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The Beilby Layer


#1

Hi all,

Is there anyone out there who can explain the Beilby Layer to me in
scientific terms. I have a chemistry degree and although a lot of my
knowledge has seeped away due to none use, I can understand
scientific speak. I have done a google search to find out what
exactly the Beilby Layer is and its significance in finishing but
couldn’t find any conclusive answers.

I’ve seen it referred to both metal and gemstone polishing and am
curious as to its scientific definition and its importance.

Thanks.
Helen Hill


#2

Helen,

The Beilby flow explanation for polishing was discredited long ago,
and no one accepts it today. it implied that with the use of certain
oxides, some mineral material get hot enough locally to allow the
layer to flow, thus creating a polish. Doesn’t happen, can’t happen.

HOWEVER, it is known that plastic deformation can and does take
place during polishing of certain stones with certain oxides, but
heat is not involved. It is a complex mechanical action involving
shear limits and plasticity of the material.

Examination of surfaces with a scanning electron microscope formerly
believed to be polished by the Beilby principle show the tell-tale
minute scratches of conventional abrasive polishing. This subject
usually comes up in connection wth cerium oxide and the quartz or
beryl family. Cerium oxide breaks down rapidly in size under
pressure and the polish that results is indeed exceptionally
fine…but it is an abraded surface not a flow layer. Beilby flow
does not exist.

Wayne Emery


#3

Oh boy…Helen, you have opened a can of worms. I thought the last
few messages on this subject might awaken the monster but…nothing
significant appeared.

Now, you have asked the single most debated question in the lapidary
discipline.

G. T. Beilby’s hypothesis was documented in his paper, "Aggregation
and Flow of Solids, Macmillian and Co, London, 1912! Since then a
great deal of research has been done, especially in the late 1990’s
and early 2000s. The best and clearest description of the polishing
process, of which the Beilby layer is part, are papers by Stephen W.
Attaway, Ph.D in the 2003 timeframe. You may still find some of this
at the New Mexico Faceters Guild site at:
www.attawaygems.com/NMFG/preston-equation.html. In several papers, he
discusses Fracture Mechanics, Plastic flow, Plastic process zones,
etc., and describes in some detail the theorys of - Finer Scratch
Theory, Flow Theory, and Chemical Polish Theory. Beilby fell into the
second. Further, Gerry Wykoff did some significant work on these
theories, and in his, now defunct, American Gemcutter letters,
stongely supports the latter of the three…describing polishing as
an electrochemical process. HHMMm interesting to say the least.

Supporting Dr. Attaway’s research is work by Dr Scott R. Wilson,
Ph.D whose papers are also available at the above site.

I hope this gives you what you need to understand the process.
Beilby’s theory currently is not highly regarded though he is given
recognition for a possibly a correct theory for the wrong reasons.

I find that John Sinkankas doesn’t even mention Beilby in his
famous, Gem Cutting books describing polishing as a controversial
subject on which few agree. On the other hand, Robert Webster gives
some non-scientific explanations of it in his “Practical Gemmology”,
N.A.G. Press Ltd. London, 1978 reprint.

Chances are you will never fine a truly scientific explanation of
this theory other than in Beilby’s original paper and that is, at
best, a theory.

Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry and where the July classes have commenced at
the Boca Raton Museum of Art School!!


#4

I think the current consensus is that it was an interesting theory,
but turned out to be wrong. It’s pretty much discounted among
faceters now. One of the other faceters on the list might have some
references at hand. If not I’ll try to dig up some. I’ll also look
for an article that I remember written by Dr. Stephen Attaway,
describing another theory of polishing.

The basic problem is that the “finer and finer scratches” theory of
polishing doesn’t fit well with facts like polishing quartz and
beryl with CeO2, which is softer. That’s why we have people
theorizing about why it works so well.

Al Balmer
Sun City, AZ


#5

A Beilby layer is a thin layer of amorphous material, usually about
15 nanometres thick on the surface of a polished material. It is
caused by the displacement of the atoms (dislocation) during
polishing, whether mechanical or chemical or by ion bombardment.
Essentially, this layer has lost its normal atomic structure. It
gives rise to problems with characterisation of many of the new
materials used in making the next generation of semiconductors and
the like which are grown epitaxially on a particular crystal
orientation of a substrate. In the polishing of metal for a super
high shine finish it is a plus because it hides the small differences
in reflectance of the different parts of an alloy. Try making a
copper/silver alloy from a molten state and cool it slowly. You will
get separate dendritic grain growth of the silver and copper metals
and when you polish this will be visible. Repickle the piece of metal
and polish it with a lot of pressure. the surface will then smear and
flow slightly. The visible result of this is a brigher and even
surface. Electropolish (bomb) the surface and it will show its
crystal grains again because the Beilby layer has been removed.

Nick


#6

I can give it a try as it relates to jewellery.

