A Lucite lap is a polishing lap fashioned from a disc of polymethyl methacrylate (PMMA, sold variously as Lucite, Perspex, or Plexiglas), used by faceters to polish soft to medium-hardness coloured stones. Lucite is one of several plastic and resin lap materials in modern faceting practice, alongside tin, lead, ceramic, BATT, and Vol-J laps; each has its own grip characteristics, optimal abrasive carrier, and behaviour against different stone species.

Properties and behaviour

The principal advantage of Lucite is its softness and conformability: the disc takes a light charge of fine diamond or oxide abrasive readily and yields under the stone, producing a smooth polish on materials that scratch or chatter on harder laps. It is most often used for stones in the Mohs 5 to 7 range, including apatite, fluorite, calcite, sphene, kunzite, and some tourmalines, as well as for soft synthetics. Above Mohs 7.5 the lap wears too quickly to be efficient, and tin or ceramic laps are preferred. Lucite is sensitive to heat and to solvent contamination; cutting fluids must be selected with care, and the lap must not be allowed to overheat under aggressive cuts.

Use

Faceters typically charge a Lucite lap with a fine diamond suspension (50,000 mesh or finer) or with cerium oxide for the final polish, applied sparingly. The stone is presented at a light load, with the lap turning at moderate speeds (around 200 to 500 rpm). Periodic re-truing with a flat reference and gentle resurfacing with fine sandpaper extends the working life of the disc. Most faceters reserve a Lucite lap for a single material or a single class of materials, since cross-contamination between species can produce surface defects on the next stone.