REFLECTED GLORIES

Viewing instruments in different kinds of light has become standard in documentation and assessment. Leonhard Rank explains how, in addition to ultraviolet light, researchers can now use infrared, and even parts of the visible spectrum, to reveal even more secrets

Violin by David Tecchler under (left–right) visible light; blue (460nm with filter); cyan (490nm) and ultraviolet light (320–400nm)

Since as early as the mid-20th century, historical art objects such as stringed instruments have been examined and appraised not only in visible light (VIS), but also with light in other spectral wavelength ranges. This has nowadays become a significant part of detailed examinations, as it makes visible several features that would otherwise go undetected, to visualise and interpret the condition of an instrument more fully. Information gained in this way can be helpful in determining the instrument’s condition, both for the purposes of documentation and also as an aid to restoration work.

The most common technique, which has been used almost exclusively until now, is examination with ultraviolet light (UV). It lies in the range that is invisible to the human eye, and in the commercial lamps we use is usually restricted to the ‘UVA’ wavelength range: 320–400nm. When using this light for analysis, what we can see is essentially the fluorescence induced by the light, which in this wavelength range is well differentiated and rich in contrast. An experienced observer will be able to differentiate between original varnish and retouching, damage, or the addition of new material.

A particular characteristic of this wavelength range is, put simply, the ‘obscuring’ behaviour of fluorescence. This usually leaves only the uppermost fluorescent layer visible, and covers up the underlying layers. The effect is very helpful in restorations where, for example, the removal of retouching or excessive protective varnish is required, because it makes it possible to define the exact transition between original varnish and nonoriginal additions.

This overlying fluorescence can, however, often be a disadvantage in establishing the quantity of original varnish and in pinpointing its location. As soon as the original varnish is no longer unpolished and without any evidence of retouching (as is the case with only a very few instruments), the overlying fluorescence of these additions makes it difficult to assess its state of preservation precisely.

In this respect, documentation is usually only moderately informative, and therefore sadly unsatisfactory, when this conventional method of analysis in UV light is used – especially given that the properties described here can be very different in other spectral light ranges, as we shall see.

In 2019 we embarked on a comprehensive restoration and documentation of the c.1690 ‘Barjansky’ cello by Antonio Stradivari (see The Strad, May 2021). The research was subsequently published in 2021 by Jost Thöne Verlag in cooperation with Art & Strings. In this project, we introduced a further wavelength that makes use of the less obscuring properties of fluorescence. For this, we used forensic lamps that extended the wavelength range from 320nm to 840nm, i.e. from UV right up to the infrared (IR) light spectrum (figure 1).

FIGURE 1 Treble side f-hole of the c.1690 ‘Barjansky’Stradivari cello under UV light (320–400nm)

FIGURE 3 Comparison of ‘Barjansky’ Stradivari cello under visible light, UV (320–400nm) and cyan (490nm)

These lamps (Lumatec Superlite S04 and Superlite M05) made it possible for us to examine the glazing and fluorescent characteristics of the varnish within the widest possible spectral range (figure 2).

For our purposes, many of the images that emerge from the different wavelengths can only be interpreted with a moderate degree of accuracy. Images in these colour concentrations do not correspond with our visual expectations. When illuminated with a pure blue light (460nm), for example, bright purple fluorescence is visible in the varnish. Such an image is almost impossible to interpret, especially for an inexperienced observer.

However, a surprising observation could be made at a wavelength of 490nm, the colour range between blue and green: in other words, cyan light. This clearly showed that, even when viewed with the naked eye, retouching, protective varnish and patina mostly do not fluoresce, or fluoresce differently.

In contrast, the original varnish beneath these overlying layers shines forth with a distinct orange-coloured fluorescence.

The ‘obscuring’ fluorescences, which rendered the original varnish invisible when observed with UV light, have disappeared.

When one compares the fluorescences of the cyan wavelength (490nm) with those of UV light (320– 400nm) in suitable areas of varnish, it is noticeable that the two methods consistently identify the colour varnish the same way. Only the colourless ground coat, considered typical of Stradivari and his contemporaries (luminous white in UV light), is no longer visible at this wavelength without the use of optical filters (figure 3).

An important tool for such investigations is photography, as the highly sensitive camera sensors offer an advantage over viewing with the human eye. Furthermore, the adaptation possibilities in raw format (for example TIFF files) are very important for visualisation. Using image processing that has been adjusted specifically for an instrument (white balance; colour correction), the effect of the orange fluorescence can be further enhanced (figure 4, next page).Ideally, the data generated from this could be evaluated in such a way that it can be used not only for quantitative analysis of an instrument’s original colour varnish, but also in establishing the thickness of the remaining original varnish.

