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Advocatus Diaboli

'Dichroism'; in the ground or 'varnish'?

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While I've always associated the 'Dichroic' qualities of the nicer Cremonese varnish with the ground, Most of the makers (not necessarily restorers...) I know associate it with the varnish layer.  Do folks have an opinion about this?

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I've actually alway wondered what people mean with this dichroism...

If it's the changing colours I'd associate it with the wood-ground partnership, the varnish layer enhancing this. Or adding more complexity in some cases.

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Wood changes colour when it is viewed with different light directions - little with spruce, much more with the flaming of maple, for example.

 

Varnish changes colour when viewed as a thin or thick section depending on incident light - compare the effect when looking through a drop of varnish (my varnish goes from green to ruby red as a drop on a glass rod).

 

I have not noticed any directional colour change uniquely associated with the ground or sealer. Tinting and pigments will have an effect to enhance the wood colour changes, but this is very unlike what is seen with varnish.

 

I will let Magic Molnar define the terms and apply the correct one.

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There are several different things going on with varnished wood, as Janito described, and trying to ascribe the whole effect to either ground, wood, or varnish is oversimplifying things.

 

The most obvious effect is directional reflection of the grain, creating most of the contrast in the flames.  This is affected by a few things:

-Lighting and viewing conditions

-Ground:  how well it wets the wood, and how deeply it penetrates the cells

-Wood darkness:  new, white wood allows more wood to penetrate deeper and scatter back out randomly, reducing flame contrast

 

True dichromatism is a pigment or dye related effect, with different apparent colors depending on film thickness.  So that would be in either the ground or the varnish.  

 

I'm not sure about the intensity effect combined with dichromatism (Mike?), but I think that the directional reflection of the grain combined with a dichromatic varnish would show dichromatism too, even though the varnish thickness is not different.... just the light intensity that you see.

 

There is also another thing to confuse the issue:  filling the wood with colored material.  This would slightly "burn" the flames, but produce some other color effects.

 

Even just the varnish is more complicated than rocket science.

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The term is indeed dichromatism. William gave a great reference. You can see this effect with Indian Yellow pigment found in artist tube pigments. The color varies by the "optical depth". 

 

Don is right that there are other secondary effects that make the wood sparkle and glow as it were.

 

Members of the VMAAI saw my paper on this topic. Don presented it on my behalf when I could not make it to Tucson.

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I like Mikes' approach, read the paper !

Since old Cremonese fiddles have less varnish  on them it makes sense 
that you'd associate the dichromatism with the ground and less with the varnish. 

People often talk of the Cremonese ground as 'shimmering' etc. 
Poetic words can be hard to evaluate and intimidating. 
I like to think that humans (Strad was) made violins back then, so anything
they did can be seen as human effort, not magic. 
Padding talks about traditional alchemy in a historical context, puts it to bed. 

So I'll re-read the section about dichromatism opalescence chatoyance and the other stuff he mentions. 
I think the gist is the layers of varnish & ground & sealer combine to hold the light 
in them, reflecting to send back different / changed nuances to the eye. 
The depth of wood surface itself fluctuates (maple more than spruce) in it's orientation to the plane at 90 deg to the eye.
As Padding notes, your eye could be very close to the varnish and still not be able to isolate the direction
of effects emanating from the object. 
The eye sees what the brain can't identify, pinpoint. 

  

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I like this topic but a picture is worth a thousand words.  I would be quite interesting if some of you  would post a picture that illustrates dichromatic appearance in either  ground, varnish or wood.   Wood itself is not dichromatic.  Don't confuse chatoyance with dichromatism. 

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I agree.  A video would be much better.   That's why I usually do a short youtube video on the rare occasions when I actually make something.    A video in natural light can't be beat!

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I'm not sure about the intensity effect combined with dichromatism (Mike?)

 

After doing a little more research on this, I think the answer is that intensity of the light does not affect the color... objectively, as would be measured by a spectrometer.  However, what we perceive may be something else.  Dim orange would look brown, I think.

 

This video is NOT DICHROMATISM, but pure chatoyance, as it is bare planed maple.

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I like this topic but a picture is worth a thousand words.  I would be quite interesting if some of you  would post a picture that illustrates dichromatic appearance in either  ground, varnish or wood.   Wood itself is not dichromatic.  Don't confuse chatoyance with dichromatism. 

