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Advocatus Diaboli

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

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The sensitivity of your camera's chip is fairly easy to figure out. Look up your camera's information sheet, figure out the model of the CMOS (or CCD) chip inside, google that to find the chip's datasheet, and it will most likely be there. What would worry me is the processing done inside the camera. Pictures are normally saved as jpegs so there is image compression going on plus whatever on the fly adjustments are done to the image to make it look better. It would be better to use a simple camera like the ones used for digital astrophotography. Those ones save a non-compressed bitmap image that is easy to work with in programs like Matlab. Plus it is super easy to find spectral information for the chips inside them.

 

If you are electronically inclined there are small integrated circuits with pin diodes and multiple color filters that are designed specifically for colorimetry.

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This is a flat spruce surface where using  tungsten light a picture is taken and another is taken about 180o, essentially opposite direction. Note the pencil dot along one pencil line along the edge where a dark reed is a little longer and almost runs to the pencil line. When viewed on the other photo the reed line of slow growth appears totally different. I guess you can call this dichroism. Hope Mike or Don can comment.

The coating is made with a mix of 4 to 1 ratio of rosin to oil, the film cooked with copper that Nagyvary found as a ground coat in  Cremonese inst's. It is also very reflective.

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Looks orange-yellow all the time to me (not dichromatic), just brighter when the wood fibers reflect in the right direction.  I don't know if you'd call that chatoyance or not in spruce, but it's reflective too.

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It looks like this is just due to texture. The hard late growth lines have soaked up less varnish or were smoother before varnishing. After varnishing the late growth has a smooth varnish that produces specular reflection, the softer earlier growth has a rougher surface and is giving diffuse reflection. Unrelated to dichromatism. At least that is how it looks to me in the pictures.

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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?

What you are describing is more like dichroic material. However, the optical path length can be lengthened if there are (internal) reflections compounding the actual path length. This would describe dichromatism.

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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?

Simple refraction.  The drop is acting like a prism and showing a spectrum as you change the angle.  The same effect gives us rainbows, and the brilliant "fire" of diamonds, CZ's, etc.

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I have a rather typical looking very old German violin in my shop for restoration. Well, it seemed typical until I started trying to color match the varnish. When the pseudo-Stainer archings are viewed from the side, the varnish appears warm, clear, Amber with the grain beautifully reflective. When viewed straight on from the back, at the arching rises it appears deep brown. I have never honestly seen anything quite like it, and I have seen many hundreds of instruments. I think this points to the distinction between dichroism and dichromaticism. Does this makes sense?

Can someone suggest a procedure for color matching the varnish on a through patch for this instrument? I have tried darkening the spruce wood with potassium bichromate and tannic acid, and then oil varnish with Hammerl Dyes. I'm on my second try and it is not going particularly well. I think it is an issue of refraction in the varnish. The original varnish changes a great deal in various lights, but my color matching attempts look somewhat good only in daylight. Most challenging to me is trying to achieve that quality of color variation and transparency according to viewing angles. Diffuse light, incandescent light and other flourescent light makes the original varnish look less interesting, but th new varnish looks redder in these conditions even though no red has been used. (I have also experimented with tiny amounts of black and also catechu. This has helped color wise but has taken me further from the clear refractive qualities)

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I have a rather typical looking very old German violin in my shop for restoration. Well, it seemed typical until I started trying to color match the varnish. When the pseudo-Stainer archings are viewed from the side, the varnish appears warm, clear, Amber with the grain beautifully reflective. When viewed straight on from the back, at the arching rises it appears deep brown. I have never honestly seen anything quite like it, and I have seen many hundreds of instruments. I think this points to the distinction between dichroism and dichromaticism. Does this makes sense?

Can someone suggest a procedure for color matching the varnish on a through patch for this instrument? I have tried darkening the spruce wood with potassium bichromate and tannic acid, and then oil varnish with Hammerl Dyes. I'm on my second try and it is not going particularly well. I think it is an issue of refraction in the varnish. The original varnish changes a great deal in various lights, but my color matching attempts look somewhat good only in daylight. Most challenging to me is trying to achieve that quality of color variation and transparency according to viewing angles. Diffuse light, incandescent light and other flourescent light makes the original varnish look less interesting, but th new varnish looks redder in these conditions even though no red has been used. (I have also experimented with tiny amounts of black and also catechu. This has helped color wise but has taken me further from the clear refractive qualities)

If you have a photo server or a website you might try linking us to a photo...  You can upload to this site after 10 posts are approved, but you have a ways to go there. 

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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?

Sorry for coming in so late! :)

Dichromism depens on many things, however the main factors are the transparency of the varnish and the internal structure of the wood. The sharpness of the tools is also a factor.

