Raman Spectroscopy in Archaeology and Art History

Volume 2

Peter Vandenabeele, Howell Edwards, Peter Vandenabeele, Howell Edwards

• Publisher: Royal Society of Chemistry
• Year: 2018
• ISBN:
  9781788015653
• Edition: 1
• Topic: Naturwissenschaften
• Subtopic: Spektroskopie & Spektralanalyse

Ten years after the first volume, this book highlights the important contribution Raman spectroscopy makes as a non-destructive method for characterising the chemical composition of objects with archaeological and historical importance. The original book was ground-breaking in its concept, but the past ten years have seen some advancement into new areas, consolidation of some of the older ones and novel applications involving portable instrumentation, on site in museums and in the field.

This new volume maintains the topic at the cutting edge, the Editors have approached prominent contributors to provide case-studies sorted into themes. Starting with a Foreword from the British Museum Director of Scientific Research and an Introduction from the Editors, which offer general background information and theoretical context, the contributions then provide global perspectives on this powerful analytical tool.

Aimed at scientists involved in conservation, conservators and curators who want to better understand their collections at a material level and researchers of cultural heritage.

CHAPTER 1

Analytical Raman Spectroscopy of Inks

Howell G. M. Edwards^*

School, of Chemistry and Biosciences, Faculty of Life Sciences, University of Bradford, Bradford BD7 1DP, UK
^*E-mail: [email protected]

Research on ancient manuscripts has dealt with the identification and use of pigments in their historiation and the degradation of the vellums, but relatively little has been undertaken on the composition of the inks. This chapter traces the use of carbon-based inks, the so-called Indian inks, through to mediaeval iron-gall or gallotannate inks and, more rarely, coloured inks made from logwoods and cinnabar. Recent interest in ancient ink composition has arisen from the significant destructive effects noted particularly in the use of iron gallotannate inks on vellums and paper. Following an historical survey of ink through the ages and a discussion of ink preparation and composition, this chapter considers three case studies in which the analysis of inks has been instrumental in providing information for assessment by conservators and historians: these are the Vinland Map, the Voynich Manuscript and the Beato de Valcavado manuscript. In particular, the role of Raman microscopy in the discovery of anatase in the ink of the Vinland Map is highlighted in the ongoing controversy surrounding the importance of this manuscript in the discovery of North America, and the interesting Raman spectroscopic analyses of inks in the Beato de Valcavado are discussed.

1.1 Introduction

Many of the earliest studies of manuscripts involving Raman spectroscopy focused upon the analysis of coloured mineral pigments in historiated manuscripts from which much novel information could be obtained about their composition and preparation technologies. However, complementary studies of these inks have generally been less frequent, yet as will be seen, this can often provide interesting and supportive information on the holistic preparation of manuscripts and printed works of art. The distinction between a paint and an ink is not easily defined as both contain pigments dissolved or suspended in a carrier liquid or solvent with added binders and drying agents to produce a coloured fluid: the major difference, therefore, is one of application rather than composition and in some cases printing inks can be used as paints and vice versa with an adjustment of pigment content. Generally, paint is applied more thickly than ink, and often the drying process is chemically different, depending upon the ingredients used in the ink or paint.

The adoption of fluid inks and reed pens for their application has been attributed to China around 2700 BCE; hence, ink can be accredited to almost 5000 years of human history. The name ink is believed to originate from the Latin incaustum and the later mediaeval French encre; as was experienced with pigment minerals, both have been found to apply to the earliest carbon-based inks and to the later iron gallotannate (iron gall) inks.^1,3 The earliest ink, formulated in China from carbon black suspended in an aqueous solution of water and gum arabic, is now known as Indian ink because the best carbon supplies at that time for this purpose were sourced in India; the carbon was synthesised by burning wood such as pine, in which the partially combusted resin helped to bind the sooty deposits, in a limited amount of air under an upturned iron or ceramic dish. The gum arabic had a multifunctional role in keeping the carbon particles in suspension whilst also thickening the writing fluid for ease of application using quill or reed pens. Other carbons were sourced from the calcination of animal bone and ivory, which produced a deeper black colour – these contained residues of calcium phosphate derived from the hydroxyapatite component and provide a useful analytical Raman spectroscopic signature for the detection of bone black or ivory black, with the characteristic wavenumber of phosphate ion stretching at 960 cm⁻¹, which can be used to discriminate between vegetable and animal origins for the source of the amorphous carbon used in the manufacture of ancient inks.⁴

The discovery of natural mineral oils and petroleum afforded another opportunity for the production of a deep black sooty residue upon combustion in a limited supply of oxygen. A hierarchical basis for recipes for the production of carbonaceous soot existed in which certain botanical materials were highly prized for combustion to form the blackest inks, such as peach stones, almond shells and vine twigs. There was much empiricism in the formulation of early inks as evidenced by the universal adoption of gum arabic as a binding agent; in addition to assisting the suspension of the insoluble carbon particles in the aqueous ink medium, the water-soluble gum arabic modified the viscosity of the ink, so assisting in the writing flow when applied with reed pens, quills and brushes and also improved the adhesion of the ink to the writing substrate. However, the addition of too much gum arabic resulted in a brittleness of the applied ink when dry and a tendency for the writing to flake off – hence, the debris found between the leaves of ancient manuscripts frequently contains particles of ink from the associated script that can provide a rich source of sampling to derive analytical information without involving the further destruction of the manuscript text. Different names have been recorded for carbon black inks through the ages, dependent upon their formulation or source, such as bistre,⁵ an extract from sooty fires that possessed a warm brown colour, and sepia, a dark, semi-transparent ink from cuttlefish, which was much used by the Roman scribes.

