Varnish affects the sound of a violin17 March 2016

Varnishes protect works of art and woodeninstruments from environmental damage. However,until recently, little research had been carried outinto the effects of varnish on the sound of violins.Empa researchers have now investigated thisrelationship and have published their initialfindings.

Leonardo da Vinci's oil painting "Mona Lisa" looksrather gloomy with its dark green and browncolours. It is a little-known fact that the maestropainted the picture in bright colours - so the subjectappears bathed in the light of a sunny day. Whatcaused this difference in colour? Da Vinci coveredhis oil painting with varnish to protect it. Thevarnish acts like a filter, causing the picture tobecome darker than the artist originally painted it.

It has the same effect on the spruce used to makeviolins and other stringed instruments. In its rawstate, it is white with a touch of gold. The woodlacks durability, so violin makers protect it fromenvironmental damage using a varnish. It is thisprotective coating that gives the violin its elegantdark brown colour.

Violin makers pass their varnish recipes down fromone generation to the next. It is rare for anyone

outside the family to find out how exactly the mixtureof resins and oils is formulated. The chemistry ofvarnishes has been scientifically researched for along time. In contrast, little attention has been paidto the physical properties of varnishes - wrongly inthe opinion of Marjan Gilani from Empa's AppliedWood Research Laboratory. The scientist and hercolleagues suspected that varnish affects thevibrational properties and therefore the soundcharacteristics of violins, and they began toinvestigate this relationship. The first stage of theresearch was concluded with the publication of ascientific paper in the journal Applied Physics A.The findings support the Empa researchers'hypothesis.

This violin was made in the course of the project with thesupport of the Walter Fischli Foundation. Its exterior isnot very different from that of the violin from Cremona,which is nearly four hundred years old (see previousparagraph). The violin maker Michael Baumgartnerformulated the varnish, which gave the violin a texturethat was true to the original.

Four varnishes, 60 wood samples, eleven hoursof UV irradiation and conclusive results

For their research, Gilani and her team cut 40 woodsamples in the direction of the grain. 20 of thesesamples were taken from the young wood and the

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other 20 from the heartwood of a Norway spruce.Gilani and her colleagues also cut a further 20samples from the same spruce across the grain.Based on these three types of test samples, theresearchers were able to study the soundbehaviour of different wood structures. JohannaPflaum, a trained violin maker, student ofenvironmental sciences and member of Gilani'sresearch group, created two varnishes in the lab.The research group also received two furthervarnishes from German violin makers. Gilanicoated the spruce samples with these fourvarnishes. For each type of sample, five of thetwenty pieces of wood were coated with the samevarnish and were dried and cured for eleven hoursusing UV light.

However before this, the researchers measured thestiffness and sound insulation of the wood samplesin their raw state and the mass of the varnish. Thepurpose of the measurements was to allow theproperties of the bare wood to be compared withthose of the treated wood. After curing, the expertsmeasured the same parameters again. They alsoascertained the sound propagation of the samplesat the start and end of the eleven-hour UVirradiation process. Sound propagation describesthe way in which tones leave the body of the violinand move towards the audience. Higher soundpropagation values result in clearer tones.

X-ray tomography images show the microstructure ofwood samples treated with four different varnishes(pictured in blue). The red, yellow and green colouringindicates the wood structure, the concentrationscorresponding to the growth rings. It is also evident thatthe varnish penetrated much deeper into the wood in thesecond sample than with the others. This led to acorrespondingly greater decrease in the soundpropagation values for this sample.

In the 40 samples in total that were cut along thegrain, the varnish increased the sound insulation ofthe heartwood and young wood and reduced itsstiffness compared to the raw state. This may bedesirable because, up to a certain point, the moreelastic the wood of the violin, the warmer and softerthe sound, particularly in the case of high notes,which can sometimes be perceived as unpleasant.However, this change is accompanied by a loss ofclarity and sharpness of the violin sound, whichwas evident in the laboratory as a reduction insound propagation. The findings were different forthe samples cut across the grain. With these, boththe sound insulation and the stiffness increased.Similarly, the sound propagation values alsoincreased. An audience in a concert hall wouldtherefore perceive clear, sharp sounds with strong

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acoustic presence if the instrument was made in thesame way as the samples cut across the grain.

Results of X-ray tomography

The wood researchers also analysed part of thesamples cut along the grain at the Empa Center forX-ray Analytics. They discerned from the X-rayimages that the varnishes formed layers of differentthicknesses on the wood and had different effectson the density of the pieces of wood. In addition,some varnishes penetrated deeper into the woodthan others. The samples with the highestpenetration had been treated with the varnish withthe highest oil content. They were also the woodsamples that experienced the largest drop in soundpropagation - i.e. their tone lost the most clarity dueto the varnish.

Following on from these measurement results, arelationship was established between the varnishcomposition, the microstructure of the wood/varnishcomposites, their mechanical properties and theirsound behaviour. Gilani and her research groupintend to refine these findings in other studies. Anadditional scientist is joining the research team inApril 2016 to assist with this work.

Antique violins at the Empa Center for X-ray

Analytics

For comparison purposes, Empa researchers alsoanalysed two antique violins at the Empa Center forX-ray Analytics. One was made by a violin maker inCremona in the 17th century. This town is locatedaround 80 kilometres south-east of Milan andproduced a number of famous violin makers,including Antonio Stradivari. The second violin wasmade in around 1920 in the German state ofSaxony. Scientists took samples measuring 1.5cubic millimetres in volume from both violins. Theviolin from Saxony had fine cracks in the varnish,impacts on the surface and woodworm damage.Compared to this violin, the violin from Cremonahad a higher density of wood/varnish composite. Inthe images from the Center for X-ray Analytics,high densities are shown in yellow, green and red,and low densities are shown in blue. Stiffness and sound propagation values also increase withdensity - the violin from Cremona will thereforehave produced clearer tones than the violin fromSaxony.

Provided by Swiss Federal Laboratories forMaterials Science and Technology

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APA citation: Varnish affects the sound of a violin (2016, March 17) retrieved 15 November 2021 from https://phys.org/news/2016-03-varnish-affects-violin.html

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