In order to make material ( gold, silver, gems ) to reflect light,
the size of the irregularities on the surface must be smaller that
the 1/2 of the wavelength of light used to observe the surface.
Visible spectrum is between 3500 and 6500 angstrom. This means that
if we want to achieve reflective surface using abrasion, the
particle size of our medium must be less than 1750 angstrom. Diameter
of the atom approximately 10^-11, 1 angstrom is 10^-10, so we talking
about a particle made of less than 17500 atoms. Very tall order
indeed.

I do not know if it is possible to manufacture such medium with the
present technology, but it was definitely out of reach in the
beginning of 20th century and before that; but craftsmen were able
to produce reflective surfaces nevertheless.

G. T. Beilby was first to propose theory to explain how reflective
surface can be achieved without use of super-fine abrasives. Beilby
was trying to explain mechanism used in polishing crystalline
material. At first his theory was rejected simply because it cannot
be proven, but later 2 scientists Bowden and Hughes accepted his
theory and it was published as Beilby-Bowden theory of polishing.
That theory was challenged again and finally rejected after Crowcroft
published Demise of Beilby-Bowden Theory of Polishing. However,
Beilby theory still holds in polishing of metals. Beilby correctly
observed that polishing generate heat. Heat causes high points of the
surface to melt and flow into lower points thereby creating smooth
surface capable of reflecting light. As it relates to gem polishing
the theory was rejected because in gem polishing water is used which
is very efficient heat conductor and hypothesized melting would
never take place. Present theory is based on the idea that in the
presence of water the hydroxyl formation take place, microscopic
fractures develop in hydroxyl layer, that causes flaking of the
hydroxyl layer and the surface is reduced to reflective state. But in
metal polishing no water is used. The heat generated does cause the
changes proposed by Beilby and resulting layer is called as Beilby
layer.

Practical importance:

If we finish surface using abrasive medium, under magnification the
surface would look like a surface of the moon. When such surface is
heated, the oxide formation takes place; the oxides are dissolved by
the flux and the result is deepening of the surface irregularities.
After pickling the surface becomes matte no matter how well it was
polished prior.

If Beilby layer was achieved on the surface, as long as re-melting
and re-crystallization would be prevented, the surface will remain
in the original state after soldering.

Problem is that achieving Beilby layer uniformly requires long time
and the process does not fit within cost structure of most
businesses and therefore it is not taught in modern schools. This
also
explains why it is so difficult to polish platinum, since the melting
point is much higher then gold.

Leonid Surpin.


#7

On a polished surface it is a layer that looks like the surface has
been possibly melted and reflowed.

At the point of contact with the "polishing " it appears that the
surface temperatures are near the melting point but this temperature
is not perceptible to our senses. The area where these effects are
studied is called Tribiology ( includes lubrication) – search this
term. Galling ( welding and then sheared) which appears on similar
metal surfaces in rubbing contact is part of this. Tribiology is a
relatively new term which I believe originated in the UK. Polished
gem surfaces show the same effect under high magnification. There are
a number of engineering books on the subject published in the last
few years. It is a very important area of study in wear prevention
(lubrication) and in the area of planarization-- surface ( leveling)
for manufacturing semiconductors.

jesse


#8

Nick,

Very interesting to see a different opinion. Most seem to think such
a layer does not exist but your explanation seems to support it,
although as “coralnut” said, Beilby is given credit for perhaps a
correct theory but for the wrong reasons.

I like the sound of the copper silver alloy allowed to cool and the
separate dendritic growth patterns. That sounds rather attractive.

Thanks again Nick.
Helen


#9

Greetings

The Beilby flow explanation for polishing was discredited long
ago, and no one accepts it today. it implied that with the use of
certain oxides, some mineral material get hot enough locally to
allow the layer to flow, thus creating a polish. Doesn't happen,
can't happen. 

I’m not a scientist, but I remember while polishing an opal with
cerium oxide I watched the surface of the stone ‘flow’ to a perfect
polish.

I don’t know if that was ‘the beilby effect’ or what.

KPK


#10

I was told a long time ago that the Beilby theory was the
explanation behind the burnishing of non-ferrous metals - ie. the
pressure of the burnisher created heat of sufficient temperature to
micro melt the surface and cause the “flow” resulting in a polished
surface.

Since, however, burnishing is usually done with a lubricant - oil,
soapy water, saliva, whatever - which also acts as a coolant the high
heat theory never felt right. My experential explanation for this
metalurgical metamorphisis of surface is compaction. Simply put, the
tighter the atoms of the material are against each other the less
surface irregularity to break up the reflecting light rays.

Polishing on a wheel, however, seems to be of a different nature in
that poor crystaline structure cannot be improved at the surface as
it would be with judicious burnishing. I suspect, though, that a best
case scenario re the material being polished is that we’re still
dealing with a micro-burnishing effect as opposed to more exotic
transformational relationships between the metal and the media.

