Re: Nephrite - Jade Polish
Possibly more information (some true, some false) has been written regarding the polishing of jade than all other lapidary materials combined. And so, one concludes that jade is difficult to polish. The answer is "Yes" and "no". Let us consider the mineral itself. There are three separate and distinct minerals that are known under the general term jade: Nephrite, Jadeite, and Chloromelanite. Nephrite is the one found in BC, Alaska, and Wyoming. This is the one we shall consider here.
Let us first consider the nature of these rock-forming minerals. They are described as being fibrous, which simply means that structurally, they are a mass or network of tiny fibrous hair-like crystals, much like straws in a straw stack. They are so fine in nephrite that they cannot be seen even with a powerful magnifier. A peculiarity about crystals is that they are generally softer parallel to the crystal growth, and harder across the crystal. It is just this simple characteristic that creates all the difficulty in applying a polish, if one uses the same procedure as for example, agate. However, provided one has the proper equipment and uses it in the proper manner, nephrite jade is easy to polish. And now for the job!
Proper sanding is of vital importance. Improper sanding mainly through use of unsuitable equipment and technique, is the greatest cause of difficulty.
We will assume that you have your cab ground to shape, and coarse-sanded on a disc or drum, using 120 or 220 grit. The 220 is slower, but otherwise it matters little which is used. Recommended sanding speed is about 1000 RPM for a ten-inch disc. And not too much pressure, for more reasons than one. We have all read of using a well worn sanding cloth for the final touches. This is O.K. for quartz minerals, but in my experience does not apply for jade, nor any other fibrous material. Crystalline Rhodonite, for example. It is obvious that sanding will tend to wear away the fibres parallel to the direction of rotation of the sander at a faster rate than the fibres at right angles. This pesky behavior is called undercutting, and must be avoided. If we use the well worn technique, we will probably decide that the old cloth isn't doing much,.. so a little extra pressure is added. What happens? In the first place, the stone is liable to be burned, creating nasty white spots, which, of course, must then be ground out. And that cab looks a little on the thin side already: But what is just as bad, if not worse, the sander (which has a small amount of resiliency) will be forced to plow out the parallel fibres and leave the cross-fibres already mentioned. What do we have now? Just plain orange peel. The obvious remedy is to use a sharp sander. No matter what number grit we are using. This will have a planing action which will remove material without undue pressure. After our cab has achieved a fine satin-like appearance all over, we may proceed to the fine sanding operation. But first, scrub the stone and dop stick to remove all traces of coarse grit. And make sure there is no grit (let's call it dust) anywhere- near the fine sander. 320 grit will be found quite satisfactory (and fine enough) for the final sanding. But here again, it must be new, sharp grit.
Do not depend on this 320 to remove much material. As stated before, have a nice satin finish before leaving the coarse sander. The 520 grit will soon produce a mirror-like surface that 'night easily pass for a polish. Any areas that appear to have a "frosty" look are not sanded enough, and this frost must be removed. Actually, They are areas of small pits and depressions that are filled with sanding debris.
We now have a semi-polished stone. This must again be very thoroughly scrubbed, particularly behind the stone where the dopping wax is located. And doubly important, this time, all dust must be removed from anywhere near the polisher. For polishing jade, we find that heavy harness leather, at least an eighth inch Thick is most suitable. Do not try to use light leather. A piece of felt floor covering makes a good cushion behind the leather. Chrome oxide is the most satisfactory polishing agent. Slow speed, not Over 350 RPM for a ten-inch disc works best. Quite' heavy pressure is generally required. Particularly for flat surfaces. And just enough water to keep the leather moist, applied with the chrome oxide in the form of a thin cream. Considerable pull will be felt as the leather dries cut. It is only while this pull or drag is felt, that actual polishing takes place.
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From Laphound Liftfinger - August 12, 2007 at 19:30:46