Improving the tonal quality of wood may allow modern instrument makers to match the legendary quality of violin master Antonio Stradivari.
It is the dream of every young violinist to play on a Stradivarius, but for the vast majority that remains an unfulfilled fantasy. The price tag for a Stradivarius starts at about a quarter of a million pounds, making it unaffordable to all but a privileged few.
The manufacturing techniques employed by the Stradivari family in the late 17th and early 18th Centuries has long been a bone of contention, but what is generally accepted is that at the heart of the instrument is wood of superior quality.
Did the great violin maker use a special primer or varnish, did he treat the woods he used with minerals, or was it a fungus that gave the wood its special tone characteristics? Ever since, violin makers have tried to find the reason behind the differences in tones and paid careful attention to the quality of the wood used.
Only as recently as 15 years ago did scientists become interested in the resonance properties of spruce fir, the most commonly used wood in violin and piano construction.
The wood - spruce for the top, willow for the internal blocks and linings, and maple for the back, ribs, and neck - grew during the Maunder Minimum, characterised by harsh winters and short summers that led to slower growth and more uniform annual rings.
It is difficult to find comparable wood today, but that may be about to change thanks to research by Professor Francis Schwarze at Empa, the Swiss Federal laboratory for materials testing and research.
Preparing the wood
The germination of the idea came in the early 1990s when Prof Schwarze was working on his PhD-project evaluating the application of different diagnostic devices to detect decay in urban trees.
One of the methods he used was a device that measures the speed of sound in trees. The speed of sound increases with the stiffness - the resistance of an elastic body to deformation by an applied force - of the material, and decreases with the density. All wood decay fungi reduce density, but the majority also reduce the speed of sound.
"However, during my PhD I identified a few members of the Xylariaceae (Ascomycetes) that reduce wood density without altering the speed of sound," Prof Schwarze explains. "When I met violin maker Martin Schleske he explained that a high ratio of speed of sound to density in material quality is a characteristic feature of superior tone wood and has the most significant impact on the acoustic properties of a violin. He also informed me that due to global warming it is becoming increasingly difficult to find superior tone wood even in high altitudes.
"From my experience, I proposed the working hypothesis that it should be possible to apply a range of wood decay fungi to improve the acoustic properties of tone wood. On this basis, Martin and Empa patented our method in 2005."
Using wood decay fungi for biotechnological applications in the forest products industry has been studied for several decades. The specificity of their enzymes and the mild conditions under which degradation proceeds make them potentially suitable agents for wood modification. Fungi, for example, are successfully used in the biopulping or biobleaching of kraft pulp or in bioremediation and detoxification of preservative-treated waste wood because of their tolerance and ability to degrade creosote, toxic polyaromatic hydrocarbon compounds, and pentachlorophenol.
The alterations in the woody cell wall structure reflect the plasticity of the degradation modes of wood decay fungi and can be used for the purpose of wood engineering. During the early 1960s, industrially cultivated white rot fungus was used in the German Democratic Republic, mainly on beech wood for pencil or ruler production.
"More recently, we have investigated the potential of a range of wood decay fungi for biotechnological applications in the forest product industry," Prof Schwarze says. "In Switzerland, 65 per cent of the forest stand consists of Norway spruce and European silver fir. To be used outdoors the wood of either of these species requires preservative treatment, which involves impregnating the wood cells with chemical preservatives or wood modification substances to suppress colonization by wood decay fungi."
In most cases, the substance is infused into the wood cells using vacuum pressure impregnation, but the wood of difficult-to-treat (refractory) species must be incised to enhance the uptake and distribution of the chemicals in the wood. Incising is a pre-treatment process in which small incisions, or slits, are made in the wood surface to increase the exposed end and side grain surface area.
Bioincising is a biotechnological process that has been developed to improve the permeability of refractory wood species by incubation under controlled conditions for short periods with a white rot fungus, Physisporinus vitreus. "Our studies show that isolates of Physisporinus vitreus have an extraordinary capacity to induce substantial permeability changes in the heartwood without causing significant loss of impact bending strength.
"In fact, wood durability is enhanced by the bioincising process, which is a promising technology for efficiently distributing wood modification substances, promoting desired improvements in wood properties, as well as leaving the wood surface aesthetically pleasing and the mechanical wood properties unaltered."
Prof Schwarze uses the vegetative state of two fungi, Physisporinus vitreus for the top plate and Xylaria longipes for the bottom plate, with thread-like cells that actively colonise the wood and secrete enzymes which ultimately alter the wood structure and its acoustic properties.
Once an optimum wood density loss has been induced in the top and bottom plate by the wood decay fungi the wood is sterilised with ethylene oxide, killing the bacteria and fungi.
Ethylene oxide is traditionally used to sterilise substances that would be damaged by high temperature techniques such as autoclaving. It is also widely used to sterilise the majority of medical supplies such as bandages, sutures, and surgical implements in a traditional chamber sterilisation method, where a chamber has most of the oxygen removed to prevent an explosion and then is flooded with a mixture of ethylene oxide and other gases that are later aerated. After this sterilisation process the fungus can have no further effect on the physical or acoustic properties of the violin.
"We verified the latter hypothesis showing that incubation of wood with two species of decay fungi caused marked density losses and cell wall thinning; that is, the partly degraded wood resembled superior resonance wood grown under cold climate conditions," Prof Schwarze explains. "The significant increase (P 0.05) in the damping factor (340 per cent in the radial direction) that was recorded after incubation of 20 weeks can be attributed partly to selective degradation of pit membranes. This is an important side-effect of the fungal treatment as this reduces the often irritating high notes of a violin and makes the instrument sound warmer and mellower."
The consequences of this processing is that wood density is reduced; damping is increased, while the modulus of elasticity remains unchanged, by avoiding degradation to the middle lamellae. The method allows improving the acoustic properties of resonance wood (value added product) in times where it is becoming increasingly difficult to find superior resonance wood due to the impact of global warming.
The proof here is in the listening. In 2009, two of Prof Schwarze's fungul violins were blind tested against a Stradivarius, and the panel and audience preferred the tonal quality of the biotech instrument.
To produce more of these biotech violins, Prof Schwareze and his team are now working on up-scaling the biotechnological process of fungal modification by defining standardised conditions to achieve reproducible results in a project funded by Walter Fischli, the founder of biotech company Actelion.
For the benefactor, Fischli, a biochemist and biotech entrepreneur, the synergy came from his own love of music. "I'm a biochemist, play the violin and have a special interest in large Italian instruments, so I called Francis Schwarze out of curiosity," he explains. "During the very first conversation I learned that, even though the results were very promising, the project could not be continued for financial reasons. It would have been unforgivable to let this interesting project, which combines science and violin-making in an ideal way, fade away.
"It combines two passions of mine: science and music. I find it extraordinarily exciting to track down the secrets of why violin makers such as Stradivari and Guarneri were capable of producing such extraordinary instruments. Certainly, their craftsmanship was crucial, but apparently the wood they used also played a major role. For me, it's interesting to be able to scientifically explore these materials-related aspects of violin making. The results could shed light on the origin of the exceptional tonal qualities of antique instruments."
If the process can be made highly repeatable and cost-effective for commercialisation, the portents are good for musicians. "If the outcome of the project is successful, in the future, every talented musician will be able to afford a violin with the same tonal quality as an expensive Stradivarius," Prof Schwarze says. "The biotechnological tailoring of resonance frequencies and damping should open the way to an improved, in-depth understanding of how the wood contributes to the global tonal quality of instruments." *