You might consider reading this article by Steve Attaway
of the New Mexico Faceters Guild.
Hello Al, Lester, Dave, and the fine members of the Orchid
community,
Thanks for the great compliments!
The question of what causes polish has interested faceters for a
long time and has been the subject of numerous debates. Polishing
theories generally fall into three categories:
Finer scratch theory: The surface is removed with smaller and smaller
scratches until the scratches cannot be seen. Flow theory: The
surface deforms and flows to fill in and level the surface. Chemical
polish theory: The surface is removed atom by atom through chemical
reactions.
Listed below are several technical articles related to the mechanics
of gemstone polish that may help explain the possible mechanism for
polish.
The first article is by Dr. Scott Wilson regarding research that he
did some years ago when he was polishing telescope mirrors and used a
Nomarski microscope to measure their surface characteristics. The
March/April, 1996 issue of the New Mexico Facetor published a
condensation of Scott’s presentation that related this research to
faceting, given at the Rocky Mountain Faceting Symposium held May
11, 1995 in Albuquerque. His results were most revealing. It showed
how damage that left from previous coarse grinding and not removed
during the pre-polish would appear during polish.
Surface_Characteristics SURFACE
CHARACTERISTICS OF SOME FACETED GEMSTONES, MANY WITH POLISHING
HISTORY Scott R. Wilson. NMFG, March/April 1996
Another related topic is an article on grinding, published in the
November/December, 1997 issue of The New Mexico Facetor, and the
article on the Mystery of Gemstone Polish, published in the
January/February, 1999 issue of The New Mexico Facetor.
http://www.attawaygems.com/NMFG/cabinet_makers_and_chain_saws.html
Fine Cabinet Makers Don’t Use Chain Saws, Stephen Attaway, NMFG
Newsletter, November-December 1997.
http://www.attawaygems.com/NMFG/mystery_of_polish.pdf The Mystery of
Gemstone Polish Part 1, (pdf file) Stephen Attaway, NMFG Newsletter,
January-February, 1999.
In the NMFG newsletter article, “Fine Cabinet Makers Don’t Use Chain
Saws”, the grinding damage layer thickness for each grit size was
estimated using a multiple of the diameter of the grit size (4X as
the minimum and 10X as the maximum). Note that all of the damage
cannot be seen. As damage occurs, compression forces may force the
cracks and fractures closed, making them impossible to see without
the use of special optics.
The damage thicknesses listed above were only estimates. Has anyone
measured damage? Yes, they have. In “The Mystery of Gemstone
Polish,” Stephen Attaway reviews the latest efforts by leading optics
researchers to understand the mechanics of damage. Not only have
researchers measured the damage layer thickness from grinding, they
have also estimated the compressive stresses in the damaged surfaces.
Even more interesting is the behavior of some materials for small
grit sizes.
One of the obstacles to understanding polish is that it is very hard
to design experiments that will reveal what is really happening
during polish. In reality, polishing occurs on a very, very small
scale. The surface is actually inaccessible during the polishing
process. No one has yet designed a sensor that can be used to make
measurements during the polishing process. Therefore, what is known
about polish is often inferred from indirect experiments and
observations.
Experiments that have been used to characterize the behavior of
glass and the associated inferences that the researchers have made
from these experiments should/could provide insight to gemstone
polishing.
Summary of the findings: 1) Experimental evidence for size effects in
glasses and other materials. 2) Experimental evidence for two modes of
grinding: ductile and brittle. 3) We inferred that size effects
control the transition between ductile and brittle grinding. 4)
Experimental evidence (Twyman effect) that grinding introduces stress
in the surface. 5) Experimental evidence that grinding with very
small grit generates more stress than grinding with larger grits. 6)
From the stress measurements using the Twyman effect, we inferred
that a thin plastic deformation layer leads to high stress under
ductile grinding. 7) Experimental evidence shows that polishes like
Linde-A and cerium oxide leave a stress-free surface. 8) For glass,
three modes of material removal may be at work: brittle, ductile, or
chemical. 9) We have some evidence that some gemstones may have three
modes of material removal.
By closer examination of the mechanics of fracture and plasticity,
researchers today do not believe that the material actually melts
but simply behaves plastically.
Four material properties have been identified that might control the
=91mechanical=92 aspects of polish: Young=92s Modulus, Poisson=92s Rat=
io,
fracture toughness, and Vickers hardness (yield stress). You may
find it interesting to know that these same properties also play a
role in the cutting and polishing of metals. As far as I know, no one
has measured these properties for a wide variety of The
chemical aspects of polish are much less understood. More testing
will be needed to fully understand the subtle aspects of chemical
polishing.
Yet another good faceting topic is how several scientists at Sandia
National Labs found that water generates microcracks and penetrates
glass, as well as some This topic regarding brittle
material fracture and fractography was published in the July/August,
1999 issue of The New Mexico Facetor.
Enviromentally-caused Fracture and Crack Growth in Glass, Bruce
Bunker, NMFG Newsletter, May-June 1999.
http://www.attawaygems.com/NMFG/program_speaker_bunker.html
Dr. J. Glass covered relating to her work on ceramic and
glass failure analysis, and she correlated the data to correspond to
the faceting of natural
Brittle Material Fracture and Fractography, Jill Glass,
http://www.attawaygems.com/NMFG/program_speaker_Jill_Glass.html
As far as lap speed and pressure are concerned, you may find the
Preston equasion useful. See:
http://www.attawaygems.com/NMFG/preston_equation.html
We invite Orchid members to access the New Mexico Faceters Guild web
site at: New Mexico Faceters Guild
Steve and Nancy Attaway