I recently started to work opal. My main focus has been Mexican
fire opals. I cab the stones with a Diamond Pacific Genie. I’m
doing pretty good, but just don’t seem to be able to get a good
final polish on my cabs. I have tried cerium oxide on a rough
leather polishing pad, but the results were disappointing,
especially if the opal has any small fractures. Then I just seem to
pack the polishing compound into the opal.
Any advice this group could offer would be greatly appreciated.
Jed - Sounds as though you are getting the stone too hot at some
point in the process; other than some boulder opals you should have
NO small cracks to deal with at all. My best suggestion is still to
follow Paul Downing’s cutting/polishing instructions.
Hi Jed, try using a wet slurry of Cerium Oxide on a felt buffing
wheel. The wet slurry helps keep the opal cool and prevent cracking
or crazing. Opal is SUPER heat sensitive. I have not worked any
Mexican opal but this method works very well for me on my Australian
Opal. I try to keep my felt very wet and dont use much pressure when
I am polishing.
I get a mirror finish on opals using a soft leather pad laid over
cork. The lap is slightly convex which helps when working higher
domed stones with somewhat flatter sides. The surface of the pad
should be kept slightly damp and do not overload with polish powder.
As Opal is silca base, cerium oxide should do well but I find Linda A
does better since it is a finer micron powder. Run the wheel at low
speed…say 200 to 400 RPM and use only medium pressure.
Regarding the fractures…there is little you can do in such a case
except work the stone down to 50K diamond on a Crystalpad and when
you go to the polishing wheel, only give it a short/light lick and a
promise to finish the process. I don’t recommend working on a stone
with fractures…natural pits or matrix ok, but not fractures.
Remember, ALL diamond powders, no matter what grit or mesh, work by
abrading while polishing oxides work on an electrical level. The
former leaves the surface slightly greasy or’smeered’ ooking while
the latter provides a clearer and brighter surface.
Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut1
I have tried cerium oxide on a rough leather polishing pad, but the
results were disappointing, especially if the opal has any small
fractures. Then I just seem to pack the polishing compound into
the opal.
Hi Jed,
I also use a DP Genie… a dream machine. Fractures will always pack
with polishing compound. Nature of the beast. Use fracture-free rough
(preferably), or use a toothbrush and water to clean after
polishing. You could experiment, but I don’t think you’ll find a
better compound for opal than cerium.
Are you sure that you’ve thoroughly sanded the stones at the higher
grits, to remove the super fine scratches? With opal, you’ll want to
spend little or no time on the coarse wheels, and more time on the
finer ones. You won’t achieve a fine polish unless all the fine
scratches are removed. The stone should appear reasonably well
polished after the final wheel, before the formal polishing with the
cerium.
Mexican opal isn’t considered to be among the better opals, except
maybe for contra luz opal or fire opal, which is usually faceted and
doesn’t have the famous “play of color”. In all my years of playing
this game, I’ve only seen one Mexican opal I would consider fine gem
quality, and it was set in an heirloom piece.
Not to be offensive, but one of the first rules of lapidary (and
computing) is GIGO: Garbage In, Garbage Out. It is not possible to
create a superior gemstone from inferior rough.
Jed, You may consider joining a lapidary digest. If you send a
request to this address: lapidary-request@caprock-spur.com you can
take part in dialog regarding the polishing aspects of Opal and many
other stones.
Remember, ALL diamond powders, no matter what grit or mesh, work
by abrading while polishing oxides work on an electrical level.
The former leaves the surface slightly greasy or'smeered' looking
while the latter provides a clearer and brighter surface.
Hi Don.
“polishing oxides work on an electrical level” haven’t heard of
that before. Could you please explain further how that works?
"polishing oxides work on an electrical level" haven't heard
of that before. Could you please explain further how that works?
I’d be interested in hearing interpretations of this, as well. My
level of understanding might be a little dated, as far as the science
goes. As far as my goes, its a subject for debate
amongst serious scientists as to what actually happens when a stone
is polished. As opposed to just a successive series of finer grits,
resulting in finer scratches, a polish on a stone is different than
metal. I referred to this on a recent post as a “formal” polish, as
opposed to a really fine diamond grit finish.
As Don stated, the polishing compound is usually an oxide… chrome,
cerium, aluminum, etc. Magic stuff. What my limited mind allows me to
accept as true is that there is an actual molecular flow that occurs,
known as (I believe) the Bielby Process, allowing the surface to
“glaze over”, eliminating the finest scratches and providing a true
polish.
