||Lapidary Polishing Pads and Points
by Dick Friesen
To start, it may help to describe what is probably the most common polishing process.
After the stone has been properly prepared for polishing by a series of sanding steps, the polishing compound, generally a metallic oxide in powder form, is mixed in a liquid, usually water,and applied to the polishing pad. The stone is held firmly against the rotating pad until a slight pulling is felt, then the stone is moved against the pad until the entire surface has been polished. If the polishing compound dries out during the process, more compound and water is applied to keep the pad in a "damp/dry" state.
Obviously, there are several variants of this process, but the major issue involves which pad works the best. I don't think any one pad is really best for everything, but different ones will work better depending on the kind of stone being polished, the technique used, and the shape that must be dealt with. So how does one make the choice?
To start with, it helps to remember that all the pad is there for is to hold the polishing compound. It does not DIRECTLY do anything to the polish. Almost anything you can think of that will hold the compound can, and has been, used as a polishing pad. Wood, especially hard maple, works very well; many people like hard felt; and soft leather is commonly used. Phenolic (a type of plastic) wheels and points are replacing maple ones for use with diamond compound (especially in gem-carving). One dealer recommends the back, or paper, side of sanding disks, canvas pads are often used, and there are several sources of urethane pads as well- the list is almost endless.
When polishing stones in a tumbler there is no pad at all, although sometimes leather scraps are used to keep the stones from hitting each other. An interesting technique is used sometimes when critically shaped objects are being polished. The compound is suspended in a moving liquid surrounding the object. This technique is sometimes used in the laser industry for lenses. For small-scale lapidaries, the pad is primarily a vehicle for the polish which also helps the liquid evaporate down to the damp/dry state as the friction heats the stone, thus speeding up the polishing process. One could polish in the wet state; but it just takes too long. So anything that does these things might work, at least on some stones.
Just because the pad itself doesn't do anything and all these different things work does not mean one can just use anything that comes to hand. The pad may not directly do anything but it certainly can mess things up. In polishing agate or harder stones one can experiment with almost any pad material, but for anything softer one must be more careful, due to the inherently abrasive qualities of some materials.
Tin oxide on soft leather is a good choice for gem quality turquoise but hard leather can scratch it. If one is using stabilized turquoise, even the soft leather can scratch it. Tin oxide on soft leather also works well on African malachite.For turquoise a safer choice is ZAM on a soft muslin buff; the same kind that is used to polish silver. ZAM, by the way, is a mixture of chrome oxide, aluminum oxide, and a binder. This mixture sounds like it would be appropriate for hard stones rather than turquoise, but the softer pad seems to keep things under control. ZAM on muslin also works very well on most plastics.
I like aluminum oxide on soft leather for tigereye. I find this results in fewer problems with the stone's fibers pulling out than with any other combination of pad and polish. I think that the pulling, or not pulling, of these fibers is more a function of the pad than of the polish.
The harder pads, such as wood, hard leather, and phenolic help to maintain a smooth surface on stones that tend to undercut. I find though, that I sometimes have to follow these harder pads with a soft leather pad to get the final polish I want. This doesn't always work but it works often enough to be worth considering on softer stones. I use 0.3 to 1.0 micron aluminum oxide first on the hard pad, then follow with 0.2 to 0.1 micron aluminum oxide on the soft pad.
Since I'm switching between twenty or more different polishing compounds, I tend not to use the urethane or felt pads much. It isn't that they don't work, but they are just too expensive to use in quantity. I have been able to get leather scraps from leather shops, sometimes for free, that I cut into four inch circles. I use Crystalite's four inch Flexodiscs and replace the pads as needed. Frequently there are 12" x 24" leather scraps available at the Quartzite gem show for a buck, one can cut a lot of 4" circles out of them.
One problem I have had with the leather pads comes up when I add vinegar to the polish. Vinegar seems to speed up the polishing process on some stones but it also appears to harden the leather, thereby shortening the pad's life. The vinegar- hardened pads don't hold the polish as well and leave the pad unpredictable in terms of what it will or will not polish. As long as I can get cheap pads it is worth the shorter pad life to speed up the polishing, although I still can get several months life from the vinegar-hardened pads. I do use hard felt knife-edge wheels to polish quartz or agate when I am carving. They get into grooves well, but the resultant polish is no better, or worse, than the leather pads give.
Actually, individual technique is probably as important as the pad itself and the best answer is to try many different pads to see what works the best in different circumstances. Sometimes changing ones technique to suit the pad, rather than the other way around, is the way to get the best results.