Heat treated bridges

[DiemViolins]

As far as I can see on the info sheets is this a product to treat decayed and rotting wood. If you want to increase the E-modulus (or stiffness) why not simply use a coat of linseed oil? The only thing you have to do is wait till the UV-rays have polymerised your linseed oil and you'll have your increased stiffnes with a minimum of added weight. Minwax is based on solvents which evaporate within 30 minutes leaving behind a hard coating. As you know linseed oil is without any solvents and is a natural product.

[DiemViolins]

Mike,

if the goal is to make the ratio of E/d as high as possible than should Balsa wood be an excellent selection: balsa has a density of only 200 - 280 g/cm3 whilest spruce has a density of about 440 g/cm3. However I've never seen a bass bar of balsa wood or bamboo. Maybe that the E-moduli of balsa and bamboo are not high enough? Does anybody know why these types of wood are not applied in instrument making?

[Michael Darnton]

Oded, do you have the figures handy on what application of linseed oil does to stiffness?

[Oded Kishony]

Michael,

This is what Martin Schleske has to say abou the use of oil including linseed oil " There is a strong increase in the cross-grain fiber loss factor. The velocity of sound in the alonggrain fiber direction was significantly reduced" In other words the acoustical effect is negative, a fact well borne out by makers that have tried using linseed oil in this way. Martin did not measure the added mass.(measured after 9years drying time!)

From a practical point of view; soaking in linseed oil causes the bridge to become filthy after a short time. It would take several years before the oil would dry. Most instruments aren't stored in direct sunlight which is what you need to accelerate the drying of linseed oil. (which needless to say would be catastrophic for the violin

In my opinion it's about the worst thing you can do.

Oded

[JimMurphy]

Hi all,

A very important physical difference between hardwood & softwood fiber structure is that hardwoods have "vessels" - in addition to hollow cellulose fibers - while softwood conifers [spruce, Cedar, etc.] do not have vessels.

In a hardwood cross-section image, vessels appear as large diameter "pores" between the smaller diameter straw-like cellulose fibers. See images here: http://woodmagic.forprod.vt.edu/Kids/anatomy/Anat19.htm

These hardwood vessels carry not only water, but also "sap" to various parts of the tree.

As bridges are usually made of hardwood, such as Maple, I'm wondering if the "purpose" of heat treatment

may be to "seal-off" most of the large diameter vessels by crosslinking via sap and other resins. If so, this may allow for better transfer of energy between the lengths of straw-like cellulose fibers giving more "efficient" bridge response.

Just conjecture on my part FWIW.

Best wishes,

Jim Murphy

[GMM22]

In certain circumstances, I have had some success with coating bridges with linseed oil; however, these were on lower quality violins and always of the harsh and bright sounding kind. A few that I had setup and cut bridges for were still very strident even after bridge tuning. In some cases, an application of linseed oil was extremely beneficial in reducing the harshness, presumably because I was reducing the efficiency of sound transmission, which is just what certain violins require.

Violins that are inherently loud and harsh would not necessarily benefit from a high quality stiff bridge, as clearly not all violins are wanting for better sound transmission. One can even soak more or less oil at critical end grain locations to tailor the damping much the same way as one pares wood away from key areas. I have not found a problem with so treated bridges getting dirty any faster. No doubt, such bridge treatment is reserved for lower end and troublesome violins.

[DiemViolins]

Oded

In martin Schleske’s article “Wood and Varnish Analysis” his investigation learned that the goal should be to increase the Young’s modulus by strengthening the grain structure. The test was followed by the determination of the sound velocity on varnished specimen of 3 mm’s thickness after nine years. I can’t retrieve in Schleske’s articles what you are saying about a drying time of linseed oil. The nine years refer to oil varnishes with different resins dissolved in it which may include linseed oil, but this is not mentioned as such. I could not find information about linseed oil alone. In his conclusion he stated that one should prepare a varnish that increases the sound velocity and reduces the damping. Best results with known additives were obtained with Copiava Balsam and Mastix. Three outperforming varnishes were prepared by Martin Schlesk’s people and were code as “K” “V” and “P” with secret composition. Many instrumnet makers use Mastix so that is anyway a good start.

But Martin Schleske also mentioned that the thickness of his specimen of three mm's is very sensitive to variancies. Because three coats could not breng the benefit and a couple of coats extra did. As we have in our top plates graduation of thickness than the amount of layers will be very sensitive to the best result. How does one know when to stop?

Frits