Shellac?

[Links]

This has probably been discussed a million times, but I have never gotten a definitive answer.

A "French" polished mandolin (or other instrument) is finished, to the best of my understanding, by dissolving shellac flakes in a solvet (alcohol) and applied in thin layers by "building" up the finish. I understand a small lint-free rag is used to apply the schellac.

I have seen brochures for the "finest" European varnishes that is used on furniture and I assume could be used on instruments. This is already in a liquid form and is usually thinned with a solvent and also built up in layers.

Although I have heard of mandolins being "French" polished, I never see them advertised as a "shellac" finish. It is either lacquer or varnish.

What's the scoop? Are maker calling their "French" polished mandolins "varnish" as just a generic term. Help me understand!

[Michael Lewis]

Yes, shellac is a varnish, whether french polished or brushed. Brushing generally builds a thicker film than french polishing, but if you keep going with the french polish it will eventually build a heavy coat. The benefit of french polishing is to build a thin yet finely finished coat which will impede the vibration of the instrument less than a thick coat. The term 'varnish' basically means that there is a coat of something, and it can be shellac, or oil based, or just about any other medium. The violin varnishes generally are based on either shellac or a drying oil. There are MANY receipes for violin varnish, and they range from very soft to fairly hard. There are many different types of varnish currently being used on mandolins by various makers.

I think the oil varnish is preferable because it can be so thin that it doesn't impede the vibration of the mandolin. It is generally protected by french polishing a thin coat of shellac on the surface. The beauty of this type of finish is that it is very easily "distressed" in the process of application, so that the instrument looks old and very well cared for.

[Luthier]

....and here is a recipe from an old cabinet maker's book I have from 1884: The book is called "Practical Information for Cabinet Makers and Furniture Men."

....."rectified alcohol, half gallon; add six ounces of gum sandarac, three ounces gum mastic, and a half a pint terpentine varnish; put the above in a tin can by the stove, frequently shaking until well disolved. Strain and keep fpr use. If you find it harder than you wish, thin with more terpentine varnish."

....to make a yellow stain they even suggest dissolving 1/4 lb of tumeric in 1 pint of alcohol until the tumeric settles to the bottom....

Lots of cool dye and stain recipes too.

Don

[onthefiddle]

I wouldn't call a pure shellac finish "varnish", but the recipe that Don gives is one example of what Violin makers call "spirit varnish".
I wouldn't generally buy commercially prepared varnishes, as they lack the transparency desired by most instrument makers, and often contain driers that can cause problems in the finish. Sprirt varnish is very simple to make yourself.
Alternately there are some specialist varnish makers, such as Hammerl.
Another very effective yellow spirit stain is saffron soaked in whisky. There is always the problem of what to do with all the remaining whisky though!

Jon

[Links]

Gentlemen:

Thanks for your information. I suspect, that although some folks are willing to share "varnish" formulas, that in some cases it is a "trade" secret. It might be that one extra step or ingredient that sets your finish apart from others.

I actually ran into an amateur instrument maker the other day and when I asked what kind of finish he uses, he said varnish. Upon further questioning, it turned out that he uses a "varnish" from a spray can. No telling what kind of finish he is actually putting on. It may have no resemblance to instrument varnish.

I would also assume (just a guess really) that there are some professional mandolin makers who have not yet refined their "varnish" finish. If ordering a varnish instrument, is it a fair question to ask them their finishing process!

Just a few thoughts! Thanks again!

[John Bertotti]

I think if you're buying an instrument, all questions related to it are valid. My .02 cents. John

[Chris Baird]

From a purely technical perspective a varnish is a drying oil of some kind. However, in the real world and in laymans terms a varnish can mean shellac or oil. There are many different application processes and recipes. I've done a few tests on a thinned out piece of flat top wood. I cut the thinned out top into 4 sections and (tried) to apply a consistant layer of 3 kinds of finishes on the wood and then hang to dry. Shellac and lacquer both put a lot of tension on the wood and it warped quite a bit. The oil varnish had almost no effect and was only slightly warped as compared to the piece with no finish on at all. There is a mechanical difference between various varnishes.

[John Bertotti]

Hey Chris, just out of curiosity but did you finish both sides of the wood pieces. Uneven drying would cause any flat piece to warp. It is curious that some warped more then others though. I would be really concerned if both sides were finished and it still warped as bad. Granted the inside of the instrument isn't sealed by everyone either. John

[Chris Baird]

Only one side was coated like 95% of mandolins. Don't get me wrong I'm not saying that the tension (warping) is a bad thing. I'm just supplying data. I make no claims that oil is better than shellac or that either is better than lacquer. Just that they are different and effect wood differently.