FIGURE 2 Light wavelengths from ultraviolet to infrared

ALL IMAGES LEONHARD RANK/ART & STRINGS ARCHIVE

FIGURE 4 ‘Barjansky’ treble side f-hole (as in figure 1) but under cyan light (490nm)

FIGURE 5 Quantitative evaluation of the varnish in the ‘Barjansky’ cello under cyan light (490nm). Yellow–red–brown = sparse–moderate–abundant

FIGURES 4 AND 5 LEONHARD RANK/ ART & STRINGS ARCHIVE

However, the image must not be obscured by a high concentration of impurities or retouching. This was possible with the ‘Barjansky’ cello. Through schematic colour representation, we were able to obtain a particularly clear picture of the typical, predominantly natural, wear on a cello of this age (figure 5).

Examination at this wavelength of 490nm has produced interesting results, not only with Stradivari’s instruments but also with those of other historical luthiers. It is possible to see that the fluorescence definitely differs in intensity and colour depending on the instrument (figure 6).

Clearly, fluorescence also depends on the intensity of the colour saturation in the varnish. However, the question as to which substances, or combination of substances, in the varnish produce this specific fluorescence is currently just as unanswerable as it was with UV light analysis. While it is logical to assume that pigments or colouring agents and their concentration influence the intensity and type of fluorescence at this wavelength, it must also be noted that colourless or only slightly coloured components of a varnish in visible light, such as resins or drying oils, fluoresce in cyan light and can influence the results. This has been corroborated by specifically designed tests carried out with different materials typically used in the production of varnish.

On instruments with very little varnish, or where the varnish exhibits hardly any colour saturation, examination with a wavelength of 490nm is sometimes less informative. With cyan light at an oblique angle of incidence it is usually possible to distinguish a bit more (presumably since the fluorescent particles appear closer together when viewed from this angle). However, the results are mostly less than satisfactory (figure 7a). In this case it can be helpful to choose a light with a somewhat shorter wavelength. As previously mentioned, the colour representation at 460nm (blue) is visually very unfamiliar (figure 7b). This can easily be modified with the aid of an optical filter, which also provides two additional benefits.

Firstly, as with UV light, a longpass filter (such as eyewear or a camera filter) can be used for viewing in order to eliminate the incident beam that is projected on to the instrument. Only the fluorescences displayed primarily at other wavelengths – i.e. in other colours – become visible (whereas using a longpass filter only reveals the long-wave frequencies).

This optical trick is not particularly spectacular in UV light, as we cannot perceive this frequency range with the naked eye anyway, and a camera direct from the factory has an inbuilt filter that blocks out this wavelength range and, similarly, the infrared range. If, however, one uses a longpass filter when analysing at 460nm, which filters out shorter-wave light spectra up to 500nm, the result is a much more familiar-looking picture, as in the right-hand image of figure 7b. As a result, the obscuring fluorescences at 460nm increase somewhat, even with a filter, but the original varnish, which exhibits little colour saturation in cyan light (490nm), can also be much more easily identified. Another positive effect is that the uppermost layers of protective varnish, which often appear white, remain largely or even entirely transparent.

FIGURE 6 Instruments under cyan light: top row c.1690 Stradivari cello; c.1700–10 Testore violin; 1709 Stradivari violin; c. 1750 N. Gagliano violin; bottom row 1780–90 Varotti violin; c.1790 Lupot violin; 1848 Pressenda violin; 1829 Gand violin

The second benefit of this method is that unwanted reflections from the actual light source are filtered out by the longpass filter (the effect is similar to that of a polarisation filter in VIS). These reflections are a significantly pronounced disturbing factor, particularly in the blue light spectrum.

The advantageous qualities of this application are therefore once again different from those gained through exposure with just UV or cyan light. Consequently, this method is not only of value for instruments where analysis with cyan light yields insufficient information, but also as an enhancement that bridges a gap, a mixture between UV and cyan light.

These light examinations can only be used to a limited extent in determining the age of the varnish. In principle, however, this method can provide evidence of a tradition in the layering of varnish. Yet some contemporary violin makers have achieved such a high level in varnish production that, when using the previously mentioned methods of analysis, the fluorescence of the new varnishes exhibits similar qualities to those of, for example, the old Cremonese instruments (figure 8). Therefore, differentiation between the genuine and the inauthentic, or moreover the new and the old, will continue to be possible only with professional experience. The use of different light sources does make this task easier, however, and opens up a better and more reliable perspective on the history of an instrument.

