A non-invasive investigation of Egyptian faience using a long wavelength optical coherence tomography (OCT) at 2um

TL;DR

This study demonstrates that a 2um wavelength optical coherence tomography (OCT) can non-invasively image the microstructure of Egyptian faience objects down to the core, enabling detailed analysis without sampling or damage.

Contribution

The paper introduces the use of a longer wavelength OCT at 2um for non-invasive imaging of Egyptian faience, overcoming previous depth limitations and allowing comprehensive microstructural analysis.

Findings

2um OCT successfully images the core of faience objects.

It enables comparison of microstructures across layers.

The technique is suitable for large museum collections.

Abstract

Egyptian faience is a non-clay ceramic semi-transparent material, formed of a quartz core and alkali lime glaze with some cases exhibiting an interaction layer between them. Several possible glazing methods have been identified. Previous investigations have tried to identify the glazing technique by using the microstructure images obtained from polished sections using scanning electron microscope(SEM). Such techniques require sampling which is not feasible on museum collections. Optical Coherence Tomography (OCT) is a non-invasive 3D imaging technique, that produces virtual cross sections of transparent and semi-transparent materials. Liang et al. (2012a) investigated the feasibility of using OCT to non-invasively investigate microstructures of Egyptian faience, but the limited probing depth of the 930nm OCT prevented viewing down to the core of the objects, where the presence of glass was thought to be a distinguishing feature between some of the manufacturing techniques, and where the particle size of the quartz may indicate the difference in the raw material. In this paper, a unique longer wavelength OCT at 2um is used to scan a number of ancient Egyptian faience objects including ring and shabti fragments. It was found that the core of the faience could be imaged at this longer wavelength, allowing comparisons in all the layers within the microstructure, and leading to discussions about the possible glazing methods. The 2um OCT offers the possibilities of rapid, non invasive imaging of faience microstructure down to the core, allowing comprehensive studies of intact objects and large museum collections.