Articles

Duncan Miller

No, this is not about polishing diamonds, which in South Africa is illegal without a license, but about working with diamond grit or paste. For the coloured stone gem cutter, diamond paste is easier to source and to use. Loose grit and pastes are available in a range of mesh sizes, with crushed natural diamond or synthetic diamond. Synthetic diamond is made as single crystals and polycrystalline aggregates. The polycrystalline diamond breaks down with use to produce finer particles. Single crystal diamond is used for grinding, pre-polishing and polishing. Polycrystalline diamond is more suitable for pre-polishing and polishing. Pastes are available in both water-soluble and oil-soluble formulations, and recently in dual formulations that are compatible with either water or oil. For a bewildering array of available alternatives visit the Gearloose Lapidary website (www.gearloose.co).

Diamond grit sizes are measured in mesh or in microns. Mesh is supposed to indicate the number of holes per inch in a sieve, and common sizes run from 100 mesh to 100 000 mesh. The higher the mesh number, the finer the diamond. There are corresponding micron sizes, which are supposed to indicate the mean (average) particle size measured in thousandths of a millimetre. The lower the micron number, the finer the diamond. Both scales actually involve a range of sizes around a mean. Some products have a narrower range than others, although it is difficult or impossible to get details of this from the producers.

Diamond grinding grit or paste needs to be added to metal laps. Of course, with electrobonded laps and sintered bronze laps, the diamonds are permanently bonded to them or impregnated in them. Loose grit or paste can be applied to tin/lead, Batt, zinc or copper laps for facet grinding and pre-polishing. Typical grit sized are 325 mesh for coarse grinding, 600 mesh for large facet cutting, 1200 mesh for fine faceting, 3000 mesh or 8000 mesh for pre-polishing. For pre-polishing 3000 mesh is preferable on the softer metal laps, because the diamonds embed further, and 8000 mesh on the harder zinc or copper laps. The coarse grinding grits or pastes need to be spread evenly on the lap using an appropriate extender – oil-based or water – and forced into the metal with a roller, a ball bearing on a stick or screwdriver handle, or a flat face on a hard mineral like synthetic corundum. After the diamond has been forced into the lap, wash the lap thoroughly to remove loose particles. Then cut the facets using water as a lubricant. When charging a series of laps, work from the finest to the coarsest, so you don’t risk contamination, and you don’t have to wash your hands between each lap. When laps become dull, simply recharge them again to keep the grinding action constant. You do need to develop your own charging technique to avoid contamination and a mess. There are several good videos on YouTube demonstrating the process.

The finer mesh size used for pre-polishing doesn’t need to be forced into the lap, and applying it to the lap with a clean fingertip is sufficient, before you simply start pre-polishing; or you can work it into the lap first with a dedicated stone if you wish. Loose grit will need some lubricant, while pastes usually lubricate themselves. When black swarf builds up on the pre-polish lap, clean it off with a piece of paper towel dampened with methylated spirits, or water or WD-40, depending on what type of paste you are using. Use the grit or paste sparingly. A little goes a long way.

Diamond polishing is necessary when faceting hard stones like sapphire and chrysoberyl. Some people use it for softer stones as well, although for those I prefer an oxide polish. Suitable polishing laps to use with diamond include cast iron, tin/lead, Batt, BA5T and Diamatrix (the last three available from Gearloose Lapidary). First you dampen the lap with the appropriate lubricant, then add a tiny amount of 50 000 mesh or 100 000 mesh diamond paste, and polish. I use a clean fingertip to apply the paste to the slowly spinning lap, and polish at low speed. Scratching usually means you have applied too much diamond or not cleaned off accumulated swarf. Water-based paste makes it easier to clean the facet for inspection, but oil-based paste seems to work better with troublesome facets. I use Gearloose’s BA5T to polish sapphire but members of the GemologyOnline lapidary forum sing the praises of the Diamatrix ceramic composite lap. (see http://www.gemologyonline.com/Forum/phpBB2/viewforum.php?f=8).