Armature from a clock movement MIH IV-212 - Zn Al Sn Cu Alloy - Modern Times

Armature from a clock movement MIH IV-212 - Zn Al Sn Cu Alloy - Modern Times

Armature from a clock movement MIH IV-212

Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

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Credit HE-Arc CR.

Fig. 1: Broken armature (left) from the clock movement of a wooden wall clock (right) (after Seematter 2009),

Armature from the clock movement of a wooden wall clock with electric movement broken in two elements (Fig. 1 and Fig. 2) with a heavily cracked surface.

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

Complementary information

Nothing to report.

Credit HE-Arc CR.

Fig. 2: Location of sampling area on the second element of the armature,

Stratigraphic representation: none. 

Credit HE-Arc CR.

Fig. 3: Micrograph of the cross-section of the fragment sampled from the armature showing the location of Figs. 5 to 10,

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

Complementary information

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.

Credit HE-Arc CR.

Fig. 5: Micrograph of the metal sample from Fig. 3 (inverted picture, rotated by 270°, detail), unetched, bright field. A dendritic structure is visible. The micrograph of Fig. 6 is marked by a square,

Credit HE-Arc CR.

Fig. 6: SEM image, BSE-mode, detail from Fig. 5 (detail), unetched, bright field. Three phases appear: in light-grey the Zn-rich phase, in dark-grey the Al and Cu-rich phase and in white the Sn-rich phase,

Credit HE-Arc CR.

Fig. 7: SEM image, BSE-mode, and elemental chemical distribution of the selected area from Fig. 6. Method of examination: SEM/EDS, Lab of Electronic Microscopy and microanalysis, IMA (Néode), HEI Arc,

Fine dendritic structure (no cohesion between the phases)

Zn

Al, Cu, Sn

Complementary information

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.

Credit HE-Arc CR.

Fig. 8: Micrograph showing the metal - “corrosion products” interface from Fig. 3 (reversed picture, detail), unetched, bright field. We observe in white the metal matrix and dark-grey the adhering material,

Credit HE-Arc CR.

Fig. 9: Micrograph of the metal sample, detail from Fig. 8 (reversed and rotated by 90°), unetched, bright field. A dendritic structure is visible. The area selected for elementary mapping (Fig. 10) is marked by a rectangle,

Credit HE-Arc CR.

Fig. 10: SEM image, BSE-mode, and elemental chemical distribution of the selected area from Fig. 9 (detail). Method of examination: SEM/EDX, Lab of Electronic Microscopy and microanalysis, IMA (Néode), HEI Arc,

Internal cracking

zinc pest

Complementary information

Nothing to report.

Fig. 4: Stratigraphic representation of the fragment sampled from the armature in cross-section (dark field) using the MiCorr application. The characteristics of the stratum are only accessible by clicking on the drawing that redirects you to the search tool by stratigraphy representation. This representation can be compared to Fig. 8, Credit HE-Arc CR.

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.