Re: What makes up the lustre of metals?

Graham, 

Luster (or lustre) is the result of a combination of effects having to do 
with the way in which a material reflects light.  Luster is somewhat of an 
imprecise term and many materials have what people perceive as luster.  

The first effect that causes luster is simply the reflection property of the 
surface material.  That is, what happens to a ray of light when it hits the 
surface?  A surface is called "specular" if it reflects light perfectly, 
like a mirror.  In other words, the ray bounces off the surface at an angle 
that is opposite of the angle of incidence.  A surface is called 
"lambertian" if it diffuses the light, like flat wall paint.  In other 
words, the ray splits up at the surface and equal amounts of light are 
scattered in each direction.  Most surfaces are part specular and part 
lambertian, giving them their unique reflective properties by the proportion 
of each.  Generally, the more specular a surface is, the more strongly we 
perceive a luster.  Highly polished metals are specular, which gives them 
their characteristic luster.

Another effect that impacts luster is the penetration of light into the 
material.  Light will penetrate into some materials more than others.  In 
addition, some light will penetrate while other light may bounce, depending 
upon its angle of incidence to the surface.  Penetration depends upon a 
number of factors, such as the transparency of the material to various 
wavelengths of light and the orientation of the molecules within the 
material.  As with reflectance, transparency tends to be a continuum from 
very transparent, like glass, to opaque, like brick.  As a light ray 
penetrates a material, it eventually may bounce off of a molecule of a 
different material or encounter a change in molecular orientation.  The 
light may be absorbed or reflected back, or even absorbed and reemitted at a 
different wavelength.  All of these factors affect perceived luster.  
Compare the luster of a pearl to that of an opal.  Both are combinations of 
reflection and penetration and other effects.  Compare the luster of a fancy 
automobile with many layers of clear gloss lacquer to that of one with only 
a factory paint job.  The layers of gloss add to perceived luster as a 
"depth" in the paint, because some light (at low incidence angles) reflects 
off the surface, while other light penetrates and reflects "deeper" into the 
finish.

Cheers, 

Todd