FIGURES 6 AND 7 LEONHARD RANK

FIGURE 7A A 1722 Tecchler violin under (left–right) visible light, UV light (320–400nm) and cyan light (490nm)

FIGURE 7B The same Tecchler violin under (left–right) blue light without filter (460nm); blue light with filter (460nm w.f. 500nm)

FIGURE 8 Violin by Julia Maria Pasch (2017) in UV light: the layering appears similar to that of the classic old Cremonese makers

There is another light frequency that can be even more interesting for luthiers and instrument experts in everyday use. This lies in the range of the longwave infrared (IR) spectrum. Fluorescence in the traditional sense has no relevance in this context. Infrared photography has attracted much attention in the analysis of paintings, because it can reveal underlying layers of underpainting, and so provide a much better understanding of the developmental process of a picture (figure 9). Naturally, one should not initially expect this type of information when using this technique for the examination of old stringed instruments. It would, however, be extremely useful if it could detect conditions such as worm damage, cracks or even patches located in the deeper layers: in other words, within the body of the instrument or the wood itself.

Until now, examinations of this kind have only been possible with a CT scan, in which X-rays penetrate the instrument in layers. Then, using sophisticated measurement technology and a high computational effort, the data is converted into a 3D image. In particular, the significantly higher resolution offered by micro-CT scanning gives an unparalleled ability to deliver informative material for analysis. Particularly when examining instruments of the highest quality, this method has become standard and, together with a condition report, contributes significantly to a reliable assessment of the value of an individual stringed instrument.

FIGURE 9 Left A 17th-century Italian painting with a mouse gnawing at a hand. Middle An attempt is made with IR photography to make the underpainting visible. Unfortunately nothing could be found with this technique (but see also page 59). Right A linen doubling has been glued to the back of the picture

FIGURES 8 AND 9 LEONHARD RANK

FIGURE 10 ‘Barjansky’ cello in infrared light (850nm). The areas exhibiting no patina in the growth rings, or in which other impurities such as cracks or retouching do not disturb the overall appearance, appear as a homogenous and unbroken white and not, as one would expect, with the strong contrasts of the growth rings. A dendrochronology examination was not possible in this case.

FIGURE 10 LEONHARD RANK/ART & STRINGS ARCHIVE

However, CT examination is not only disadvantageous from a cost perspective, but also because of the logistical effort required.

It would be helpful to have an alternative method of examination which gives approximately the same amount of information.

During the research conducted for the ‘Barjansky’, photos were taken in incident light with intense infrared lamps (850nm, 2x Lumatec M05), as well as an IR camera (Canon 5dsr) modified for these purposes. The results provided details that were not only very interesting, but also somewhat unexpected.

The flaming and growth ring patterns of maple and spruce exhibited far less contrast than expected. A more homogenous image emerged, probably due to the vascular rays in the wood, which reflected the light very strongly (figure 10). Cracks, patina and superficial additions were more easily recognisable; however, it was not possible to gain insights into the deeperlayers of the wood, at least not to the degree that had been hoped for.

CR ACKS, PATINA AND SUPERFICIAL A DDITIONS WERE MORE EASILY RECOGNISABLE WITH THE USE OF INFRARED LIGHT

Only after further experimentation did it become apparent that this type of photography could have other technical applications, since the IR lamps must not necessarily be positioned so that they are directed towards the front of the instrument.

If a very thin piece of wood is held up to a light source, the light shining through causes the grain of the wood to appear in high contrast (figure 11). Some luthiers use this effect as a guide for carving the wood to the thicknesses they desire.

The transparency of wood in the infrared light range (not visible to our eyes) is much greater than in the visible light range, and this can be used to advantage when analysing stringed instruments with a suitable camera (figure 12).Placing the light source on the inside of the instrument, and photographing it from the outside, results in an image that in certain respects resembles an X-ray (figure 13).

Owing to the varying densities of the growth rings, flaming and vascular rays, the structure of the wood is easily discernible.This is why, in this examination, the winter growth rings of a coniferous top (which are actually dark in visible light) appear lighter, and the summer rings appear darker.

In figure 13, the bass-bar and the inner blocks, as well as the linings, are recognisable as a clearly defined shadow, which must be attributable to the thickness of the material. Cleats, whether on the top or back, are also generally visible. Fortunately cracks, in particular, appear with very high contrast even in complex areas with a heavy overlay of patina, or with dark varnish.

