Optical Coherence Tomography for the non-invasive investigation of the microstructure of ancient Egyptian faience

Optical Coherence Tomography (OCT) is a non-invasive subsurface 3D imaging technique based on the Michelson interferometer. The non-invasive nature of OCT and its speed of acquisition makes it possible to image large volumes of intact objects to yield a complete overview of the microstructure. The production methods for ancient Egyptian faience were first investigated using scanning electron microscopy (SEM) imaging of the microstructure in polished sections and microprobe analysis of the composition of the glass phases. These studies were based on original Egyptian faience objects and laboratory reproductions of faience beads made using three different production methods. The microstructure of the same laboratory samples and the Egyptian faience objects from the British Museum Research Laboratory Collection are re-examined using OCT. It is found that OCT virtual cross-section images can be used to group ancient faience objects into three categories on the basis of the morphology of the surface glaze layer and the glaze/core interaction layer. The OCT images correspond well with SEM images of polished sections of ancient faience objects and laboratory reproductions. The virtual cross-sections produced by OCT are somewhat limited by the penetration depth, which is affected by the high absorption coefficient of the material and, therefore, cannot always provide information on the presence or absence of interparticle glass that binds together the quartz particles in the core. Nevertheless, the top two layers are well imaged and since OCT images can be obtained rapidly and without the necessity of removing a sample, the method can be applied to a very much wider range of ancient faience objects than is possible by SEM examination of polished cross-sections. In summary, this paper examines to what extent OCT can assist the investigation of the production techniques of ancient Egyptian faience.

► OCT can image the glaze, glaze/core interaction layers of faience non-invasively.
► Non-invasive OCT images can separate ancient faience into three categories.
► OCT can be applied to a much wider range of ancient faience objects than SEM.