Armature from a clock movement MIH IV-212
Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)
Armature from the clock movement of a wooden wall clock with electric movement (fig. 1).
Horological object
Wooden wall clock
1902-1904
Modern Times
Indoor atmosphere
International museum of horology (IMH), La Chaux-de-Fonds, Neuchâtel
International museum of horology (IMH), La Chaux-de-Fonds, Neuchâtel
MIH IV-212
Not conserved
Nothing to report.
Stratigraphic representation: none.
This sample is the complete cross-section from an element of the armature (Fig. 2). The metal has huge cracks radiating from the centre to the outside which have deformed the armature (Fig. 3). The metal is covered with a Ni coating.
Zn Al Sn Cu Alloy
As-cast
MIH-VI-212
HE-Arc CR, Neuchâtel, Neuchâtel
International museum of horology (IMH), La Chaux-de-Fonds, Neuchâtel
2009, metal analysis
Nothing to report.
Analyses performed:
Metallography (unetched), Vickers hardness testing, SEM/EDS.
The metal is a Zn-Al-Sn-Cu alloy (Table 1) with an average hardness of HV1 105. A fine dendritic structure is observed (Fig. 5) which consists of clearly separated Zn, Al-Cu and Sn-rich phases (Figs. 6 and 7). Pb is associated with Sn but also forms tiny nodules.
Elements |
Zn | Al | Sn | Cu | Pb | Total |
---|---|---|---|---|---|---|
Metal | 67 | 17 | 11 | 4 | <1 | 99 |
Table 1: Chemical composition (mass %) of the metal (oxygen not shown). Method of analysis: SEM/EDS, Lab of Electronic Microscopy and microanalysis, IMA (Néode), HEI Arc.
Fine dendritic structure (no cohesion between the phases)
Zn
Al, Cu, Sn
Nothing to report.
The Zn-rich phase is heavily oxidised internally (Fig. 7). The corrosion has developed throughout the entire metal body, generating cracks (Figs. 8 and 9). The cracks are Zn, O and C-rich (Fig. 10) and could be composed of zinc carbonate.
Uniform - transgranular
zinc pest
Nothing to report.
Corrected stratigraphic representation: none.
The armature of the clock mechanism is constituted of a ZnAlSnCu alloy. The absence of cohesion between the different phases has led to the penetration of O during the manufacturing of the alloy. Small original cracks have developed further eventually causing the armature to break. This phenomenon, known since the 1920’s, is called zinc pest. It develops mainly on cast objects, starting with localised modifications (blisters and pits, Cramer and Covino 2005).
It appears that the armature was made of an unsuccessful experimental alloy. The IMH has in its collection a similar mechanism with the same armature but made of an another more stable alloy, suggesting then that our armature was some kind of prototype element.
References on object and sample |
References object 1. Seemater, V. (2009) Conservation-restauration d’une horloge à mouvement électrique, rapport interne HE Arc CR.References sample 2. Seemater, V. (2009) Conservation-restauration d’une horloge à mouvement électrique, rapport interne HE Arc CR. |
References on analytic methods and interpretation |
3. Cramer, S.D., Covino Jr., B.S. volume editors. (2005). ASM Handbook, 13B, 37. |