University of Pavia, Department of Musicology and Cultural Heritage, Corso Garibaldi 178, 26100 Cremona, Italy, [email protected] 4University of Pavia, Department of Chemistry, via Taramelli 12, 27100 Pavia, Italy, [email protected] Abstract – Acquiring precise measurements of histori- ness [1]. Moreover, damages on the varnishes could be a cal violins is an important task both for researchers and key factor of the material degradations, especially biolog- violin makers. Up to now, the measures are taken man- ical ones, with a consequent requirement of a restoration ually using a caliber, with limitations in the repeatabil- work [2]. A faithful 3D model of a violin can be very help- ity, since the access to these instruments is restricted ful in this field. With a 3D model it is possible to repeat and the contact with the caliber has to be limited to the several times the measures without risks and also reach ar- bare essential for avoiding accidental damages to super- eas nearly inaccessible on the original instrument without ficial varnishes. In this work we propose a new protocol opening it (e.g. the highness of the belly). for the acquisition and creation of high quality 3D mod- The digitalization of historical relics is becoming a stan- els of violins suitable for taking precise measurements. dard practice in many museums and archaeological sites. The procedure was applied to Stradivari “Cremonese” Different Imaging technologies are available (laser scan- (1715) kept in “Museo del Violino” in Cremona. The ner, photogrammetry, stereocams and so on), the final quality of the result was validated comparing the mea- choice depends on the characteristics of the object to ac- sures taken on the 3D mesh to those ones taken by the quire [3]. These different techniques may be merged to curator of the museum on the original instrument. As improve the quality of the reconstructed models, as an ex- a test case the data are then used to confirm the use of ample both photogrammetry and 3D scanning were used the “G” mould for the construction of this violin. to create a model of the Donatello’s “Maddalena” [4]. Mathys et al. presented a comparison among the differ- Keywords – 3D scan, measuring, historical violins ent techniques, including also Computer Tomography; as a conclusion the authors suggest to use laser scanners when I. INTRODUCTION only external surfaces are needed and Computer Tomogra- An important goal in the study of the morphology of an phy if it is useful to acquire internal structures [5]. Also ancient relic involves the acquisition of precise measure- the possibility to use low cost devices, such as the Kinect, ments. This task is particularly valuable in the case of his- has been demonstrated for the reconstruction of excavation torical violins, not only for researchers, but also for violin sites [6]. The acquisition of a historical violin is particu- makers, that can use these data as reference for the making larly critical, the complex morphology and the presence of of their instruments. Up to now, the classic way to measure a lot of small details require a very high precision in phase a violin with precision is the manual use of a caliber. Even of scanning; moreover the presence of highly reflective if this procedure is standard with modern instruments and varnishes made complex or nearly impossible the use of can be repeated several times, it can be performed only oc- classic photogrammetry techniques. For these reasons, the casionally on historical ones. This is not only due to the digitalization of violins are generally obtained by CT-scan. restricted access but also to the need to limit to the bare This is surely the most precise existing scanning technol- essential the direct contact with the caliber, in order to not ogy, it can acquire both external and internal surfaces of an ruin the ancient varnishes or even (in the worst scenario) instrument and provide important information, such as the chip the wood. The varnishes in particular could represent thickness of the ribs, or the presence of internal damages a limit in the application of mechanical measurments, be- and repairs [7]. More recently, the Smithsonian institution cause of their historical importance, brittleness and thick- adopted the CT-scan for the study of seven Stradivari vio- 29 section v.. II. ACQUISITION AND 3D MODELING As previously stated, the complex morphology of a vi- olin and the high reflectivity of the varnishes make diffi- cult the acquisition with a laser scanner. Even if a perfect solution does not exist some characteristics are crucial in the choice of the scanner to adopt: a high level of mo- bility, in order to reach all the surfaces of the instrument, and a high accuracy, in order to acquire as much details as possible. For these reasons we chose a “RS3 Integrated Fig. 1. Overall schema of the proposed procedure of ac- Scanner” (a linear laser scanner with a stated accuracy of 30 quisition, 3D modeling and measurement of a violin. µm ) mounted on a mobile arm with 7 degrees of free- dom (Romer Absolute Arm 7-Axis "SI") both produced by Hexagon Metrology. The suite Polyworks was used as scanning software and for cleaning the raw data. lins and proposed a detailed protocol for their acquisition and modeling. They also demonstrate the usability of such Since wood instruments are sensible to abrupt variations models for measurements tasks [8]. The use of laser scan- in humidity and temperature, that can alter their morphol- ners is instead very limited, since they can acquire only the ogy, the working laboratory was always maintained at the external surfaces. Only some notable cases are described same climatic condition of the showcases of the museum, 20 ◦ 50% in litterature, such as the analysis of elastic deformation a temperature of and a humidity of . Before the performed on Guarneri del Gesú “Cannone” (1734) [9]. scanning all the mobile parts of the violin that can inter- However, some advantages of this technology with respect fere with the acquisition are removed: the tailpiece, the to CT-Scan must not be neglected: it is more affordable; bridge and the chin rest because they hide part of the belly; it only needs one people to perform the acquisition instead the pegs because they partially hide the outer wall and the of a team of experts; it can be used directly in the museum pegbox floor of the head; the strings because they can al- without the need to move the instrument. Moreover, in our ter the light beam of the scanner. Their loss is acceptable, field of interest the limitation to the external surface it is they are not strictly relevant for measurements and gen- not critical, since the most of the important measures in- erally they are not the original ones (quite all the mobile volve the outside of the violin. parts of historical violins have been substituted during the centuries). In this work we propose a protocol for the creation of ac- curate 3D models of historical violins, suitable to take high We performed a series of tests on sample violins to tune precision measurements. The main steps of our workflow the acquisition protocol. The morphology of the instru- are summarized in Figure 1: the raw data were acquired ments and the limitations of the laser scanner were studied using a high resolution laser scanner and then refined and in deep. We identified the most critical areas, the best way validated in an iterative way. The procedure was applied to acquire them (or at least the most of them), the errors to an important instrument held in “Museo del Violino” of to avoid, and the typical corrections needed for obtaining Cremona (Italy), the “Cremonese” (1715) made by Anto- a good final mesh. After this tuning phase we applied the nio Stradivari. A comparison between measures taken with procedure to Stradivari “Cremonese” (1715). The instru- CAD software on the 3D mesh and measures taken with ment was acquired in four steps, generating many partial a caliber on the original violin was performed. As a test scans that will be aligned and connected in post-production case, the data obtained are used to confirm what mould was later: left side of the body, right side of the body, front side used by Stradivari in the construction of this important in- of the head, back side of the head. Two supports in Plex- strument. A preliminary description of this last result was iglas, a vertical one (Fig. 2(a)) and a horizontal one (Fig. previously presented in the celebrative book for the 300 ◦ 2(b)), were designed to place the instrument in the optimal anniversary of the “Cremonese” [10]. In the current paper positions for reaching as much details as possible. In par- we provide a more in deep explanation of the procedure ticular, the vertical support was used for the body (better adopted. for reaching with the laser the internal edges of the f-hole and the part of the top plate under the fingerboard); the The paper is organized as follow: section ii. describes horizontal one for the head (better for reaching the pegbox the proposed acquisition and the modeling protocol under- floor, the peg holes and the throat of the scroll).