 

Hi Mike

Generally when we get to the highly colored varnishes that we see in in the classical post 1700  Cremonese they look yellow in a thin layer or red in a thick layer. The color changes if you are looking directly into the varnish at it's narrowest or obliquely when you are looking through a greater thickness. Resins often show a big color change here.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Hi Don,  yes,  dark orange is the same thing as brown.   So intensity of light could have an effect on perceived color.    

Melvin, that's nice looking varnish.  Where the flames of the maple are dark it looks redish and where the flames are reflecting the light it's is yellowish.  Nice!   I've seen that in dark cooked rosin.  

Cross lighting from different light sources can produce interesting effects also. 

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I think that lighting intensity could have an affect on the color seen in varnish since different receptors in your eye have different sensitivities. The red receptors in your eye work poorly in low light so I would expect varnish to look less red and more brown, orange, or yellow as the light is dimmed. This would be purely due to how the human eye works, the color measured by a spectrometer or colorimeter would be the same regardless of the intensity.

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The colour temperature of a light source under which an instrument is viewed, at least in some cases, seems to influence aspects of the colour seen.  Certain Cremonese varnishes can look quite red under incandescent light but washed out/almost brown under some fluorescent lighting.  Even under natural light, the time of day, cloud cover etc., will influence what you see.

 

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I like this topic but a picture is worth a thousand words.  I would be quite interesting if some of you  would post a picture that illustrates dichromatic appearance in either  ground, varnish or wood.   Wood itself is not dichromatic.  Don't confuse chatoyance with dichromatism. 

How about the pictures at the beginning of this thread? http://www.maestronet.com/forum/index.php?/topic/320397-how-to-make-a-dichromatic-colored-varnish/ In this thread I made a dichromatic varnish by mixing several colors that were not individually dichromatic.

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Here is a copy of the paper published in the Violin Makers Assoc. of Arizona International

 

Many thanks for sharing.

 

The colour differences I have seen looking through a drop of varnish (no pigments, tints or colourants) with various angles of transmitted sunlight are quite striking (olive green to red), but I am not sure that it is solely due to the thinkness of the varnish through which the light is transmitted. Almost as if it is being selectively filtered (?refracted) depending on the incident angle of the light.

 

Is this possible?

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M.Molnar thanks for that PDF!   I only had time to skim over it just now.   In there you mention green/yellow.   According to google the most dichromatic substance is pumpkin seed oil which is green in a thin layer and then goes to red.   Maybe they figured out a way to make that a hardening oil.    Probably not.  I'm just thinking outside the box some.  

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The wikipedia definition and explanation is good, but it occurred to me that another level of complexity exists (as if we need one).   The example given is for pumpkin oil, a transparent material with a complex transmission function.  But what happens to the non-transmitted light?  Presumably, in pumpkin oil it is all absorbed.  But say we were to add a pigment to the oil:  now we could have something that not only displays different colors depending on thickness, but now would look different depending on whether the light is transmitted or scattered.  Applying such a coating to a mirror, it would look more the color of the pigment in diffuse light, but the pumpkin oil effects would take over when you reflect light off of the mirrored surface.

 

Now for a figured maple example, take some yellow (clear) varnish, and add a slight loading of not-too-transparent dark red-brown.  When a flame reflects light, the yellow would dominate; when light is not reflected (now only scattered from the pigment), you would see a dull red-brown.  This all depends on getting the pigment load and varnish thickness just right.

 

 

The colour differences I have seen looking through a drop of varnish (no pigments, tints or colourants) with various angles of transmitted sunlight are quite striking (olive green to red), but I am not sure that it is solely due to the thinkness of the varnish through which the light is transmitted. Almost as if it is being selectively filtered (?refracted) depending on the incident angle of the light.

 

Is this possible?

 

I think this may be related to my above comments.  I recall making a solution of chlorophyll, which looked green from scattered light but deep red from transmitted light.  What is transmitted is one thing, but the non-transmitted light might be scattered or absorbed, depending on the material.

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The term is indeed dichromatism. William gave a great reference. You can see this effect with Indian Yellow pigment found in artist tube pigments. The color varies by the "optical depth". 

 

Don is right that there are other secondary effects that make the wood sparkle and glow as it were.

 

Members of the VMAAI saw my paper on this topic. Don presented it on my behalf when I could not make it to Tucson.

 

I wonder if the spectral sensitivity of a digital camera chip is to be found anywhere.   Also how "red-eye" is handled by the camera processor.  Since these things seem to be adjustable to some degree,  it would be a nice way to illustrate or experiment with color sensations. 

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