Most faceters I have talked to agree that the action of diamond in polishing is best when it is a scraping action rather than a tumbling and chipping action. On coarse diamond cutting laps, where the diamond is bonded into the lap surface, you have a scratching or a scraping action. When you cut facets with a new 150 grit diamond cutting lap it cuts fast because the points of the tiny embedded diamond particles are sharp; they scratch or tear particles out of your stone. After it has been used for a while the sharp points get worn off, and it doesn’t cut as fast but it cuts a smoother surface. The scratching action of the sharp point has changed into a planing action. On a worn lap you have the effect of many tiny planes shaving off bits of your stone. It is reasonable to expect that such comparable actions of the diamond are not changed merely by the size of the diamond particles; a new and worn 1200 diamond lap will act the same as the 150. Therefor when we look at the polishing process it seems reasonable to assume the same things are going to happen.
Two important qualities in a polishing lap are hardness and flatness. The lap must have a surface which allows the applied diamond to embed or it must be made with the diamond already embedded, and secondly, it should retain its flatness for a reasonably long time.
If we use a lap so hard that the diamond particles cannot become embedded, then there will be a tumbling and chipping action on our facet’s surface. This will not produce as smooth a polish as embedded diamond particles worn down into tiny planes.
Ceramic laps are very flat and retain their flatness for a long time, but they are too hard to allow the diamond particles to embed, and are therefor unsuitable for polishing. Polishing with a ceramic lap is not well understood. The ceramic lap is made of corundum particles cast in a glassy matrix. When new there is only the rolling and chipping action of the diamond. A mixture of diamond, oil, dust, and stone dust builds up on the lap surface–I call this “crud”, others call it TMslurry”. If you wipe all the crud off the lap the diamond goes with it and you have to recharge the lap. With use, the glassy matrix between the corundum wears away. Tiny pockets develop in which the crud collects. The lap is now said to be “broken in”. In addition to the diamond particles rolling and chipping on the surface you have diamond particles rolling around in the pocketed crud and a few diamond particles lodged against the back edge of the pockets performing some scraping action. The consistency of the crud and the amount of diamond in it is always changing, making it nearly impossible to predict how your facet will polish. It becomes a constant battle trying to maintain the amount and consistency of the crud and the amount of diamond in it. These problems are further aggravated by the fact that occasionally a particle of corundum will break out of the glassy matrix and roll between the lap and your stone, producing a scratch. Also balling-up (agglomeration) can occur in the crud on the surface and in the pockets. These agglomerates can become larger than the size of your polishing particles, and cause a scratch. You cannot prevent agglomeration because you need the crud to polish with. Any crud on the lap can also cause slight rounding of the facet by turbulence at its leading and trailing edges. Your ceramic lap can be best used for setting hot pots on when cooking or serving meals.
In an ideal metallic lap where the diamond is applied externally, the diamond should embed only under firm pressure. After applying a charge of diamond, a very hard material should be used to force the new diamond into the surface of the lap. The lap should be hard enough so that the lighter pressure used in polishing does not continue to force the embedded diamond down into the lap to where it is even with or below the lap’s surface, and where it is ineffective. There are two reasons for the diamond not becoming further embedded under normal polishing: The first is economical. If the diamond sinks into the lap before it has a chance to polish for its useful life, you are wasting it. The second is that if it sinks in before it wears enough to attain a planar surface you never get the planing action, only the scratching action. It must remain embedded but with a part of the particle above the surface so that it can become worn into a plane and perform a planing action. Only then do you get your best polish.
At present I know of no lap that meets the above ideal requirements. I have John Alden’s lap which is harder than others, but when new it takes more than one charge of diamond initially, and other applications of diamond at periodic intervals, but not nearly as much as a tin lap does. I think it needs to be a little harder yet. At the ideal hardness, the metallic lap should require very little diamond, hold its flatness and seldom, if ever, need to be resurfaced.
It is not only the diamond that can embed. The softer the lap the more the particles from the stone being cut can embed with the diamond. Most metal laps being used today, such as copper and tin, are too soft, and must be continually recharged until the buildup of embedded particles causes a bridging effect to occur which slows the sinking of the diamond grit into the lap. At this point the tin lap should be at peak performance. With further embedment there is the probability that too much diamond and crud become embedded in proportion to the metal needed to hold it, resulting in incomplete embedment, with subsequent semi-embedded material coming loose and tumbling on the lap. This condition of saturation may be what Charles Hettich refers to when he speaks of a tin lap becoming “cranky”, requiring resurfacing. Also with soft laps flatness may be lost too quickly and grooving can occur.
Since it is the embedded diamond, and not the slurry, which we want to do our polishing, the lap’s surface should be kept clean and free of crud when polishing. Cleansing of the lap’s surface must be continuous. To reduce heat at the facet’s surface, the lap needs a lubricating agent. Either the finest oil or oil- kerosene mixture can be used. Clean, pure water should also work well, and may be the best. Some people advocate scoring the metal lap. The only reason I can think of for doing so is if you intend to polish with a continuously running source of water and believe the grooves help remove the crud, or if you want to use the slurry in the polishing process and think that the grooves help hold a reservoir of slurry.
If you intend to polish only with the embedded diamond, you must keep the lap surface free of crud to prevent agglomeration and rounding of facet edges. Scored grooves can hold crud and help accumulate agglomerates.
There are also laps which are neither ceramic nor metallic, like the Last Lap. I have not used these, so will have to let others tell about them, but if diamond is used the above principles should apply to them also.