Shoe 228.1955 Technical Analysis

Length 16 cm,  Width 7, Depth 2 cm 

Ultra-violet Imaging

The ultra-violet (UV) images of shoe 228.1955 show an other all blue/white fluorescence indicating the presence of a fatty bloom or a preservative coating. The  fluorescence is particularly clear within the crack in the leather on the underside of the shoe, possibly as a result of a build up of the material in these regions. 

Energy Dispersive X-ray Analysis

A small sample of wood was collected from shoe 228.1955 and analysed using Energy Dispersive X-ray (EDX) analysis. The series of images above indicate the distribution of a range of elements present in the sample. The absence of a significant distribution of chromium in this sample is reassuring, as this indicates that the leather used to make the shoe was not chrome tanned (consistent with a pre-industrial object). The trace aluminium identified in the sample may relate to the presence of embedded clay mineral particles in the leather such as kalonite. 

FTIR Analysis

The FTIR spectrum for cow leather (brown) includes a number of bands that facilitate comparative analysis of unknown samples. The relative intensity of the some bands can also indicate the state of preservation for the material. As shown above, cow leather has a strong band centred over 3300cm-1, known as the amine band, that relates to stretching of the =N-H group in the proteinaceous component of the leather. A series of three decreasingly intense bands at 1651, 1550, and 1340cm-1 are referred to as Amide bands I-III respectively, and are useful indicators of the protein content in a leather sample. The sharp peak at 2912cm-1 relates to asymmetric stretching of =NH and – CH3, –CH2 groups, while the peak at 2858cm-1 is assigned to symmetric stretching of –CH2–groups. Additional prominent peaks include one at 1742cm-1 relating to the –COOH un-ionized group, a band at 1448cm-1 related to bonding oscillations of –CH3, and –CH2– groups, and a pair of peaks at 1088 and 1028 that refer to stretching of C–O or –C=S groups (Bienkiewicz, 1983).

As can be seen in the spectra above, there is some variation between the reference spectrum for cow leather (brown) and the sample collected from shoe 228.1955 (blue). Notably the peaks relating the the proteinaceous component of the leather (3300, 1651, 1550, and 1340cm-1) are either absent, or much reduced. This suggests that the collagen component in this sample has undergone proteolysis. The peaks at 2912 and 2858 cm-1 while present, are also somewhat reduced in comparison to the cow leather reference sample. The fingerprint region in spectrum 228.1955.1 (blue) is somewhat complex, but there are a number of peaks visible in the detailed image above showing the spectral range  from 1600-600cm-1. These peaks correspond with oleic acid (pink) and linoleic acid  (green), and likely relate to the deposition of deteriorated fatty components on the surface of the leather. There is no indication of post collection contaminants in the FTIR spectrum for shoe 228.1955.  

Mass spectrum for leather sample from shoe 228.1955 showing diagnostic peak at 1105 m/z. 

Zoological Mass Spectrometry(ZooMS)Analysis

A sub-sample of the leather collected from shoe 228.1955 was analysed using ZooMS at the Department of Bio-Archaeology, University of York. In this technique the sample is chemically processed to extract the collagen component of the leather. The collagen component is vaporised using a laser in a process known as Matrix Assisted Laser Desorption/Ionization (MALDI), and the ions generated from the sample are analysed using time of flight mass spectrometry (TOF-MS).  The velocity of the ions as they pass through the drift space in the mass spectrometer is determined by their specific charge to mass ratios. Thus the ions are detected at specific points on the spectrum according to their specific charge to mass ratio. As the amino acid sequence of collagen is unique to every species, so too are the ions generating during vaporisation. The appearance of specific peaks on the mass spectrum is therefore indicative of particular species.  Spectral peaks derived from the sample are compared against a database of known species mass spectra to indicate the species of the leather. The lack of higher mass peaks in the spectrum above may reflect the proteolysis noted from the FTIR spectrum above. While it has not been possible to define the species from which the shoe was produced, the peak at 1105 m/z indicates that the family is either Bovidae or Cervidae. 

Bibliography

Bienkiewicz K (1983), Physical Chemistry of Leather Making, Florida, Robert E. Krieger Publishing.