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Dichroism revisited


Advocatus Diaboli
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There have been several discussions in the past about dichromatism and it’s presence in classic violin varnishes.  There have also been some discussions about the difference in dichromatism and dichroism.  Let’s not derail this thread into either of those topics, since they’ve been pretty thoroughly hashed out. 
 

the couple times dichroism has popped up in threads it’s usually an explanation of why the color shifting we see in classical varnishes is not caused by it.  I mostly agree.  But…there have been a handful of instruments I’ve run into that look as if some oil in the color had leaked out and formed a blue-green layer on the very surface.  I thought about that again when I went to print some labels this evening and saw a board with some dried up printing ink on it.  Linseed oil and lamp black are the only ingredients.  

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I think what you're seeing in these photos are thin film interference.  https://en.wikipedia.org/wiki/Thin-film_interference

There is a video Kevin Lee luthier where he is doing some cleaning of what he claims to be an old Cremona violin.  At the end of the video showing the varnish at one angle it's golden in color and at another angle the thin red varnish appears.  It's a very nice effect.  

Attacking a Strad with steel wool.. really?  heh  but seriously skip to time stamp of about 44:45 and check out the dicromatic effect on the ribs.

Does anyone know what instrument this is?  

 

 

 

 

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3 hours ago, MikeC said:

Does anyone know what instrument this is?  

I've not been exposed enough to Mirecourt violins to know if this is a type of that, so I'll give it the benefit of doubt and say that it's probably a very low level French maker's violin from the late 1800s. If someone wants to place it as a better Mirecourt instrument, I won't protest.

This video makes a good case for B&F's policy when I worked there of not hiring people who had any previous experience or training, preferring to work with a clean slate.

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1 hour ago, Advocatus Diaboli said:

Which is dichroism. 

That is the optical effect seen in certain gemstones such as color change sapphire, alexanderite , garnet, flourite and some others and it's pretty complicated wavelenght splitting based on composition and formation and what type of light, or they look different in daylight than they do under lamplight. Certain other things, like varnish have been know to create this effect, and it's real scinecy trying to explain all the details and variations based on material. Use google.

I have been able to make a successful dichcroic spirit varnish using a very common Plum leaf, coming from the very popular and often seen "Purple thundercloud plum" these are the purple plum tress seen all over California and many other parts of the USA. Simply take completely dry leaves and soak them in dentaured alc. for about a week, when you take out the leaves you will see that they are green and all the purple has gone into the alc.

This can then be the alc. that you will use to construct your shellac based spirit varnish. Do not use it as a ground, but after proper sealing. It will act like a pigmented spirit so one must be careful of melt through when building multiple coats, but if successfully applied the finish at first will have a stong purple cast that eventually lightfast transitions to a golden brown once cured out the finish will display in certain light conditions a subtle change from brown/red/yellow to brown/green/purple cast. It is very subtle but you can see it when playing with the lighting and angles of view .

I actually used this on the first instrument I made and have been surprised by how stable the "settled" color has been and if anything with proper storage the instrument has turned slightly darker over time, but it is a very nice golden brownish shade.

The solution in a jar demonstrates the change very dramatically where one will see a purple solution that can also be very green as you move the jar in the light.

I would like to find some red maple leaves to try the same thing, it's very easy to mess around with , you could also{not for violins} use it as an alc based stain , but I do not recommend it for raw flamed wood, it will burn the grain . 

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6 hours ago, MikeC said:

I think what you're seeing in these photos are thin film interference.  https://en.wikipedia.org/wiki/Thin-film_interference

There is a video Kevin Lee luthier where he is doing some cleaning of what he claims to be an old Cremona violin.  At the end of the video showing the varnish at one angle it's golden in color and at another angle the thin red varnish appears.  It's a very nice effect.  

Attacking a Strad with steel wool.. really?  heh  but seriously skip to time stamp of about 44:45 and check out the dicromatic effect on the ribs.