[John Bertotti]

I understand Chris, I just thought it was curious and wondered just exactly how much more stress may be added to an instrument just by the finish drying. I think it is quite interesting. I think it was a worthy experiment. Thanks John

I find myself more convinced that I want a very thin seal coat inside the instruments I make.

[Dave Cohen]

A finish by itself does not "stress" a piece of wood. It does not put one side in compression and consequently put the other side in tension. Likely your experiment caused uneven uptake of moisture when the relative humidity changed.

A similar, but more extenssive and systematic experiment was done by Martin Schleske and reported in CASJ. The reference follows:

Schleske, M; "On the Acoustical Properties of Violin Varnish", Catgut Acoustical Society Journal, Vol. 3, No. 6 (Series II), pp. 27-43 (November, 1998.

[Chris Baird]

I don't think so. Try it yourself. The warping occured within 6 hours and was far more than uneven uptake of moisture could have caused. The pieces never unwarped as long as I kept them. I'll trust my own observations over Schleske's. Regardless of why they warped differently, they did, and that is important to note.

[sunburst]

I think a film that shrinks as it dries would cause the permanent warp that Cris is describing more or less independently of moisture. What I wonder is; with the plate restrained as in a constructed mandolin, how much would the film stretch as it dried, and how much stress would be introduces into the wood? Also, would the stresses eventually subside as the film stretched and the wood relaxed? Could that have something to do with an instrument breaking in? Would an instrument with a finish that doesn't cause as much warp in unrestrained pieces sound older when it's new as a consequence?

Probably very little diference, but I like thinking about stuff like that.

[Dave Cohen]

In Chris' case, a stress may have been induced by the shrinking of the finish. But one thing that should be understood about stresses and strains is that there is NO stress in a piece of material unless it is moved or deformed. In his case, the concave side was under compression, and the convex side was under tension. Presumably, the deformation occurred across grain. The cross grain young's modulus is anywhere from 7 times to twenty times smaller than the mudulus parallel to the grain. That means that it doesn't take very much to deform a piece of spruce across grain.

But would a film finish put a stress on an arched mandolin or violin top? I think not. The fact that the top plate is fixed at the edges and is carved into an arch as well makes it effectively much stiffer and much more resistant to deformation. An understanding of the finish at the molecular level would be helpful. A film finish formulation consists of polymer(s) plus plasticizer(s) plus solvent. As the solvent leaves the mix for the vapor phase, voids are left between the polymer molecules. in the vicinity of the voids where there do not happen to be any plasticizer molecules, attractive forces between the polymer molecules cause them to collapse toward each other, i.e., to shrink. In the Baird floppy piece of guitar top experiment, the shrinkage will occur in all directions, including the direction which puts a stress on the strip of guitar top. But on a much stiffer arched mandolin or violin plate fixed at the edges, the attractive forces between the molecules have to overcome the restoring forces of the plate, which they cannot do. Consequently, the shrinkage occurs in the only direction in which it can occur, i.e., the film gets thinner. In other words, the restoring forces in the plate are much larger than the attractive forces between the molecules, so the molecules try to minimize the potential energy by moving toward each other in the only direction in which they can; the film gets thinner.

What Schleske found was that the primary effect of violin varnishes on his wood strips was damping, with some modal frequency shifts also. The paper is a good read.

[amowry]

In Chris' case, a stress may have been induced by the shrinking of the finish. #But one thing that should be understood about stresses and strains is that there is NO stress in a piece of material unless it is moved or deformed.

I'm not sure I completely understand this part Dave-- depending on the rheology of the materials in question, wouldn't it be possible to have stress that doesn't result in strain? Or at least no significant strain?

[sunburst]

Dave,
If you were to remove the top from the mandolin soon after the finish dried, wouldn't the shrunken film warp or curl it across the grain? It seems to me that the pull on the restrained plate would result in stress, and if the plate was free to move the stress would be relieved.
Am I confusing stress with strain, or just not understanding either term?
I haven't found the Schleske paper yet, but it sounds like it should be interesting.

[Chris Baird]

I will agree that the difference between finishing materials and how they effect a plate is slight, but it is there. A great mandolin is a thousand subtle tiny differences in sum. I certainly believe that the finish medium effects the tone of the instrument independant of thickness. Thickness does matter but so does shrinkage after delivery and final flexibility.

[PaulD]

If you were to remove the top from the mandolin soon after the finish dried, wouldn't the shrunken film warp or curl it across the grain? It seems to me that the pull on the restrained plate would result in stress, and if the plate was free to move the stress would be relieved.

