Replication of a Marine Chronometer Gold Alloy Helical Spring: Conservation Project

By Eliott Colinge, PG Clocks and Related Objects / MA Conservation Studies

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Figure 1: Marine Chronometer mechanism by John Lilley and Son, 1929

The marine chronometer signed by John Lilley & son from 1929[1] (Fig.1) has shown evidences of a failure of one of its major components. The helical hairspring which is co-regulating the frequency of the balance (factor determining the rate of the mechanism) was broken by fracture near its lower attachment point (Fig. 2).

Figure 2: Fracture point of the spring near the lower attachment point and remains of the broken spring

The aim of the project was to provide a replacement spring for the chronometer which would possess the required mechanical properties (toughness, elasticity, shape, composition …).

A compositional analysis was conducted with the help of Mary Davis (our lab assistant) using energy dispersive x-ray and x-ray fluorescence technologies. The main elemental constituent of the spring was determined to be gold (69.8%), followed by silver (16.2%), copper (9.1%) and platinum (4.9%).

The alloy was prepared in a CaO crucible in a specific order to insure the homogeneity of the resulting metal and to limit the interstitial oxidation. The ingot was processed using goldsmithing techniques and the understanding of the behaviour of the metal. A rolling mill and a draw plate (Fig. 3) were used to turn the 1.702g ingot into a wire of 0.5mm in diameter and 600mm in length.

Figure 3: 3D representation of the drawing of a wire through a draw plate

The wire was then compressed by a rolling mill to conform the section of the spring to the former spring (Fig. 4). The compressed wire was than coiled onto a specifically designed mandrill and strongly maintained, forcing an even spacing between the coils, using a brass wire.

A heat treatment was applied to the spring in order to maintain its cylindrical shape of the required diameter whilst provoking the hardening of the metal by a process called age-hardening (Figure 5). This was done after the use of the phase diagrams of the corresponding elements. The phase diagrams give information about the temperature of recrystallization (annealing temperature), the melting point and the temperature of age-hardening of the alloy. The spring was cleaned of its oxide layer using an etchant and was adjusted to the balance of the chronometer.

Figure 5: New spring mounted on the brass mandrill during the age-hardening/forming process

In terms of properties, the new spring maintains the frequency of the oscillator at 7200 oscillations/hours with the same number of coils as the model spring. The shape of the spring obtained was better than the initial expectations.

Figure 6: New spring mounted on the balance

Table 1: XRF comparative analysis of the elemental constitution of the two springs

The crystalline structure of the springs revealed a couple differences; the initial spring presents a finer graining although a clear orientation of the grains supposes that they were both made using an extrusion or rolling technique.

Figure 7: Longitudinal metallographic section of the two springs after etching with Aqua Regiae and magnified by an optical microscope 40x. New spring (left) and initial spring (right).

Recommended literature:

Goldsmithing:

Brepohl, E. (2001) The Theory and Practice of Goldsmitihng, USA: Brynmorgen Press.

Metallurgy:

Higgins, R.A. (1993) Engineering Metallurgy, Applied Physical Metallurgy, 6th ed., UK: Arnold
Callister, W.D. (2007) Materials Science and Engineering, An Introduction, 7th ed., USA: John Wiley & Sons.
Mitchell, B.S., (2004) An Introduction to Materials Engineering and Science, For Chemical and Materials Engineers, USA: John Wiley & Sons.

[1] Mercer T. (2003) Mercer Chronometers, History, Maintenance & Repair, UK : Mayfield Books