Does anyone know what instrument this is?  

 

 

 

 

He' s wearing a red jacket and handle above a red carpet. It helps much to get red reflection with a gopro in the back.

I personnaly have a mirecourt with red varnish.

Light on right

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Light on left

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red in both angle.

No Dichroism exemple.

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Dichromism is seeing different colors depending on the angle and light path.  So, see the color at the wood from high one angle short light path, and seeing the varnish color through a lower angled longer light path.

 

Inteference colors are from some physical distance/gap being near in length to a light wave length.  Barbs on butterfly wings or bird feathers, and oil films being classic examples.

 

These are two unrelated and physically completely different color phenomena.

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13 hours ago, MikeC said:

 "and now we can see what is called the dichromatic quality of the famous Cremonese varnish".

The varnish alone is just part of the effect.  IMO the lighting and the contrast/reflectivity of the wood under the varnish are much bigger players in what you see.  It looks like there's almost no varnish left on most of the images anyway.

Here's some deeply flamed bare wood under different lighting conditions.  With ground and varnish, I'm sure it will be much more dramatic... but primarily due to the wetting/penetration/transparency of the coating allowing the optical properties of the wood to show best, rather than the varnish adding some magical optical effects on its own.

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I think we should just replace the term with "color change" .Again strictly speaking of just the film of varnish, not the wood, there are varnishes that if you could pull the film off and examine it like a fruit roll up that would look like a nice golden color sheet of plastic, then there are some that can/will "color change"  and demonstrate the properties of the solution in the pics I posted. The film can be brown/red/purple, it could be gold/yellow/green or it could be a combonation of the two dependent of angle of veiw and type of lighting.

So not sure if Cremona instruments display that or not, but to me there is nice wood, there is nice wood covered with a non color change varnish, and then there is nice wood covered with color change varnish. They are 3 distinct things. 

So the holographic effects of the chatonoyance and grain celebration imo are a completrely separate property that will "do what they do" after a qaulity varnish has been applied and then there is "all that" with a qaulity varnish that has the ability to change color and effect the "cast" of the varnish dependent on light and angle and that is completely independent of the woods chatonoyance 

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21 hours ago, David A.T. said:

He' s wearing a red jacket and handle above a red carpet. It helps much to get red reflection with a gopro in the back.

I personnaly have a mirecourt with red varnish.

Light on right

20210911_210912.thumb.jpg.3a2f472b62698a8791e02c00e73df90d.jpg

Light on left

20210911_210906.thumb.jpg.d032a386d3c99cf4e71945dcea8806ac.jpg

red in both angle.

No Dichroism exemple.

Yes, I don't want to pan your instrument or it's varnish, but it is a good example of what we don't want, or it is very monochromatic in it's appearance, has universally washed out the "cast" where the "stripes" are a darker version of the field in between, and there is no "golden underwood" delineating the the stripes, thus helping them to loose the holographic effect. It will still "flip" dependent on angle and view, but because the cast of the film is so "red" and potentially not independently sealed wood ,we get a real mono tone display from the film. I think this is a typical example of knowledge lost once factories started pumping them out.

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Iridescences and thin film effectives are completely different mechanisms than the dichromism in classical violins.  There is absolutely no connection.

 

Dichromism (two colors basically) is much more related to how the sky air looks blue when the sun is over head and the light path is shorter and more direct.   But the air can look in the reds to yellows and oranges when the sun is low and the light path through the air is much long.

What happens still not the same as varnish dicrhromism, but it's much closer related.

Dichromism in varnish involves the wood structure, transparency into the wood and reflection from it, and color of the varnish, color of the wood, and color in the ground.   And light paths and angles.

It's sort of an enhancement involving the wood texture and curls of what basically happens with any lake of transparent pigment.

It is normal to any lake of color in painting that the apparent degree of coloring versus transparency depends not just on how you apply and make the color lake, but also on the angle and intensity of light (and viewing angle). 

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