Unfortunately there just isn’t one speed that works best. The glass industry has published a lot of research work on polishing glass and they found if you can keep everything else equal, the faster the surface contacting the glass runs, the faster the polish happens. Now before someone jumps all over me for that statement, I’m condensing a lot of pages into one sentence and everything else is never equal in the case of gemstone polishing. If nothing else, it’s hard to keep the heat down with increased speed and some stones don’t like to get hot.
The bottom line is; it isn’t the speed of the wheel, it is how one uses it. I don’t know how many Genies Diamond Pacific has sold but they all spin at about 1725rpm and somebody has probably polished every stone there is on one. Conversely, somebody using one has probably had trouble with every stone as well.
I know that I can’t get the results I want with a single-speed polisher. I have tried almost every combination of equipment I could think of to get the speed control I wanted and I finally decided to spend the money on a 1/3 horse DC motor, which smoothly accelerate with an increase in voltage. It cost more than I wanted to spend, but I got the results I wanted. I can spin almost any wheel up to 8” diameter from 200 to 2000 rpm with no rpm lost from polishing pressure. If a variable speed DC motor is not a possibility, fitting a belt-driven arbor with a series of pulleys will also make a number of different speeds possible.
While the glass industry has been able to quantify how all the parameters of polishing work together, they have the equipment to do repeatable work. For us, the technique becomes almost an art rather than a science. For example, there are curves that show that as pressure is increased, one should change the type of pad to match the pressure. This is just one way different people can get different results with the same equipment, pointing out why it is very hard to tell someone else how to duplicate my work.
A general recommendation, if one can’t control the speed, is to work with the dampness of the polishing compound. Polishing actually takes place before the pad dries to the point of pulling and its associated heat, but it is much slower. One can also try using less pressure: changing dampness and pressure will help control the amount of heat build-up that takes place. On the other hand, some stones seem to need the heat to create a polish. Jade and jasper seem to like a lot of pressure and will forgive a lot of heat—but I have burned both.
I wish there was a table of stone vs. speed vs. pressure vs. polish etc., but for every table I have ever seen I could show how, by making a change somewhere that the chart didn’t take into account, its recommendation could be made not to work. There’s really no substitute for experimentation.