In mediaeval times, iron gall ink replaced carbon black ink as the favoured medium of writing; although iron gall ink has recently been detected⁶ using X-ray spectrometry on the Codex Eusebii Evangelorum (the Vercelli Gospels), the oldest existing version of the Gospels written in Latin and dating from the 4th century CE. Iron gall ink, more correctly described as an iron gallotannate, was the first water-based ink to be made from a chemical reaction between aqueous solutions of iron(ii) sulfate and extracts of oak galls with the addition of gum arabic. Oak galls are spherical, nut-like protuberances resulting from the egg-laying of wasps on oak trees. The best galls were those fully developed from which the emerging wasp larvae had hatched. As encountered with the carbon-based inks, there are many empirical recipes in existence for the manufacture of iron gall inks; indeed, as we have noted above, the chronology for the first appearance of iron gall inks actually predates the mediaeval period and Pliny in the 1st century CE describes in detail the preparation of aqueous gall solutions that blacken in the presence of copperas, an iron sulfate ore. However, Pliny was specifically referring to the detection of the adulteration of verdigris by the addition of cheaper copperas through the formation of a black colouration on exposure to an infusion of nutgalls. Outside of the Vercelli Gospels, the first record of the use of iron gall ink as a writing medium seems to have occurred in the Dead Sea Scrolls, from the late 3rd century CE.

The preparation of iron gall ink was a rather complex alchemical procedure, as indicated by the following ancient recipe for the manufacture of the highest quality iron gall ink:

8 oz powdered Aleppo galls; 4 oz logwood chips; 4 oz iron sulfate; 3 oz powdered gum arabic; 1 oz copper sulfate or verdigris (basic copper acetate); 1 oz sugar; all heated and triturated in 12 pounds water followed by filtration and the addition of alum, ammonia, beer, lemon juice, oil of cloves, ground walnuts, lavender, wine, boiled oil, and extract of amber or shellac in brandy to minimise the growth of mould.

An understanding of the chemistry of the preparation of iron gall inks reveals the roles of the iron complex and its formation: the enzyme tannase from the fungus Aspergillus niger in oak galls releases gallic acid, a triphenol carboxylic acid, C₆H₂(OH)₃COOH, and glucose through the catalytic hydrolysis of gallotannic acid ester. Iron(ii) ions from ferrous sulfate then form a dark grey 1 : 1 iron gallate complex, which releases hydrogen ions and is then oxidised by aerial oxygen on the manuscript to a ferric pyrogallate complex that is black in colour. An excess concentration of iron(ii) causes the ink to gradually fade, a problem experienced with the multifarious recipes in existence in the Middle Ages, and this also stimulates the release of hydroxyl ions and the formation of hydrogen peroxide through a Fenton reaction. It is this last property that causes the destructive damage effects noted on ancient manuscripts involving iron gall inks.

Iron gallotannate inks quickly became the medium of choice for mediaeval scribes because, unlike the carbon-based inks they replaced, they interacted physically and chemically with cellulose substrates, conferring better adhesion and permanence of the writing on the script.^7,8 Even when used with parchments and vellum, the iron gall inks had a noteworthy adherence to their substrate and could only be removed by mechanical detachment and scraping, unlike the carbon-based inks which could be more easily erased and washed off. However, this improvement in the writing permanence of iron gall inks caused severe corrosion problems for paper manuscripts in particular. In some cases, this process resulted in the formation of holes in the manuscript (lacunae) in place of the writing; many manuscripts have suffered irreversibly in this way and pose problems for their conservation and the preservation of their integrity.^9,10 It has been found that arresting the decay can be achieved by the application of calcium bicarbonate, lime, magnesite and calcium phytate, but generally, irreparable damage has been done to the original script and text.^11,13

The corrosive effect of iron gallotannate inks upon cellulosic substrates can be related to the iron-catalysed breakdown of cellulose in an acidic environment.¹⁴ It will be seen below that the formation of the iron(ii) gallate complex releases hydrogen ions and decreases the pH significantly to about 2; in this process, excess Fe²⁺ ions then react with acidic decomposition products of the cellulose to form hydroxyl and oxygenyl radicals, from which the subsequent creation of hydrogen peroxide destroys the cellulose substrate and oxidises the iron¹⁵ to Fe³⁺. Hence, it has been suggested¹⁴ that measurement of the Fe²⁺/Fe³⁺ ratio in an ancient iron gall ink could provide a means of assessing its age, although clearly the actual rate of degradation of the ink would be dependent upon several environmental factors, not the least of which would be the recipe and formulation of the original ink, which was certainly not standardised in any way. The elemental migration of iron atoms into the substrate also can provide a measure of the age of the script, as determined from Auger spectroscopy, but pitfalls can be encountered particul...