Everyone wants to know what the “best” polishing compound is and how it is used. Unfortunately I don’t think there is a single “best”—just a better one for the stone in question, depending on your technique. But here are some of my thoughts on the subject.
The Fundamental Problem
Any recommendation for polishing difficult gems needs the added statement that the technique used is as important as the polishing compound selected. The techniques are not well-described in any book, and I have found many books to be hard to follow or contradictory. Most books describe techniques for agate and that is about all. Since agate is about the easiest material to polish, that is not too helpful.
In any discussion on polishing you can’t really separate the sanding and polishing steps. If you are trying to polish a stone with magnesium (nephrite, serpentine, etc.); manganese (rhodonite, rhodochrosite, etc.); or any stone that has areas of differing hardness (lepidolite, charoite, etc.) expect to spend some time developing your sanding technique if you expect good polishing results. Most lapidary material will polish relatively easily if it has been properly prepared.
When reading this, please remember: the information herein is based on the results achieved using my techniques. Your results with your techniques may be different. When I cut I usually work 50 to 100 stones at a time and patience is not my long suit. I sand with a lot of water. I normally use Crystalite 8″ × 3″ diamond belts with moderate to heavy pressure at 1140 rpm. Polishing is done with 4″ Flexodiscs and my compound of choice on soft leather at 1140 or 1725 rpm. I normally mix my polishes on the thin (watery) side and apply them with a spray bottle. Then I let the heat from polishing dry the polish until it starts to pull. I buy scraps of leather and cut my own pads. I have a fairly complete set of Diamond Pacific, Crystalite, Hi-Tech, Raytech, and 3M diamond products, most of them set up so I can quickly try different wheels to see which is working best on whatever I am cutting. My normal sanding sequence is: 220, 600, 800, 1200 or 1800, and 3000 or 3500 with steps left out depending on how the particular stone is cutting.
Here is a quick overview on the state of polishing theory as I understand it. Refer to Barry T. Kilbourn, CERIUM A Guide to its Role in Chemical Technology (1995) Molycorp, Inc.
Polishing Theory, As Applied To Glass
“Polishing is the act of producing a sufficient degree of surface smoothness so that light, transmitted or reflected at that surface, is not disturbed by surface irregularities. The polishing mechanism is still poorly understood at the chemical/molecular level. Polishing results in glass removal and does show a dependence on chemical properties of the glass. The nature of the liquid present during polishing is crucial and only if active hydroxyl groups are present, in alcohols for example but especially in water, does the polishing phenomenon happen.
“When a glass-typically an alkali silicate-is in contact with water, a complicated series of steps take place: ion exchange, dissolution of glass constituents and possible structural changes. A surface region of the glass is modified and it is this softer hydrated layer that is removed or reformed during glass polishing. Classic abrasives produce an improvement in surface finish but leave a fine but definite roughness, the scale of which relates to the grain size of abrasive used. Several not-so-hard oxides are reasonable polishing agents and can remove and/or reform the soft hydrated layer. In general, optimum polishing rate coincides approximately with a Mohs hardness for the polish of around 6.5, very close to the hardness of most glasses.”
Acidity and Alkalinity
There is an argument that using a polishing agent that has a high or low PH will enhance the formation of the hydrated layer and speed up the polishing process. This argument says that for best results, you should add vinegar to polishes for the quartz family to lower the PH and add borax or use colloidal silica on stones containing magnesium or manganese. to raise the PH. Unfortunately it does not explain the fact that industrial process use cerium oxide in water, an alkali, to polish glass. Regardless, I have used vinegar when polishing agate and obsidian and it does seem to help. I think it shortens the life of the leather pads, though.
There is another argument that says that the correct answer is to lower the surface tension and that that is all the vinegar or borax is doing This theory says that the best additive is ethyl alcohol. Reducing surface tension reduces the surface-to-surface drag which eliminates chatter, resulting in a smoother surface. The surface tension of water is 73 dynesper centimeter and ethyl alcohol is 22.3 dynes per centimeter. (I have not tried this yet.)
Hydrated Layer Theory
There are those who argue that the finer and finer scratch theory, when using diamond, is wrong and diamond polishing is just another way of removing the hydrated layer. I believe that several years ago I saw a paper showing scanning electron microscope photos of the surface of agate. The agate had been polished with diamond compound and the SEM photos showed the scratches. I have been unable to find a copy of this paper and I am relying on my memory for this. (Unfortunately, at my age, memory is the second thing to go—I don’t remember what the first thing was.)
I think in reality there is more than one way to accomplish a polish and there is probably some validity to all of the theories.
The Old Stand-bys
The oxides of cerium, tin, and chrome, as well as Zam (a proprietary mixture of aluminum oxide and chrome oxide with an unidentified binder) have been used for years. With patience almost all gem materials can be successfully polished with them. But newer polishes are available and in most cases are superior.
