MAKING MATTERS

Our first article on the uses of ultraviolet light in violin making (October 2021) focused on the differences in radiation strength of the three different types – UVA, UVB and UVC – and their effectiveness in tanning wood. In this second article we examine how the various methods can be used in varnish drying – even though it is much more difficult to give a definitive scientific statement on the matter. As well as the type of UV radiation, other factors will affect the drying process such as the ground, layer thickness, oil-to-resin ratio, or the presence of pigments and siccatives. Many violin makers have found their personal solutions, and we can only share our own long-term experiences with the various UV sources.

Within the fluorescent tube family, the UVA/B solarium tubes have shown the best drying effect for our oil varnishes, whereas the blacklight tubes (‘BL’ and ‘BLB’) were less effective. With the LED strips we tried many products from different manufacturers: ultimately, both of us were impressed most by the 365nm UV LED strips from Waveform Lighting, which dried the varnish well and gave off little heat. UV radiation of around 365nm accelerates all possible polymerisation processes; 365nm emitters are also used in industry and at the dentist to cure plastics and adhesives.

When it came to drying the varnish, we were primarily interested in the comparison between the 365nm LED strips and the solarium tubes. The varnish tested in London seemed to dry a little better and deeper with the tubes than with the strips, but there was no detectable difference between the two with the varnish tested in Hamburg. Our colleague Felix Krafft in Berlin, who recently started using the LED strips instead of tubes, noticed they gave a slightly longer drying time for his varnish, but for him this disadvantage of the LEDs was far outweighed by their advantages – namely, their lower energy consumption and heat generation. They use around 80 per cent less energy than the tubes, and the cabinet only heats up to 22C over the course of one night. So the strips can be positioned much closer to the instrument without having to worry about warping, bubbles or micro-cracks.

We have sometimes heard luthiers advocating the use of UV light in regular intervals: three hours on and three hours off. The theory behind it is that when drying, the varnish polymerisation continues in the dark, having been started off by the UV light. We decided to put this theory to the test, and in our separate tests we both found that continuous UV cured varnish faster than the ‘rhythmic’ method: after a day of drying, the three-hour on/off samples were consistently softer.However, when testing a varnish with siccative added, after four coats and a final weekend of UV the differences were no longer obvious. The exact same procedure with a siccative-free varnish resulted in significant differences in softness. We therefore would encourage makers to do their own tests with their own varnish, to establish whether the energy saving could outweigh the potential loss of time.

The wood on the left has been tanned using continuous UV light while the one on the right was tanned using the ‘rhythmic’ method

Violin photographed under UV light

LED lights glued to an aluminium stand

Our results were more consistent when we tested the ‘rhythmic’ theory in regard to tanning wood. After a week of tanning we could see no real difference between continuous UV exposure and the three-hour method: both were equally tanned. We would add that using the ‘rhythmic’ method not only reduces the energy consumption by 50 per cent but also allows the instrument to regain some of the moisture it lost during the UV exposure, which in our opinion would be good for the instrument.

Independent of your UV source, we strongly recommend placing aluminium foil reflectors all round the instrument. Polished aluminium reflects about 85 per cent of UVA, which might well double the effectiveness of the cabinet. Another good tip is to add a humidity source in the cabinet. This could be a commercial humidifier or just a cloth hanging in a bowl of water. Most effective would be to place the cabinet in a cold, damp basement.

It is a good idea to monitor both temperature and humidity right by the instrument. Combined digital thermohygrometers are available with remote sensors that can be placed directly in the cabinet. They are inexpensive and the safety of your instrument is well worth it.

One more application of UV light in violin making is in fluorescence analysis. When exposed to certain light (such as UV), many materials convert it into light witha longer wavelength (such as visible light). This effect is called fluorescence, and can be used to check the authenticity of postage stamps, banknotes and minerals as well as to analyse paint surfaces. We want to discuss the various light sources briefly.

USING THE ‘RHYTHMIC’ METHOD ALLOWS THE INSTRUMENT TO REGAIN SOME MOISTURE

While analysis lamps with a radiation peak of around 360nm were being used in the 1970s, in their 2010 book Stradivari Varnish Jan Röhrmann, Stefan-Peter Greiner and Brigitte Brandmair used UV lamps with radiation in the 315–400nm range, and a radiation peak at 366nm. Some violin makers use lamps with small BLB tubes (peak at 370nm), or UV blacklight LED torches (peak at 395nm) for lacquer analysis. It seems that the fluorescence effect occurs with all types of UV light. As long as you only want to compare whether all parts of an instrument belong together, or where the original varnish is covered by retouching, you can use many types of UV light. Only if you want to compare or exchange photos of different origins is it essential to agree on a defined UV radiation source with the researchers and photographers involved, because the emitted visible fluorescent colours differ depending on which UV light colour is used.

An important aspect to consider with UV tubes is their relatively short lifespan. Most tubes last around 800 hours, after which they still emit light but the radiation output has fallen to below 70 per cent, so tanning/ varnish drying will require more time. The service life of LED strips, on the other hand, is estimated at around 15,000 hours, and with good cooling one can expect them to last even longer. Gluing the strips to 15 x 15mm square aluminium tubes allows the heat to dissipate well. If these tubes stand on small wooden bases they can easily be moved depending on the size of the instrument.

It should also be noted that UV tubes contain gaseous mercury, which is highly toxic. A used tube must be disposed of properly; if it breaks, the room must be vacated quickly and thoroughly ventilated.

However, the most ecological way of drying varnish is used by our colleague Alkis Rappas in Texas. The sun shines so strongly near the Gulf of Mexico that Alkis can hang his violins outside in the sun after a short initial time in the UV cabinet. Alkis swears that no artificial UV light source has such a good drying effect as the pure, clear sun. This is a variant that the present authors can only dream of in rain-shrouded northern Europe. ●