The skeptic asks, how can the molecules flow without an extreme
amount of heat and/or pressure? I dunno! That amount of heat would
certainly melt my dop wax. Granted, it seems that a degree of
friction comes into play… a semi-dry buff, with just enough
moisture to prevent heating up seems ideal for many stones.
I’d love to hear an elaboration on this from any G.G.s or lapidaries
out there!
Dave, It has been many years since I read a paper and several
articles about the polishing theory but will try to give you the
essence of what I remember.
First, the old Beilby theory has been out of vogue for about 10
years now. In that theory, the heat caused by friction caused a very
thin layer of the stone to melt and ‘flow’ causing a glassy like
surface several microns deep. Thats where you got the molecular idea
Dave.
That was proceeded by concept, like in the glass industry, that the
oxides ‘planed’ (not abraded) the high areas of the surface down
until they were all the same level thus creating a smooth glassy
surface (sound familiar?) that reflectes light evenly…thus it is
’polished’.
About 6 or 7 years ago, a researcher did some studies involving an
electron scanning microscope and way out physics. His paper
concluded that the polishing process actually occurs at a sub-atomic
level through an exchange of positive and negative ions caused by a
very low electrical charge. In effect, it was believed that the ph
differences between the stone, usually alkaline, and the oxide powder
mixed with a few drops of vinegar which is then acidic, work as a
battery and cause the the voltage. When this happens, the stone
surface undergoes a change and becomes level.
Hey guys, this is the best I can do until I can locate the
paperwork. Some of you may remember Jerry Wykoff had several
articles on this subject in the now defunct American Society of
Gemcutters monthly newsletter. He also discussed the Beilby theory
and the smooth surface theory in his book, Master Gem Polishing.
That, however, was published in 1994 and this other info didn’t come
out until a bit later as I recall. I’m sure this will get a lot of
different opinions!!
Cheers from Don at The Charles Belle Studio where simple elegance IS
fine jewelry! @coralnut1
G’day; I’m going to stick my silly head out again. I’m so
sceptical about that comment up there that I don’t and can’t believe
it.
Having polished a good deal of NZ jade using tin oxide on a leather
buff I’m inclined to wonder about the ‘molecular flow’ idea I heard
about many years ago. You finally polish jade on a leather disc buff
at about 800 RPM with tin oxide and water slurry. But you don’t get
that mirror finish until you begin to press quite hard, the moisture
has begun to dry off the disc, and you feel a definite drag on the
workpiece.
Far longer ago than I care to think about I had the job of polishing
steel specimens. These were ground as flat as possible, set in a hard
resin, and polished by hand on a series of wet 'n dry papers on a
sheet of thick glass with water as lubricant. When one got to
1500grit carbide, one continued on a slowly rotating disc containing
15,000, and finally 50,000 mesh diamond grit. The result was a truly
brilliant mirror finish. But put it under a microscope at 150x
magnification and one can see the micro scratches where the ultra
fine abrasive removed the surface.
And what did we do with those brilliantly and laboriously polished
metal specimens? Why we put them in strong acid and etched away the
polish! Stupid? We needed to do that to make the fine crystal
structures visible under the microscope.
I still don’t know whether to believe in molecular flow with jade or
not!
Dave, Just the other day I read where the “Bielby layer” theory has
been completely discredited. I haven’t a clue as to what replaced it.
Jerry in Kodiak
As far as I know, the Beilby layer isn’t theory. Beilby proved his
theory by noting a scratch pattern, polishing it away and causing it
to reappear by etching. This was further validated under the electron
microscope by G.I. Finch in 1937. I have read more of this in
Scientific American in the last couple of decades. The Beilby layer
may not form under all polishing conditions. It has been noted that
50,000 grit diamond doesn’t produce the Beilby layer thus John
Burgess is correct when he notes that the steel that he polished
exhibited the scratch pattern. This info is pretty well covered by
Vargas.
I’d sure like to read what everyone is talking about that says that
Beilby was a theory.