Getting good quality cerium oxide can sometimes be a problem. Cerium oxide has been the polish of choice for the glass polishing industry. It has the advantage of being inexpensive and it can be formulated specifically for several different industrial processes. Unfortunately much of the cerium oxide that is available to lapidaries apparently comes from manufacturer’s “over-runs”. Most, if not all, of the cerium oxide polishes are “optical grade” because that is what the industry uses them for most commonly. Unfortunately, not all of it is formulated correctly for lapidary use. You sometimes hear of “contaminated” polish but I think this is rarely the case. I think most lapidary supply houses get their polishes from a distributor, not directly from the manufacturer. I don’t think we use enough for a manufacture to produce a run just for us. What I think happens is that the supply house asks the distributor for “cabbing grade” and the distributor, not knowing what that is, says he has this good buy on “optical grade” and that’s what we get. Unfortunately, I can’t help with a current source of cerium oxide, I got a large supply many years ago and I have not needed to replace it yet.
Cerium oxide is the beginners’ polish of choice for silicon dioxide (quartz family minerals). How it polishes glass, and therefore agate, opal and obsidian, has been relatively well researched. It is tolerant of a wide range of pressure and I have used it on hard and soft leather, wood, felt, and synthetic pads. I won’t say that there were no differences but the differences were not large enough to worry about. I put it on relatively wet and let the heat of polishing dry it until I feel it pull.
I use tin oxide on soft leather on stones softer than obsidian and harder than plastic. On plastic (stabilized turquoise) I use Zam on a muslin buff. Chrome oxide has been recommended for years for stones that undercut or contain magnesium or manganese. For a long time I used chrome oxide mixed one to five with a 50/50 water-vinegar solution and 10% Linde A on hard leather. Now though, I find I have better luck with aluminum oxide and I rarely use chrome oxide any more.
Colloidal polishes fundamentally are just the polish mixed with a dispersant to keep the particles from settling out. (I know a chemist would have a fit over that definition, but that is what the manufacturer said.) Because these compounds hold the abrasive in suspension better, they should require less polish to be effective, and should be faster then the standard polishes.
Colloidal silica has a higher PH than most of the oxide polishes and should be one of the better polishes for lapis and rhodonite. I have tried to find the actual PH for polishes but I have not had any luck. The only information I have is that most (all?) of the oxides are alkalis and that colloidal silica is more alkaline than the others.
What I have seen so far has not been encouraging though. These polishes were developed for faceting and may work better in that application, but I’m not a faceter—for cabbing, I have not been too impressed. They will polish, but I don’t think they’re any better than the non-colloidal version.
Colloidal polishes of diamond, alumina, silica, cerium, tin, and chrome are available from: Rick Ford at mAji, PO Box 426 Beavercreek Oregon 97004-0426 (503) 632-3653.
There are several different aluminum oxides and each has its place. The one one hears about most is Linde A. This is a .3 micron polish that has been around for a long time. Linde also makes a .05 micron polish called Linde B and a 1.0 micron polish called Linde C. The Linde B is a little softer, 8 vs. 9 on the Mohs scale. There are industrial process that use the B and C polishes but they are not commonly used in the lapidary field.Linde A could be listed as an old stand-by except for its price: all of the Linde polishes are about $100 a pound.
Reynolds has a set of polishes available through Diamond Pacific (Diamond Pacific Tool Corp., 2620 W. Main St., Barstow, CA 92311 (800) 253-2954) and I have had very good luck with their 0.2 micron polish at about $23 a pound.
FitzCorp Inc. (P. O. Box 565 Point Blank, TX 77364 (409)377-2409) has a 0.1 micron polish that they have a lot of research into. I have had good results with it. At $12.50 a pound I think it is a “best buy”.
Myers 0.3 micron Rapid Polish ( P.O. Box 646, Keller, Texas 76244 (817)379-5662) gets my vote as the best jade polish I have tried yet. I wrote an article in the June 1998 Rock and Gem magazine. (Rock and Gem can be reached at (805) 644-3824 if you are interested in back issues.) There is something about its structure that controls “orange peel”—the pitting resulting from some attempts to polish jade and other difficult stones—better than other polishes.
There are several other suppliers of aluminum oxide polishing compounds and I think if you compare equal particle sizes they are all about equal in their results—but I have not tried all of them. I find as time goes on I am using aluminum oxides oftener and the old standbys less. Again technique plays a large part in this. I normally mix it thin and apply it with a spray bottle, then polish until the stone pulls. I have tried thick pastes applied with a brush and mixtures of water and vinegar and found very little difference in the polish; so use whatever technique works for you.
For difficult stones, this has been many cutters’ polish of choice for a long time, but I am not one of them. I think the extender fluid hides the surface too much and it takes too long to get the polish I want. However, I know others use it and like it.
Diamond Polishing Belts
There are several manufacturers of diamond polishing belts and wheels and all do an excellent job on agate. If you do mostly agate I think the convenience is hard to beat. But with the harder-to-polish stones I always seem to do better with one of the other compounds.