Lapidary Tips For Polishing Girdles
Although my general gemstone polishing technique has been improving, I’m having difficulty polishing girdles. Is it absolutely necessary to polish girdles? I’m just a beginner, but is it poor form for a professional gem cutter to leave them unpolished?
I’m having trouble getting the arm of my faceting machine down to a level where the stone is in good contact with the lap. I’m using a Facetron. I can pull the splash ring down and lower the arm at 90° but can’t seem to get the stone very far onto the disk. This is leaving a black/silver residue on the girdle and doesn’t seem to polish at all. I’ve only managed to get a nice polish once or twice. I’ve just stopped polishing girdles until I can figure this out.
I’d appreciate any comments or suggestions.
John, Corpus Christi, TX
Advice For Polishing Girdles On The Facetron
As far as leaving girdles unpolished, it really boils down to a personal preference. My preference is to polish. To me, polishing girdles completes the stones. I feel that a custom-cut gemstone deserves that. If I can take the time to polish all the other facets, I can take the time to polish those on the girdle.
I also use a Facetron, and what you point out is very true. Unless the diameter of your stone is larger than the diameter of the quill, the amount of lap that you have access to is limited to the length of the dop, about an inch or so. If the diameter of your stone is not larger than the standard diameter of the dop, ¼”, then you’re limited to about ½” or so. I haven’t found a way around this. However, polishing the girdle facets normally is pretty quick and easy and doesn’t take a lot of lap surface.
That black/silver residue you’re getting may be the heart of the trouble you’re having polishing girdles. You didn’t mention the type of material you’re working with or the type of lap you’re using. However, getting “black/silver residue” sounds like you’re smearing your lap metal onto your girdle facets. This could be caused by not having enough polish on your lap. Remember, the lap doesn’t do the polishing, the polishing media does. The lap is just the carrier for the cerium oxide, diamond polish, or whatever. The metal from your lap should NEVER rub off on to any facet, including the girdle.
There are two other possible sources of trouble you should consider. Perhaps you’re running the lap too fast. The speed may be slinging the polish off the lap. I polish using a very slow speed on my Facetron. The slow speed helps the polish stay on the lap. Or, you may be using too much pressure on the stone. Try using less pressure. Polishing girdles should not be a chore. It should be quick and relatively easy.
I usually wait to polish the girdle until after I have transferred the stone. This keeps the stone from shifting during the transfer and aligns it perfectly for completing the remaining facets. I also use the vertical micro-adjustment on the mast so that only the edge of the girdle facet polishes. This means that your quill will be set at a very, very, slight angle (past 90°) and that the girdle facet will contact the lap at a very, very slight angle. (You’ll be able to see the polish coming gradually onto the facet from the edge).
Here’s another trick for polishing girdles you could try. Sometimes, I’ve cut the girdles at 85° or even 80°. It raises the quill angle which may solve some of the problems you mentioned. It’s an interesting concept and creates a unique stone. BUT, the problem is creating a setting for a stone that has a knife edge, 80° girdle!
Hand Polishing Girdles
Personally, I feel polishing the girdle makes a significant difference in the appearance of the stone. This is especially true if the stone will be mounted using prongs, since the girdle would be very visible.
I’ve gone through the same fight polishing girdles with my Facetron. I no longer use the machine to polish the girdles of my stones.
For setting the machine at 90° and cutting the girdle, I’ve used the barn door method of cutting down into the catch basin about 5/8” and then outward in both directions about 1 ¼”. Even though it resulted in a permanent modification of the catch basin, I’ve never regretted it. (I started by trying to clip the thing down and out of my way. That was an annoyance).
I started hand polishing girdles somewhat by accident. Once I forgot to polish the girdle but only realized it after I’d undopped the stone. So, I decided to try holding it by hand on a Crystalite Last lap. I found that it was actually easy to do, at least on a flat-sided stone. That particular lap is noisy, and I can hear the stone quiet down when it’s riding flat on the lap. That nice noisy lap, by the way, also makes a great device for realigning the dop arm after transferring the stone. Often I will polish the edge of just one girdle facet on that lap to get everything lined up again so I can cut the crown.
I’ve found that all stones with curved sides posed a different challenge. Now, I take all such stones and actually hand hold them (on the dop) at 90°+ and rotate them on a finer grit lap (1200 usually), just enough to smooth off the edges so I’ll end up with more of a round “round,” for example, rather than a 16 or 32-sided one. This especially improves the appearance of ovals, trilliants, and cushion cuts. I polish harder stones (such as tourmaline, garnet, and sapphire) in the same fashion, usually with a cotton polishing pad and 50k diamond paste. I use cerium oxide and water on leather for softer stones (such as opal, quartz, and sunstone). This is fast and easy and does a decent job.
One thing to watch out for is orienting the stone so that the spinning lap is polishing into it from the bottom to avoid rounding off the edges of the break facets on the top of the stone. Polishing on a softer surface like this does tend to round off sharp edges of facets near the girdle. Still, it’s probably a good thing that the edges of the girdle itself are a little “soft,” as it should help to reduce the odds of chipping when the stone is set.
Using a Fast Lap For Polishing Girdles
The fellow who taught me to facet said that polishing the girdle increased dispersion.
I use a Facetron and a Raytech Fast lap for polishing pretty much everything I’ve faceted. I originally charged it with 50k diamond paste. Now I occasionally give it a squirt of 50k diamond spray or just a couple of drops of Microsol oil for lubrication.
Before I put on the Fast lap, I slip the rubber splash guard completely off the Facetron. I also don’t set my angle at exactly 90° but at around 89.7°. This way, I’m only polishing the portion of the facet closest to the girdle. If you’re like me, at this point, the stone you’re cutting looks like a pencil stub. When you flip the stone and start grinding off all that material from the eraser (table) end, it doesn’t make sense to polish all that area just to eradicate it. Plus, it takes a lot longer if you polish your girdle at exactly 90°.
Unless I’m lucky enough to be cutting a larger stone, the taper of the dop keeps me on the outer ½” to ¾” of the lap. Any closer than that and I start polishing the underside of the dop instead of the stone.
I hope some of this helps,