To understand this phenomenon better, we undertook a number of experiments: a freshly glued crack in a specially prepared spruce board initially exhibited no recognisable shadow in transmitted IR light (figure 14). The crack itself tended to appear somewhat lighter, presumably because the glue in its freshly applied state transmitted the light better.

In the course of the experiment, the wooden board was heated to simulate the ageing of the glue. Afterwards, in transmitted IR light, the crack was recognisable as a clearly defined shadow.A plausible explanation for this could be the increased brittleness that comes with the natural ageing process typical for glue.

The effect produced by IR light is intensified by impurities within the crack, such as small amounts of dust or debris that can still be present in the deeper layers of a crack after restoration, even if it has been cleaned. In most cases a combination of both of these factors contributes to the overall image (figure 15).

That a glued crack is completely invisible, whether in maple or willow, poplar or spruce, has not yet been possible to document. In the case of linings or inlaid pieces of wood, this method may be less informative. In the event that a patch has been extremely well fitted, and the glue has been uniformly absorbed into the wood – therefore when no residue from chalk (commonly used when fitting a wooden inlay) or a layer of glue is present – this method can be less precise. This is especially true with maple because of its flamed structure, which can be additional source of disturbance.

FIGURE 11 Rib of a 2020– 21 cello by Anton Somers of Antwerp, in transmitted light from a daylight lamp

FIGURE 12 Viewed from the outside: ribs in the bottom-block area of a Tecchler violin, illuminated from the inside with infrared light (940nm)

FIGURE 13 Oblique view of a cello in transmitted IR light (940nm). Even the soundpost is clearly visible on the top

Above Without crack: the growth rings and small reflections in the wood are easily discernible

With crack: Additionally, the crack in the centre of the image is visible (recognisable as a dark line)

FIGURE 10 LEONHARD RANK/ART & STRINGS ARCHIVE. FIGURE 11 ANTON SOMERS/DARLING PUBLICATIONS. FIGURES 12, 13, 14 LEONHARD RANK

FIGURE 14 Examining the properties of a crack along the grain of a 3mm thick spruce board, created for the purpose of this experiment, in transmitted IR light. The sample was heated in order to simulate the ageing of the glue.

FIGURE 15 Nicolò Gagliano violin under (left) transmitted IR light (940nm), and (right) visible light

FIGURE 16 Worm damage on the back and ribs

FIGURES 15, 16 LEONHARD RANK/ART & STRINGS ARCHIVE. FIGURE 17 LEONHARD RANK

Wormholes can usually be made very clearly visible (figure 16). Of course, this depends on the state of the worm damage. A truly hollow wormhole will appear very bright. It can be more difficult with worm damage that has been filled in with wood, since this can look similar to an inlaid patch.

All in all, examination with transmitted infrared light can be recommended as an easily applicable method of investigation for violin experts, auction houses and research institutions.

The findings obtained from images in this way are extensive, and offer a simple way of gaining clarity when analysing the condition of stringed instruments (and naturally also plucked instruments and others with thin-walled wooden bodies) in a practical day-to-day working environment. On stringed instruments with very dark varnish, such as those of the Viennese school, the use of infrared pictures makes a dendrochronological examination assessing the age of the wood much simpler, because it is not necessary to open up the instrument in order to get a clear picture of the year rings.

To conclude, the Old Italian painting shown in figure 9, on which the underpainting could unfortunately not be found in incident IR light, should not go unmentioned. By contrast, the examination with transmitted IR light reveals entirely new information which, due to the linen doubling glued to the back, has remained undiscovered until now.

One can clearly see that there is writing on the reverse side of the original painting (figure 17). The word ‘CARAVAG’ has been written there, which can hardly be interpreted as anything other than the beginning of the name ‘Caravaggio’. Sadly, art experts have not been able to confirm an attribution to this Old Italian master in spite of – or rather because of – the discovery of this inscription. C’est la vie!

FIGURE 17 Only in transmitted IR light can the writing ‘CARAVAG’ on the reverse side be seen

For providing instruments and their support, the author would like to thank Alago Art & Strings GmbH, Thilo Kürten, Benjamin Schilbach, Lumatec GmbH, Miharu Inayama, Rudolf Hopfner, Julia Maria Pasch, Martin Bek, Marcel Richters, Lueder Machold, Matthias Krause, Jost Thöne, Werner von Schnitzler, Anton Somers, Andy Lim, Darling Publications, Julius Niemann, Jason Price, Arjan Versteeg and Johannes Loescher. The monograph ‘Antonio Stradivari Cello c.1690 ‘Barjansky’, from which much of the information in this article has been summarised, is available at The Strad Shop: bit.ly/3dA2gKk