Official Luthiers Forum!

I don't recall referring to water on wood (I still want a scratching head emoticon) but none the less; Refraction is the bending or alteration of light as it passes from one medium to another. Dictionary, encyclopedia and like Al said wood is not a pure solid the cells do refract light. It is also a diffuser so at a given density it will defuse enough to stop a given amount of light from being visible on the other side. The amount refraction and diffusion being subject to both density of the wood and intensity of light.

Look at it this way you can put a lamp inside a guitar and view the braces. If it did not refract light, If it was purely opaque, this would not be possible. Now you’re going to say but you can’t see the braces on the back. Well you can, if your light source is intense enough. But with that much intensity come heat that just might create an issue.

I do agree that the wood would (???) transmit light. But in a different manner due to the porous nature of wood. Take a large bunch of flat black marbles and shine a beam at the bunch. Some of the light will leak through the other side by virtue of clear path and diffuse reflection (mostly that). But the light will not refract. Maybe its a "diffraction" effect as well!

Same with wood. I am sure I am not making this point well. I thought refraction had more to do with the crystaline properties (dielectric). Yes, fluids and gases refract as well.

Perhaps the surface effect (enhancement) when water is applied has more to do with multi-reflection paths induced by the surface water.

Google "refractive index for wood" and "refractive index for glass" and compare the hits. Perhaps its just a linguistics thing...

Mike

Refraction most certainly is affected by the crystalline properties.

Question; Could one thin ebony enough so that a 60, 30, 10, or even 1 watt focus beam of light would be visible from the opposite side of the ebony that the emitter is on?

If the fibers had no translucency the energy rays from the bulb would not be visible.

If it had no refractivity and only pores were allowing straight line light through then all you would see is the straight line rays through the pores if viewed head on. There would be no glow on the view side of the ebony at all just a bunch of points of light.

Now viewed straight on with the piece of thin ebony what really will happen? We would see intense light equal to the diameter of the beam the beam and a noticeable halo of rapidly diminishing intensity slightly larger than the beam

This halo affect is due to what???????? ....... you got it....Light being Refracted!!!!!!!!!! If I can't win you over with that scenario then

I am too tired to respond intelligently... but do me this, explain the crystaline nature of wood.

Mike (who believes wood does not refract light)

PS: what did your google search turn up? And don't forget the effect of "diffraction"

Maybe you can use beer instead of egg whites for the top of the guitar. Then when you get thirsty you just do a few hot licks

"Hey honey, my refractive index is low - give me another beer !!!"

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Don't they have little blue pills for refractory indexes - excessive ones...?

I'm using Ever Clear in my FP for the very same reason.

Really?

Somehow I thought that shellac was a common base for nitro.

If you say it Alan, I believe.

_________________
Horton, MI

I've always shot nitro over shellac.

Well, I choose to continue taking this seriously. I believe the original comment that stirred this up was about matching the refractive index of the finish to that of the wood. This is what brings the luster out? Look at metal flake painted cars. Before the clear coat goes on, the paint is dull and lifeless. Add the clear coat and the paint looks 3 feet deep. Why?

This is my theory: (which means it is open for discussion and criticism and in no way means I am correct)

Certainly not refraction of the paint. This, IMHO, is a multi-path reflection problem combined with the "grating" effect of the wood grain (which may imply a diffraction & polarization effect). It DOES depend on the refractive properties of the coating. But it also depends on how well the coating creates internal reflections off the coated grain surface back to the mirror-like top of the surface (and back down again, etc). The more diverse the surface grain, the richer the reflective pattern.

The "refractive" property of wood plays little or no part. There are no data out there for wood because it is not transparent (in the crystal or liquid sense). Yes, it is made of celulose, and refined celulose has a measurable refractive property. But those peroperties do not necessarily extend to materials in their unrefined state... no more than peroperties of metals and their ores.

To extend my argument, consider a finish before buffing. What is it about the buffing that causes the surface appearance to gain depth? Its the mirror effect creating multi-path reflections inside the coating. I do not deny that the refractive properties of the coating play a role... they do. The optical clarity of the coating is important too. The level of polishing is important. The thickness of the coating is crucial. The grain pattern (grating) is immensely important. I think if one were to apply this finish to a common pine board (wide grain) the effect would not be near as impressive as with figured hardwoods and fine grain spruces. Finally, the refractive property of the wood (if you could even find a value for it)plays no role.

Mike

While I do not concede I shall retire

by the way wood refractive affect on the visible light reflected from under a finish was never part of my summation.

Michael,

OK, I agree this has gone on too long . Sorry for beating a dead horse.

I do think it is an interesting subject.

Mike

I have no regrets

Rats! Don't I get the last word?

Hmm,,I wonder where Ervin Somogyi stands on this?

Joking of course.

But back to the suggestion of egg white as base coat, this is very interesting and could work very well to seal against colour leeching for those wishing to shellac the inside of a guitar as a buffer against RH swings.

Cheers

Kim

I have just sealed my guitar top with egg white (used it on my previous build) with the intention of French Polishing over the top.

The final egg white wash was a little tinted and as I don't want an streaked "amber" appearance, I sanded down through to near bare wood again- and was pleasantly surprised to notice the stiffness of the soundboard- the stuff acting like "starch" is the right description.

Hmmm. Vertical polarization makes the chubby waves disappear?

OK, back to the egg wash. I want to FP my next guitar. I have all the materials (except the ever clear, and I hear thats readily available locally)

First, the egg wash is for the top only, correct? Next, do I pore fill first? (the epoxy stuff), then egg wash? Or what? After egg wash, do I sand down?

BTW, I still wonder about cornstarch... I did research on the stuff in college. Amazing material. When heated in water, the granuels explode to form a a viscous fluid (of course, as any of you making pudding or gravy know). If you freeze dry the product, it forms a hard, glassy material. This material, when finely ground back into a powder can be lightly humidified and mixed with mineral oil to form a fluid that thickens on command under the influence of an electric field! They also use this powder to make "packing peanuts" which I throw into my sink and dissolve. Very eco friendly.

Mike