FACETIP – POLISHING REALLY TROUBLESOME FACETS
October 25, 2019Duncan Miller
Polishing soft gem materials, Mohs’s hardness 5 and less, and facets near the cleavage of some harder materials can be very difficult with commonly used polishing laps. Some years ago, Gearloose Lapidary (www.gearloose.co) introduced the Lightside™ lap, intended specifically for polishing soft materials. It is a ‘reduced-friction’ composite lap, used with diamond or oxide slurry to produce flat facets without significant edge rounding. It is described as a ‘durable, predictable replacement for wax laps’. I have found that it works well for some materials, and not well for others. For instance, I have used it successfully to polish fluorite, cuprite and rhodochrosite, but not cerussite. For that I use a commercially obtained, relatively hard wax lap, supplied by Rob Smith of African Gems & Minerals. Originally, this wax lap was a bit warped, but it flattened nicely under a heap of books. It has an intentionally rippled surface, but the tin oxide slurry I use on it has embedded and with light pressure it polishes facets with minimal rounding of the facet edges.
I have another wax lap, a homemade one, that I used previously for polishing cuprite and some troublesome facets on apatite. It is an old Crystalite electrobonded lap with the reinforcing aluminium ribs on the back turned down in a lathe, leaving just the central hub and a raised outer rim. This is placed on a sheet of aluminium foil on a kitchen stove plate and chips of ordinary candle wax melted in the hollow, until it is filled. When the wax has solidified, the lap surface is trued in the faceting machine, using an old lathe cutter tool held in the quill at an appropriate angle to skim off any irregularities due to uneven contraction of the wax as it cooled. Both laps are illustrated below, the homemade one on the left and the commercial one on the right.
The wax polishing lap on the left is homemade, using the hollowed out back of a Crystalite faceting lap, filled with molten candle wax. The wax lap on the right is a commercially obtained one, with tin oxide slurry impregnated in the surface through use.
Wax laps are not only useful for polishing really soft gemstones, but also a useful remedy for really troublesome facets. Facets near a cleavage plane sometimes refuse to respond to anything you do on a conventional lap. They continue to pit and score no matter how you reduce pressure and rotational speed, change angle of attack, change polishing oxide or slurry consistency. Then it is time to resort to a wax lap.
A good example that I cut recently is an ‘orthoclase’ feldspar—actually sanidine—from Itrongay in Madagascar (https://www.mindat.org/loc-2273.html). The facets on one side of the crown were very close to the plane of perfect cleavage. While the other facets on the crown polished easily with a little chromium oxide on a Gearloose Greenway™ lap, some facets remained matt and pitted, even at slow rotational speed on a Lightside™ lap. I resorted to polishing all the facets on one side of the crown on the wax lap with tin oxide. See if you can notice any facet rounding in the photograph alongside.
Sanidine (‘orthoclase’) feldspar from Itrongay, Madagascar. The 26,19 ct stone is 19 mm wide. The yellow colour in the reflection is closer to the true body colour of the stone.
For a detailed and well-illustrated history of faceting and faceting laps, see Justin Prim’s article at https://medium.com/justin-k-prim/lapidary-technology-through-the-ages-laps-and-polish-59c29f05a11a.
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