All Had to jump in here Ive read just about every thing u
can imagin about polishing from cab with jade to yugo
saphiers I lost the last bit of info when they did the
study with electro magnifiers or what ever also after Wycoff
era Rick Ford carried on a very infomational paper on the
subject and also sold and got into the colidial polishes which
were great his place burned down I think and the news letters
ceased there are more studys on polishing than anything else that
I have read Note: pre polish pre polish is the main answer to
all polishing problems clyde
Hi, all- I have to admit that this thread has me baffled, stymied and
stumped. If the contention is that the various oxide polishes work by
causing the ions in the surface of the stone to dissociate, in turn
causing the surface of the stone to liquify and flow, and then harden
again as the ions reassociate, isn’t it improbable that this same
reaction would occur with all stones regardless of their chemical
composition? One contributor mentioned vinegar in the oxide slurry as
a component of this process, the ions of which interacted with the
alkaline stone, but if this were the case we would be ending up with
the formation of a new mineral as the vinegar and the alkaline would
interact to form a salt. And if we are talking about a surface flow,
why isn’t some smearing or distortion of pattern visible, at least
microscopically? And what about those of us who get a polish just
fine without the addition of vinegar to our slurry? And if polishing
stone is a different process than polishing metal, which process
occurs when I am working hematite?
Why is polishing by abrasion so unlikely a hypothesis, particularly
with the fine grade polishes currently available? Nobody contests
the fact that the use of progressively finer abrasive media prior to
the final polish is abrasion. Has Occam’s razor been forgotten here?
Those of you who are better informed scientifically, please help me
out here, because I am just not getting it.
All, I have read all the post about polishing opals and agree with
most of it. Two exceptions. I use an ultrasonic to clean a lot of
stones that get cerium oxide in the pits and cracks. Even opals
that have small mud spots. I use very dilute mixture of dishwashing
detergent and water. About one teaspoon full to a quart of water.
Usually the ultrasonic knocks the cerium out in just a very short
time. One minute or under. Secondly if you are trying to polish the
Mexican opals still encased in their rhyolite matrix, cerium is the
wrong way to go. I use diamond all the way to finish at 50,000.
Then I go back in with a felt buff on my Foredom and lightly touch
the opal portions with cerium oxide at a relatively slow speed. Do
not hit the rhyolite or the cerium will immediately eat it up. Off in
10 days to Las Vegas. Drop by G+LW and say Hi.
Although the University of Florida says the Beilby Layer is no
longer a valid term, this Tanaka fellow has what appears to be an
extensive study that explains it in detail:
Dave and All, I will not get into the controversy over exactly what
takes place in the polishing process. I do know that the most
precision polishers I have every met worked in the optics industry.
Specifically in the optics for precision guided missiles,
telescopes, and super snooper cameras. They would not tell me their
secrets, but one of them slipped one day and mentioned that they use
electricity in the polishing process.
A note to all about cabochon polishing. I have changed my sequence
of grinding from the standard 100 grit wheel to 220 grit belts to
600 grit belts to 1200 diamond to polish. I now go from a 100 grit
grind to 220 grit silicon carbide to 600 grit diamond belt to 600
grit diamond pad to 3,000 grit diamond pad to polish. The results
are fantastic. The polish seems to jump on the stone, as well as
every scratch you missed glares at you. My main polishes are cerium
oxide on wet felt and 50,000 diamond on a pad. Anything that will
not polish with these probably is not worth polishing. Yes, I even
do jade on diamond.
I don’t understand what all the chatting about the scientific
speculation about how stones are polished relates to doing it.
Getting a good polish does not depend on knowing what causes it. The
technique used to polish does not relate to how it happens. I
personally do not get anything from this discourse except to
exercise my delete button.
My personal opinion is contributions to this board should be about
personal experience for processes that we know work that we can share
to help others.
The speculation and maybe its this, maybe its that, it could be this,
i heard it was that is boring to me. Mental masterbation. That’s the
way I feel about this.
Well Dave, In fact the understanding of how it happens has everything
to do with how to do it in this case. Scientific speculation today
may be scientific fact tomorrow. In reality, we use many kinds
chemicals and varying techniques in the polishing of stones and those
facts can tell us a great deal about how to properly use them to
effect the results we want. Few of us have taken the time to create
charts, graphs, or to make notes on specifically what powder,
carrying medium, lap speed and pressure works best on which stones.
Most of us work by gut feel in this respect, especially if we limit
ourselves to only one kind of stone. The work done by researchers
can give us important perspective, valuable data and documented
details about how to employ materials and techniques to achieve our
results more quickly on more diverse material and to a higher degree
of perfection.
How to properly polish gemstones results from how gemstones polish
just like how to solder gold, silver or platinum results from studies
on how solder functions or how alloys alloy.
Sorry if it means nothing to you but, I teach gem cutting and I
believe my students deserve to now the mechanics of what happens when
they do something so they can be better cutters.
PS; All, I have found the articles I was looking for and will
provide a better explanation than I gave previously just as soon as I
can digest it all again.
Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance is fine jewelry! @coralnut1