In the light of experience

Most violin makers will have a UV cabinet in their workshop, but how many realise the differences between the light sources? In the first of two articles, Andreas Hampel and Andreas Hudelmayer examine several of the available options

FIGURE 1 Relative radiation distribution from different UV light sources. Note: the sources’ radiation intensity is not shown in this diagram

MAKING MATTERS Points of interest to violin and bow makers

Violin makers use ultraviolet light in a variety of ways: to tan wood, to dry oil varnishes, and to examine varnished surfaces in fluorescence analysis. For the first two applications, strong sunlight can be the source of the UV radiation. For fluorescence analysis, however, only artificial UV light can be used. For violin makers based in northern Europe (such as the present authors), wood tanning and varnish drying are both also inconceivable without artificial UV light.

There are now many kinds of UV lamp on the market, which differ greatly in their light spectra and intensities. They also vary in terms of their service life, energy consumption, heat and environmental effects. We wanted to investigate the differences between thes light sources, and determine which light is best suited for which parts of the violin making process.

There are three types of ultraviolet light: UVA, with a wavelength of between 315 and 380nm; UVB, 280 to 315nm; and UVC, 100 to 280nm Visible light, by contrast, has a much greater wavelength of between 380 and 750nm (750nm = red; 380nm = violet). To compare the different types of radiation from the various UV light sources we can examine the spectral distribution curves, which show the relative radiation intensity in various wave ranges.

THE TUBES GIVE OFF A LOT OF RADIANT HEAT, AND HANGING THE INSTRUMENT TOO CLOSE CAN AFFECT THE ARCHING

Figure 1 shows the spectral distribution curves of different UV light sources. For this, we simply took the curves published by the manufacturers and superimposed them on the same graph. The spectral curve for sunlight was taken from a PhD thesis, which measured the light at noon in the last week of June, on the German island of Sylt. It shows strong parts in the UVA and UVB sector, whereas UVC and all short-wave rays below 295nm are filtered out by the ozone layer and the atmosphere. The maximum radiation of sunlight is outside the area of the graph, at around 500nm (blue–green light).

The best known artificial UV lamps are the so-called ‘blacklight’ tubes (also known in the US as ‘BL’). These tubes have a narrow radiation peak of around 375nm, and also emit some visible blue light. For effect lighting in theatres and discotheques, as well as for banknote checking devices, there are special blacklight tubes (designated ‘BLB’) that emit no visible light. Instead they produce special lighting effects through fluorescence. These tubes are similar to normal blacklight tubes in terms of their light spectrum and technical structure; they can be recognised by the dark coating inside the glass tube, which absorbs the visible part of the light spectrum (above 380nm). UV tubes  built for suntan salons use a complex mixture of luminous gases. They try to emulate the spectrum of sunlight and have a broad peak at around 350nm. Typically, more than 90 per cent of the UV light they emit is UVA, along with up to 1.5–2 per cent UVB along with some visible light.

UV LEDs have been around for a number of years, and now that they are mass-produced they have become affordable. We have tested some of them and have had very good experiences with the LED strips from Waveform Lighting. Figure 1 shows the spectrum of 365nm LED strips from this manufacturer.

RADIATION STRENGTH

As far as we can tell, so far there has been no study comparing the intensities of the different light sources against each other. To compare them ourselves, we performed a series of tests on the first field of application: wood tanning. To begin with, we tanned freshly planed spruce with various UV light sources for six days. Our results were as follows:

• Spruce 1 was left untouched in the window of a workshop in Hamburg. With practically no UV radiation, there was practically no browning.

• Spruce 2 was kept in a lightbox, at a distance of 15cm from a brand new UV solarium tube (Philips TL 20W/12). It gave a good tan, with good preparation for varnishing.

• Spruce 3 was also tanned in a lightbox, at a distance of 15cm from the abovementioned UV LED strips by Waveform Lighting giving out UVA with a wavelength of 365nm. This gave a slightly weaker tan than for strip 2.

• Spruce 4 was irradiated with the 365nm LED strips, but this time with a distance of just 10cm between the wood and the strip. The sample was then cut in half, so we could better compare the pieces against the others. The browning of Spruce 4 corresponds roughly with that of Spruce 2: another good tan.

FIGURE 2 Results of the wood tanning experiment with UVA/UVB light

ALL IMAGES COURTESY ANDREAS HAMPEL

We used 15cm as a standard distance for these experiments, as it is appropriate for irradiating instruments. The tubes give off a lot of radiant heat (the tube temperature reaches 45C to 65C), and hanging the instrument too close to the hot tubes can affect the arching, the neck angle and the gluing, and of course we want to avoid drying cracks. UV LED strips, however, get much less hot (max 30C), which meant we could place them closer to the spruce sample without having to worry about heat damage. Therefore, when using UV LED strips, you can safely reduce the distance to 10cm.

At violin makers’ meetings, we often hear the use of UVC mentioned as a top insider tip. For that reason we wanted to test out UVC light sources as well – but with a note of caution, since UVC damages the eye retina. Protective goggles must be used at all times! Figure 3 shows the results of our tanning experiment with UVC light. The top and bottom sections were covered with aluminium foil and received no UV radiation at all. The second section was irradiated for six days with a 265nm UVC LED strip, again at a distance of 15cm. From this, we could only determine a slight tan.

The third strip was irradiated for six days with the above 365nm UVA LED strip, again at a distance of 15 cm. As with Spruce 3 in figure 2, this led to proper pre-browning of the wood.

FIGURE 3 Results of the tanning experiment with UVC light

In summary, we found that while the tanning effect was strongest with the fresh UV solarium tube, a similar result could be achieved with the 365nm UV LED strip after the distance between the wooden strip was reduced to 10cm. We would also cautiously say that, despite the avowals of several luthiers, in our opinion the use of UVC light for wood tanning gives unsatisfactory results, though further research may prove otherwise. Finally, when it comes to buying new equipment, it is worth going to companies that can provide you with the UV radiation curves for each product; there is a wide amount of variation, particularly when it comes to BL and BLB tubes, which could affect their wood tanning abilities.

In a second article we will examine the various light sources’ abilities regarding varnish drying and fluorescence, as well as comment on their service life and environmental impact, and on how UV can be applied ‘rhythmically’.