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Question about acid polishing stones


#1

I have read references to the use of acid to “polish” gem stones and
mineral samples. Unfortunately I cannot find any technical
on this subject. Can anyone point me to a good reference
book on this technique?


#2
 I have read references to the use of acid to "polish" gem stones 

Hi Michael, I’m a little skeptical. I have been involved in lapidary
work, on-and-off, for over twenty years. I’m not familiar with acid
polishing. Oxalic acid is used to clean minerals, usually of rust
(iron oxide) stains. I have also heard of people adding a small
amount of acid (oxalic or vinegar?) to their polishing slurry to
improve performance, but have never felt the need. Aside from pouring
a liquid polymer over something, every polishing method of which I’m
aware uses some sort of physical action of a polishing compound on
the surface of the stone.

Dave
Dave Sebaste
Sebaste Studio and
Carolina Artisans’ Gallery
Charlotte, NC (USA)
dave@sebaste.com


#3

Acid is normally used in small amounts in the drip tanks of faceting
machines or cabochon polishers. The amount is usually "a few drops"
of acetic acid, into approximately a quart of water. I have tried it
in both mediums, and frankly, I cannot see any difference in the
stone. However, the machines really don’t like it! The only true way
to get a fine polish on stones it to pay attention to the sanding
stage.

Richard, in cold Michigan…Wonderful!.


#4

Hi, All-

Acid is also used in another capacity- to dip-polish the banded
calcite commonly called onyx, and other calcites as well. If you
have seen the onyx chess sets, ashtrays,etc coming out of Mexico,
those have often been polished by giving them a quick dip in acid, in
which the calcite dissolves quickly.

Lee Einer
http://members.cox.net/appealsman/


#5

Hi Richard,

Acid is normally used in small amounts in the drip tanks of
faceting machines or cabochon polishers.

I’d say the operative word here should be ‘sometimes’ not
’normally’.

If an acid is used, it’s a very weak, dilute acid like vinegar.

You’re are quite right when you say it’s hard on equipment. Many
faceting machines & lapidary machines have a large number of parts
made from aluminum or aluminum alloys. Aluminum is very suceptible
to corrosion caused by acid.

The acid causes the cutting/polishing action to improve on 'some’
stones by altering the Ph.

Dave


#6

Back in the 60’s I added oxalic acid to a slurrey of green polishing
compound to polish B. C. Jade cabochons on a hard leather wheel.

Phyllis Richardson.


#7
 I have read references to the use of acid to "polish" gem stones

In Mexico all the “onyx” chess sets, small “onyx” carved animals (
man w/ cactus & burro), and other “onyx” trinkets are polished with
acid. This “onyx” occurs in many natural shades of orange, white, &
green, etc.

The real name for this material is massive crystalline calcium
carbonate of which there are several high quality deposits in
Mexico.

The massive calcium carbonate will react vigorously in acid,
removing tool marks and other rough areas, leaving a much smoother
finish. This is the Mexican method used to obtain a quick easy
"polish" on these small cheap items.

Polishing with this method is particular for this type material, and
is based on it’s vigorous reaction with acid. There are other
materials i.e. “onyx marbles”, “oriental alabaster”, in this same
family which will may also be polished by this method.

Hope this helps.

Steve Green
Rough and Ready Gems www.briolettes.com briolettes in over 50 gem
materials.


#8

Hi Michael, Dave and others, Actually, what’s usually meant by the
phrase “Acid Polishing” is the process used to polish many of the
materials with incipient fractures and/or cleavage planes, like
Topaz, Kunzite, Fluorite and Calcite, to name a few. In these cases,
the process involves using hydrofluoric acid to literally melt away
the surface of a stone, once it’s been ground and sanded to a
medium-fine (i.e. 600 mesh) finish. The great news about acid
polishing is that it’s quick and 100% effective… the not-so-great
news is that, if you accidentally come into contact with even a
small splash of the acid, it immediately begins leaching the calcium
from your tissues and bones, which leads either to an immense amount
of nerve pain and a protracted recovery period (at best), or the
same, leading to a very painful death. Ironically, there’s been a
parallel discussion of acid polishing techniques, this week, on the
U.S. Faceting Guild’s “Faceters List” listserv; if it weren’t
considered “bad form” to do so, I could post some truly hair-raising
acid-related recollections from some of those list members!

Suffice it to say that, as with many other so-called “better ways”,
there are hidden dangers that we’d all rather not mess with. In the
case of acid polishing, even those who must deal with this stuff
on a daily basis tell how much they fear it; for me, I’d just as
soon never even see the stuff!

(Just my 1.985 cents worth.)
Doug


#9

Hi Doug (and everyone),

  In these cases,the process involves using hydrofluoric acid to
literally melt away the surface of a stone, once it's been ground
and sanded to a medium-fine (i.e. 600 mesh) finish. 

I stand corrected (of my skepticism)! That’s one of the things I
love about Orchid… the vast talent and experience this community
has as a whole! I guess there’s a good reason I haven’t heard of this
process: it’s certainly not for the small studio or faint of heart.
Sounds like a industrial-type process best reserved for the gem
factories who can deal effectively with the health and safety risks.

Thanks for enlightening me, Doug!

Dave

Dave Sebaste
Sebaste Studio and
Carolina Artisans’ Gallery
Charlotte, NC (USA)
dave@sebaste.com


#10

Dave, You are probably right about it being an industrial type
process…especially if one is doing a lot of stuff.

I remember however, many years ago, hearing about acid polishing
being done in Taiwan. I asked my very good friend, Tory Wu who was
one of the best coral carvers in town and, while he would confirm it
was being used on red coral, he would not tell me what the acid was.
He did indicate that they just ‘painted’ it on and then neutralized
it (I guess with soda).

Still a mystery to me. Cheers from Don at The Charles Belle Studio
in SOFL where simple elegance IS fine jewelry! @coralnut1