As I was reading this, I was wondering the same thing. It would be interesting to reproduce Chris' experiment with the strips of wood pinned to some substrate. After the finish is fully cured, the strips could be removed from the substrate. If, as Dave suggests, the finish thinned itself rather than creating tension I would expect the strips to remain relatively flat.

Another thought I had (maybe due to something I missed) relates to whether the shrinkage Chris observed was across the grain or longitudinal to the grain. If the strips predominantly cupped across the grain I could see it being a moisture difference between the sealed and unsealed sides. If it cups longitudinally, this wouldn't make sense to me since there is very little movement in wood due to moisture change in this direction; the warpage would almost have to be induced by tension in the finish. The wood is stiffer longitudinally anyway, so cupping across the grain doesn't prove that it's moisture related.

Paul Doubek

[Dave Cohen]

A stress is a force per unit area, which means that there has to be a force. The force comes from the shrinking of the finish, which in turn comes from the forces of attraction between the polymer molecules. Once the molecules have collapsed toward each other enough to minimize the potential energy, the shrinking stops, and there is no more force. If the forces were still there, the piece of wood would keep on cupping until it ultimately failed.

A polymer film finish by it very nature is inelastic. It has to be in order to accommodate the moisture and temperature related movement of the wood. That is why finishes get thinner with age. So after the major portion of the outgassing of solvent and plazticizers is done, the finish comes to equilibrium as a thinner film and the forces are gone. In the case of Chris' cupped strips, the cupping does not go away b/c a return to a flat configuration would cause a restoring force toward the new equilibrium (cupped) configuration. Eventually, the wood relaxes into that configuration too. Necks warp under string tension for the same reason, namely that they achieve a new equilibrium configuration (for a time) due the cells sliding past each other in the lignin matrix.

[Desert Rose]

To go back to the original topic, in my lifes experiences regarding finish

Shellac is just that shellac disolved in alcohol

Varnish is shellac disolved in alcohol with all the endless variety of things violin makers have tried over the years.

This being the difference

Scott

[Stephen Perry]

So after the major portion of the outgassing of solvent and plazticizers is done, the finish comes to equilibrium as a thinner film and the forces are gone.

Isn't difficult to use an oil varnish without solvents or any added plasticizers. My oil varnish is fairly stiff to brush cold. I heat it in water in a double boiler (well, a crock pot) and brush on quickly. The lack of solvents and other junk really helps it set up nicely.

I don't like to have to add solvents.

And about recipes that call for heating alcohol - be very careful!!!!!!

[PaulD]

I noticed in this month's Woodcraft flyer that they are selling Behlen's Luthier's Lacquer which claims to be elastic to allow for the expansion and contraction that musical instruments endure (we'll ignore the fact that all wood shrinks and swells). Since this thread was fresh in my mind I started wondering if this is such a good thing. If it really remains elastic, I would expect it to create constant tension on the finished side of the wood. If regular finishes thin and harden, I would expect that "crackle" that old finishes get as the finish is stretched and compressed, but there wouldn't really be a constant tension. Maybe it's elasticity is just marketing hype... has anybody used this stuff? Is it any good?

Paul Doubek

[sunburst]

Lacquers are generally plastisized to one degree or another to control hardness.
A harder film is more likely to check and crack. Yes, all wood moves, but a table top in the home doesn't move as much as a guitar or mandolin top that gets played indoors and outdoors in all seasons and in various weather conditions.
In general, wood finishes for indoors are harder (less plasticizer) and finishes for out doors are softer (more plastisizer). Wood movement is more of an issue out doors.

A lot of instruments built by individuals 20 or so years ago have lacquer finishes that are full of cracks. Lacquers formulated for instruments were harder to find in those days, so the builders used what they had; lacquers formulated for the furniture industry.

Plastisizers gradually evaporate and leave the finish, and the finish continues to harden over time, and will likely crack eventually in a lot of cases, and enough cycles of wood movement will likely crack any lacquer finish, but the "elasticitry" that they claim is probably for real, and is probably a good thing.
I haven't used that particular Behlin finish, but I've used what they used to call "musical instrument lacquer", or something like that. I don't know how much difference there is in the two products. They are a good company, though, and I suspect it is a good product.

[Gavin Baird]

John,
A very good friend of mine and a very excellent professional guitar player, insists that the instrument isn't sounding its best untill the finish, especially on the top, has crazed. His opinion on this is that when the surface checks this indicates that the top has been relieved of its stress induced by the finish and is more able to generate sound...G

[Bill Snyder]

In regards to the Behlens lacquer it is a popular product amongst luthiers that spray lacquer. I believe that quite a few of the BIG American string instrument builders use Behlens products.