When you cut wood, you expose the the shape of the reeding or cell ends. theses shapes look different from different angles. The shape of the end grain cells are more pronounced in curly maple  or other irregular grain structure.

Simply put: when you draw a perfect circle on the floor it looks oval from an angle and a different ovals from different angles.

So transparency of the varnish, the clearness off the cut and finish are the critical points.

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Hi Don, thanks for the comments.  I guess having a reflective surface one way and an ordinary darkish line another way is not  dichroism.  I've stripped an old violin and i'll see how it behaves as a ground coat with a colored rosin oil varnish over it. It is a very nice gold yellow, apologize for the orange.

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Does it look like that to your eyes or is that from the camera?   I've gotten that intense bright gold in photographs but it has more to do with the photography than a real effect.   

 

Sort of dichromatic-ish along with chatoyance. 

 

  post-31367-0-18445900-1452390855_thumb.jpg

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Here's another example, The Betts.  Picture taken from iPhone with no adjustments to the picture.   The pictures are an accurately represent what the varnish looked like in person. 

 

-Jim

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If you have a photo server or a website you might try linking us to a photo...  You can upload to this site after 10 posts are approved, but you have a ways to go there.

Thanks, Jeffrey. I will see if hat is possible.

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The sensitivity of your camera's chip is fairly easy to figure out. Look up your camera's information sheet, figure out the model of the CMOS (or CCD) chip inside, google that to find the chip's datasheet, and it will most likely be there. What would worry me is the processing done inside the camera. Pictures are normally saved as jpegs so there is image compression going on plus whatever on the fly adjustments are done to the image to make it look better. It would be better to use a simple camera like the ones used for digital astrophotography. Those ones save a non-compressed bitmap image that is easy to work with in programs like Matlab. Plus it is super easy to find spectral information for the chips inside them.

 

If you are electronically inclined there are small integrated circuits with pin diodes and multiple color filters that are designed specifically for colorimetry.

thanks Wm.  I think I would not do well finding information about the compression of jepg etc.  Maybe something could be done with filters and experiments.

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Seeing by eye direct is the only way to sense what's really going on, as Padding
explains in detail, the human eye has a built in exposure compensation and works with
the brain to produce an interpretation of what exists....that is what matters, not super high or 
super crap resolution debates. 
The inevitable 'here's my latest effort' photos bear that out and only eye to eye
can anyone really tell what is what. 

Bruce C generally has excellent photos to share and often explains them in context very well, 
hence his inclusion in the Padding book. Even there, eye to eye is best. 
:-)

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It's the wood ;)

 

Doing some tests for my next one at the moment, just some (boring) photo of maple with ground. I'm against staining wood and burning the flames and think that you have to adapt the grounding/varnishing to the wood.

 

post-37356-0-96563600-1452441802_thumb.jpg

 

 

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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.Zamiast tego przetłumacz z Nie moge zgodzic sie z tak plytkim podsumowaniem. Nie "m

 
I can't agree with such a shallow summary. It is not "magic" but "chemistry" - this is creating visible changes. You need to have the appropriate materials, time and imagination to "work magic" of wood many different features, color, texture, polymerization wit a wood. Then you can create miracles...

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Peter K-G, thanks for showing what I dismally failed in an earlier post. This is another try, moving the light and camera and using a neon-like tube bulb. It could be just light reflectance as Don suggested, but isn't light shift the alternate of the surface moving to create the change. I did put on another coat.

 

I think top wood is the challenge and I wish this is also shown in posts.

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A scroll in direct and indirect sunlight at once to illustrate...

Full_Size_Render_2.jpg

Interesting photo.  Reminds me very much of what you get from fluorone dyes like the rhodamines.  Low energy reflected light is reddish, above the excitation threshold they emit brilliantly in the yellow.  Could something like that be happening here?  Anybody tried fluorones?  :)

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Peter K-G, thanks for showing what I dismally failed in an earlier post. This is another try, moving the light and camera and using a neon-like tube bulb. It could be just light reflectance as Don suggested, but isn't light shift the alternate of the surface moving to create the change. I did put on another coat.

 

I think top wood is the challenge and I wish this is also shown in posts.

 

Fred, there may be several things happening in your photos.

 

With spruce, incident light direction can have an influence on what you see.

 

Attached below are a couple of photos of spruce (N.Amati) taken under an illuminated magnifier.  In one the incident light direction is along the grain and in the other, across the grain.  In each case the cellulose at right angles to the incident light direction "lights up".

 

In your photos the different light position will be creating a different incident light direction which may be creating a lighting up/highlighting of different aspects of the cellulose structure; i.e., more medullary versus along grain for one light position and more along grain versus medullary for the other.

 

I'll mention another possibility in another post.

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