Imaging behind glass (1/3)

Part 1: Introduction

Ancient papyri are most commonly stored and displayed in glass frames, which protect the papyrus and at the same time permit the display of the written surface. The papyrus fragments are basically sandwiched between two glass sheets held together by tape.

Various types of glass and adhesive tape are used across different collections, with the materials being based on a series of factors, including cost, the display conditions and environment, access to suppliers, etc. (Kaye 2019). Window glass (soda lime glass) has been used traditionally, while the more chemically stable borosilicate glass is gaining ground in recent times and UV radiation protective filters are necessary for papyri that are often on display.

Given the fragile state of surviving papyri, curators are reasonably hesitant to open the glass frames for materials analysis. So, a common technique for preliminary analysis of mounted papyri involves imaging with a portable microscope (namely using a DinoLite microscope with UV and IR light sources) above the glass frame.

This methodology, developed by the research team of Ira Rabin and colleagues (BAM), makes it possible to rapidly scan the manuscript and get qualitative information about the inks based on a simple principle: carbon inks remain black across the spectrum, iron gall inks become less intense (the lines become thinner) under IR illumination (set to 940 nm in the commercially available model of the DinoLite), and plant-based inks tend to disappear at 740 nm (Colini et al. 2021). Moreover, for plant based inks, the contract between the ink and the substrate is expected to increase (ibid.). By comparing the opacity and thickness of the lines, information about the makeup inks can be extracted. 

However, this technique relies on the (rather subjective) ability of the researcher to recognise these variations between the three predefined modes of illumination (Vis, IR, UV) and the results are qualitative and descriptive. Hilda Debora and I were wondering if we can use the equipment of the Colourlab (NTNU, Gjovik) for the quantitative measurement of glassed papyrus with hyperspectral imaging. 

What would the impact of the glass be on our measurements?

This series of blog entries in three parts follow a pilot study organised in collaboration between EthiCodex and the Lying Pen of Scribes research project. We approached Anastasia Maravela and Federico Aurora to borrow some of the old glass frames of the Oslo Papyrus Collection. 

We also decided to make our own inks and apply them on both papyrus and parchment.

The next post (part 2) describes the process of making iron gall ink from commercially available starting materials. The last part of this series (part 3) describes the process of making carbon black ink, starting from commercially available carbon particles and gum arabic and finally the setup of the hyperspectral camera. 

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