SPIRIT LAMP
WHAT IS THE SPIRIT LAMP USED FOR?
The spirit lamp — sometimes a paraffin lamp — is one of the oldest sources of localised heat used by the watchmaker. Although contemporary workshops have progressively adopted precise, programmable electric furnaces for industrial heat treatments, the lamp remains a familiar tool of the artisanal watchmaker and the restorer, who appreciate its simplicity, its autonomy and the sensitivity of the flame it produces.
In its classic form, the tool consists of a reservoir — often in glass or brass — surmounted by a metal burner fitted with a wick of absorbent fibres. An adjustment screw controls the height of the wick and, consequently, the size of the flame. When properly maintained, the flame provides a calm and clean heat, free of spitting or excessive soot — qualities indispensable to the working of pieces whose dimensions and surface finish tolerate no contamination.
The thermal uses of the lamp are numerous. Tempering — which consists in a controlled reduction of the hardness of a previously quenched piece of steel in order to give it the desired compromise between hardness and toughness — is among the most classic operations. The part is placed on a brass plate or in a bed of copper turnings above the flame, and the watchmaker follows its oxidation tint to estimate the temperature reached. The blueing of screws and of certain hands is carried out on the same principle: the watchmaker stops the heating precisely at the appearance of the desired steel blue — generally in the region of 300 °C — so as to fix this hue, which carries a strong visual identity and incidentally contributes to the corrosion protection of the component.
Other operations likewise turn to the lamp for their source of heat: the local annealing of a component to make it more malleable before fitting work; the heating of a part to permit its shrink fitting or its hot removal; or the controlled melting of shellac used to fix the pallet stones of the pallet fork, or for repair work. For all these interventions, the spirit lamp offers the advantage of immediate readiness and of a manual regulation which, in the hands of an experienced watchmaker, remains remarkably precise.