XRF setup for Garland of Howth-banner

XRF Analysis on the Garland of Howth

The use of colour in Early Irish Manuscripts can give clues about the resources available during the creation of the manuscripts, the skills of the illuminators, and the traditions of the community.

Figure 1 XRF set up for The Garland of Howth, 8th-9th century, TCD MS 56 © The Board of Trinity College Dublin, the University of Dublin. 2015.
Figure 1 XRF set up for The Garland of Howth, 8th-9th century, TCD MS 56 © The Board of Trinity College Dublin, the University of Dublin. 2015.

As we continue with our pigment analysis we have now commenced X-Ray fluorescence analysis of the illuminations and elaborated initials in our four manuscripts. This technique like Raman spectroscopy (see previous post) can, with the right equipment, be used in-situ directly on a manuscript without risk of damage, and similarly has a long history of use for the identification of pigments.1

XRF provides elemental information about the inorganic materials used to create a painted surface. The information generated by this technique is generally for elements that have an atomic weight above aluminium. The technique is based on the detection of photons of energy emitted from a sample when subjected to x-ray energy. It follows the principle that individual atoms will emit photons of a characteristic energy or wavelength that is related to its atomic number.

Our instrument – the Bruker Tracer III-V Turbo SD – is a fully field portable analyser, with both the x-ray tube and the detector housed in a hand-held device. For the purpose of this study, the instrument is supported on a tripod and positioned just above the surface of the manuscript (Fig. 1). The x-ray beam is reduced though a nose shield to about 2mm in diameter, which is ideal for isolating specific areas of an item for analysis, although not always narrow enough for fine detail on manuscript painting.

XRF can be used as an effective preliminary technique to determine whether additional data collection is necessary, and to provide supporting information for another technique (such as Raman spectroscopy) in order to make a conclusive identification of a pigment.

Figure 2 Pigment identification in The Book of Mulling, 2nd half of 8th century, TCD MS 60, f. 12v, © The Board of Trinity College Dublin, the University of Dublin. 2015.
Figure 2 Pigment identification in The Book of Mulling, 2nd half of 8th century, TCD MS 60, f. 12v, © The Board of Trinity College Dublin, the University of Dublin. 2015.

For example for the white pigment used to paint the hands of the evangelist on folio 12v from the Book of Mulling, XRF analysis provided a strong calcium peak. This indicated that the white pigment used was likely to be chalk white or gypsum rather than lead white, all of which were used since antiquity. When we carried out the Raman analysis of the same area of white pigment the appearance of a peak at a characteristic point on the Raman spectrum (1090 cm-1), made it possible for us to identify the pigment as chalk white.

On the same folio, XRF analysis gave a strong calcium peak for the blue hair of this evangelist and allowed the use of two possible blues, azurite, or ultramarine ( from lapis lazuli) to be ruled out. Since XRF identifies the presence of inorganic pigments rather than organic materials, the potential use of indigo to colour the hair was not detected. However, indigo is typically mixed with a white pigment in order to be used as an opaque paint, or to moderate its intensity. Therefore Raman analysis was carried out on the same area of blue hair, and the characteristic Raman peaks (543, 596, 1586 cm-1) confirmed that the blue hair of the evangelist is indeed indigo (fig. 2).2

The XRF analysis of the white pigment on the Garland of Howth folio 22r also gave a very strong peak for calcium, indicating that either chalk white or gypsum was used. The Raman spectra confirmed the pigment is gypsum by the appearance of a peak at 1005 cm-1.  This pigment was also extensively used on the Book of Kells, particularly for hands and faces and also mixed with indigo.

Figure 3 White cliffs of Dover, created from chalk deposit Source.
Figure 3 White cliffs of Dover, created from chalk deposit
Source.

Chalk is calcium carbonate mineral of a variety of very fine grained limestone formed from the fossils of plankton, and deposits are very widespread. The white cliffs of Dover are the most famous chalk deposit in Western Europe (fig. 3).

The preparation of the raw material is relatively easy, requiring the deposit to be ground then washed to remove coarse unwanted material. The white powder is dried and mixed with a binder to create a paint. Chalk white can also be easily identified by optical microscopy as it will contain the distinctive features of the spherical plankton body known as a coccolith.3

Figure 4 Gypsum. Source.
Figure 4 Gypsum. Source.

Gypsum is a very common calcium sulphate mineral found in deposits associated with limestone and clays (fig. 4). Its name is derived from the Greek word ‘gyps’ meaning burned material, as the preparation involves using heat to dry the ground gypsum powder to make a stable material for the artist.4

Susie Bioletti, Keeper of Preservation and Conservation and Allyson Smith, Project Scientist, Preservation & Conservation Department, The Library of Trinity College Dublin

Footnotes

  1. A.N. Shugar and J.L. Mass (eds.), Studies in Archaeological Sciences. Handheld XRF for Art and Archaeology (Leuven, 2012).
  2. Bioletti, R. Leahy, J. Fields, B. Meehan, and W. Blau, ‘The examination of the Book of Kells using micro-Raman spectroscopy,’ J. Raman Spectroscopy, 40 (2009), 1043-49.
  3. N.Eastaugh, V.Smith, T.Chaplin, and R.Siddal, Pigment Compendium (Oxford, 2004), p. 92.
  4. Ibid, p. 179.