SAPPHIRE FILE
The sapphire file is a finishing tool made from a synthetic sapphire single crystal — corundum, Al₂O₃ — cut into the shape of a bar, plate or rod and generally fitted with a handle. Its primary characteristic, a hardness of 9 on the Mohs scale, places it immediately below diamond and makes it suitable for working hardened steels and components that have already undergone heat hardening, on which conventional steel files would rapidly lose their cutting edges.
Contrary to what its name suggests, the sapphire file does not “file” by tearing material away as a toothed file does. It acts by very fine abrasion and by superficial micro-work-hardening, engaging only a minuscule area at each pass. Its working surface — either polished, or provided with a fineness of grain controlled by lapping — allows infinitesimal quantities of material to be removed while restoring a particularly regular surface finish. The tool, practically wear-free under its conditions of use, retains its reference geometry over time: an indispensable quality when perfect flatness or straightness is sought.
Sapphire files find their most widespread uses in pivot burnishing operations and in the decoration and finishing of movement components. Anglage occasionally calls upon the sapphire file for the final pass, which regularises and lustres the edge after the roughing stages carried out with the steel file and then with cabrons. Work with sapphire, performed flat or on edge, gives these surfaces a near-mirror polish while preserving the precision of sharp edges.
Beyond decoration, the sapphire file is used in certain fitting operations where dimensional precision is paramount: reworking a bearing surface, finishing a contact face on a hardened component, or final finishing of a surface intended to receive a surface treatment. In these capacities, it complements rather than replaces the more traditional tools — the steel file, the burnisher and the cabron — and forms part of the progressive sequence of finishing operations specific to fine watchmaking.