Shoe nail - Hypoeutectoid steel - Roman Times

Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland) & Valentin. Boissonnas (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Complementary information

Burial environment: Dense sand-clay soil, humid. 

The schematic representation below gives an overview of the corrosion layers encountered on the nail from a first visual macroscopic observation.

Fig. 6: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 14.
Fig. 5: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 14.

 Metallography (Nital etched), SEM/EDX

The remaining metal is an hypoeutectoid steel (Fig. 5 and 6), containing Fe, Si, O slags inclusions (Fayalite FeO,SiO2?), and consisting of less than 1% of the total volume of the highly mineralized nail. After Nital etching, it shows intergranular corrosion on one of its side (Figs. 7 and 8).

The metal is heavily corroded and the thickness of the corrosion crust is irregular, varying between 0.8 and 3.8mm, with an average of about 2mm. The corrosion has replaced most of the metal. Between two corrosion layers the limit of the original structure is visible. Some leather seems to have been preserved as pseudomorphic structures (Figs. 9 and 10). The carbon mapping (Fig.11) shows the presence of carbon in the cracks which contain the leather fibers. It may be due to the penetration of the resin in the cracks. Cl does not seem to be present, while Si, Al, P and K can be considered external markers, helping to determine the limit of the original surface (Fig. 11 and Table 1). The outer layer of the corrosion products ranges from dark grey to orange in bright field and contains less O and Fe on its borders than the inner layer (Figs. 4 and 11, Table 1). The SEM pictures show a marbling appearance of the inner corrosion layer. Cl and S have been mapped, but are not present in the sampled nail.

 

Table 1: Elementary presences in stratigraphic layers. Y = yes; N = no; NA = not applicable (not observed). + = more compared to other layers; - = less compared to other layers. Empiric observations based upon elementary mappings (Fig.11). *probably pushed back by coating resin. **though SEM-EDX analysis showed that the metal is an iron-carbon alloy with inclusions of Si, Al, P, K.

Elements

Layers

Fe

O

C

Al

Si

K

P

S1

Y-

Y

N

Y++

Y+

Y

Y-

CP1

Y

Y

Y-

Y

Y

Y-

Y

CP2

NA

NA

NA

NA

NA

NA

NA

CP3

NA

NA

NA

NA

NA

NA

NA

CP4

Y+

Y+

N

Y-

Y-

N

Y-

CP5

Y+

Y+

N

N

N

N

Y--

NMM

N

Y

N

N

Y++

N

NA

POM

Y+

Y-

Y

N

N

N

N

SV

N

N*

Y

Y-

N

N

N

M1

Y++

N

N**

N

N

N

Y--

 

Based on the analyses carried out, the schematic representation of the stratigraphy of corrosion layers (Fig. 3) was corrected.

The nail is apparently made of a hypoeutectoid steel, with what may be pearlite phase. Roman shoe nails are forged and pushed through leather soles by hammering them against a hard foot-shaped buckering device. The steel they are made of requires to be strong but not too brittle, otherwise they are prone to breaking while being pushed through the soles or when in use while marching on hard surfaces. This nail is almost entirely mineralized. Its original shape seems to be preserved within the corrosion layers. Leather fibers seem to have been preserved in either mineralized or still organic form. 

References object

1.     Volken Marquita. "Le fer et la peau: le cuir et ses outils en milieu urbain romain". In: Chardron-Picault Pascale (dir.). Aspects de l'artisanat en milieu urbain: Gaule et Occident romain. Actes du colloque international d'Autun, 20-22 septembre 2007. Revue Archéologique de l'Est, 28e supplément. RAE, Dijon, 2010, pp.415-424.

2.     Volken Marquita, Volken Serge et Paccolat Olivier. "Les clous de chaussures du site de Pfyngut: les bases d'une typo-chronologie." In: Paccolat Olivier (dir.). Pfyn/Finges, évolution d'un terroir de la plaine du Rhône. Le site archéologique de "Pfyngut" (Valais, Suisse). Cahiers d'archéologie romande 121, Archaeologia Vallesiana 4, Lausanne, 2011, pp.315-388.