Phenolic Lap — A Bakelite Polishing Surface for Lapidary Finish Work
Phenolic Lap — A Bakelite Polishing Surface for Lapidary Finish Work
A polishing lap of phenolic resin used with diamond compound or oxide polish for the final stage of faceting
The phenolic lap is a faceter's polishing wheel made from phenolic resin — the thermosetting plastic commercially developed as Bakelite in the early twentieth century — used in lapidary practice for the final polishing of cut gemstones. Phenolic offers a semi-rigid working surface that conforms slightly to the gem under pressure, accepts charged abrasive readily, and resists warping and dimensional change under modest heating. The lap occupies a position in the faceter's toolkit alongside metal laps (tin, copper, pewter, zinc) and softer resin and acrylic surfaces, and is most often selected for materials in the Mohs 6 to 8 range where a slightly yielding surface produces a brighter polish than the harder metal alternatives.
Construction
A phenolic lap consists of a disc of phenolic resin laminated to a steel or aluminium backing plate that mounts to the faceter's spindle. Standard diameters are 6 to 8 inches; thickness is typically 6 to 12 millimetres on the working face. The resin is precision-machined flat after cure and is typically supplied with a fine surface finish ready for charging.
Phenolic for laps is selected for hardness, dimensional stability, and resistance to organic solvents that may be present in the polishing oil. Standard industrial phenolic — the same material used for electrical insulators, gear blanks, and circuit-board substrates — provides a working surface that holds charged diamond well and produces consistent polish results across a range of gem species. Some manufacturers offer phenolic laps with cotton or paper-fibre reinforcement for additional dimensional stability; others offer pure resin laps for faceters who prefer a cleaner working surface.
Use in faceting practice
Phenolic laps are charged with diamond compound — typically 50,000 to 100,000 grit for final polish — applied as a paste with a light oil carrier. The lap is dressed by working a charging stone or a piece of scrap rough across the surface to embed the diamond particles, after which the lap is ready for production polishing. Charged phenolic produces a high-quality polish on quartz, beryl, corundum, garnet, spinel, and similar species in the Mohs 6 to 9 range; for harder species such as fine corundum and chrysoberyl, faceters sometimes prefer ceramic or zinc laps for marginal performance gains.
The principal advantages of phenolic over metal laps are the slight surface yield, which produces brighter polish on materials that are sensitive to flat-surface chatter, and the dimensional stability that maintains facet flatness over long polishing sessions. The principal disadvantage is the slower cutting rate compared with metal laps; phenolic is therefore typically used for the final polishing stage rather than for pre-polish or shaping. Cerium oxide and aluminium oxide polishes are sometimes used on phenolic for quartz-family material with good results.
Maintenance
Phenolic laps require occasional resurfacing as the working face becomes contaminated with embedded debris from previous gem species or with degraded polishing oil. Light resurfacing can be done with a fine diamond hone; deeper resurfacing requires sending the lap back to the manufacturer for re-machining. The phenolic does not warp under normal working conditions, and the lap typically has a working life of hundreds of hours before significant resurfacing is required.