Les Brown
L.F.Brown Goldwork


#11

Helen I am not a chemist but I had the Beilby layer explained to me
many years ago. He was trying to understand how a relatively soft
substance like tin oxide or cerium oxide could polish hard agate. In
polishing jade he found that a well sanded peice would polish while
one with scratches generated “orange peel” He decided that certain
metallic oxides formed a molecular bont with the item being polished
and “smeared” the top layer of molecules to create a polish

Tom an old lapidary


#12

Beilby Layer, n - an altered surface layer of supposedly amorphous
material formed on a crystalline solid during mechanical polishing,
whose existence was proposed in Sir George Beilby’s writings. The
existence of such a layer is not supported by recent research, and
the use of this term is therefore considered archaic and is strongly
discouraged.


#13
I'm not a scientist, but I remember while polishing an opal with
cerium oxide I watched the surface of the stone 'flow' to a perfect
polish. 

That’s exactly what I get when polishing silver which is why I asked
the question in the first place. Don’t ask what compound I’m using
though, as I don’t know. My husband had it in the garage and he used
it to polish the exhaust pipes on my motorbike to a mirror finish
years ago. It’s a block in a sort of pale blue colour.

Helen


#14

I’ve seen a number of posts on this subject, but you folks might be
interested in going a little further and reading an article by
Merrill O. Murphy in the archives of The Eclectic Lapidary, January
1, 1998 titled, “Gems Polish Because…”

http://tinyurl.com/yr46kg

It’s a fantastic piece discussing many aspects of polishing and the
theories to explain why. There’s no absolute answer, but there are
some strong suggestions and opinions by a man of science who was
also a faceter and friend to the lapidary community for many years
before his passing in 2004.

Carol
Carol J. Bova


#15

So what am I looking at in my Transmission Electron Microscope then
when I see an amorphous layer on top of most polished and ion beam
milled samples? We also electropolish mechanically polished
specimens of metal for Electron Backscattered Diffraction Patterning
(EBSD) otherwise we cannot see the crystal lattice structures
(Kikuchi lines) because of this amorphous layer.

If it walks like a duck, quacks like a duck…

Nick


#16

Kevin,

I’ve watched it many times on many species of stones…but Beilby’s
explanation included heat, a sort of melting at the point of
contact. Doesn’t happen.

What DOES happen with some materials, is plastic deformation of the
surface. This has been confirmed again and again under the scanning
electron microscope when preparing minerogical thin sections for
viewing with a petrographic microscope.

As someone mentioned, there’s a “flow” process, just not the way
Beilby described.

Unfortunately, the resistance to plastic deformation is so high in
many mineral materials that only mechanical disruption (abrasion)
takes place. Scratch polishing. But some materials, when being
polished with the oxides, react plastically and opal is one of them.

Wayne


#17

The Beilby Layer has pretty much been rejected for gem stones by the
scientific community because of no adequate explanation of where
there energy needed to satisfy the requirements for the molecular
work function can come from, friction doesn’t hack it. When the
theory was originally proposed, work functions weren’t understood, or
known.

The hydroxide arguments are known to be valid for glass but I know
of no research that shows that it exists or would work on any other
stone, glass has several properties that make it unique.

Also heat, in general, isn’t required for polishing. Although it is
generated is many of the ways to polish Think about the
polish that can be attained by tumbling (by some people). Some work
was done several years ago at Lawrence Livermore National Lab on
polishing a ruby for a laser. The ruby had been ground to a specific
shape using a computer controlled diamond grinder but there was no
polishing step available at the time that would not have altered the
shape. The ruby was polished by circulating a polishing compound in a
liquid around the ruby, it wasn’t fast, but the required polish was
attained and no heat generated. I don’t remember the polish or
liquid.

Work with electron microscopes mostly show the scratches on polished
surfaces although there are exceptions. Add to that the fact that
stones will begin to show a good polish long before the mathematical
requirements for a polish are met and you are guarantied to be able
to get more than one opinion on what is going on.

Just to confuse the issue a little more, consider glass. I don’t
know what the scientific requirements would be for defining a liquid
but for all practical purposes glass is a liquid. Electron microscope
analyses has shown cerium oxide molecules under the surface of
polished glass. The surface had to flow for that to occur. But as a
liquid, the surface work function doesn’t apply.

But consider agate. Agate is a fibrous material, to get surface flow
you not only have to get molecular motion, you also have to get
molecular motion across fiber boundaries. I know of no adequate
explanation of how this could occur, and since agate will polish very
well with no heat applied at all, I will reject the surface flow
theory until someone can demonstrate how and under what conditions it
could work.

Dick Friesen


#18

Helen,

Metal and stone often respond differently to abrasion, ablation,
burnishing, etc. Please don’t compare what might happen with silver
to what might happen with opal or corundum. There are huge
differences in how these materials respond.

There is no such thing as the Beilby flow effect.

Wayne, dead horse beater


#19

Nick,

It’s plastic deformation, but NOT a Beilby layer, which called for
melting and re-solidification, which does NOT happen.

Wayne


#20

Wayne,

I know they’re completely different but a lot of what I’ve read has
referred to metal. And I’m aware that what I observe on silver is NOT
the surface melting but rather the wax carrying the abrasive agent
melting and carrying atoms of silver to different areas.

Helen