Analytical Methods

The technique of taking paint cross-section samples, often less than a millimetre in size, has been employed for over a century. It is used to understand and identify the layers of materials employed to create artworks such as oil paintings, wall paintings and polychromes. Commonly the technique involves embedding these tiny samples in resins and polishing the resin to bring the samples to the surface enabling them to be viewed by light microscopy. Through magnification, these tiny samples are revealed as preparatory ground and paint layers and sometimes the underlying support, often a textile.

The use of this technique to study the construction of painted barkcloth has not been exploited before now. However, recent samples from a Hawaiian painted cloth (GLAHM E.667), part of the Hunterian Collection, have been embedded in an acrylic resin and viewed using light microscopy. Sampling is carried out in typical area where paint has been used to decorate the barkcloth fibre. In the cross-section sample shown here, it is clear that two paint layers have been used – a lighter and a darker red. The top darker red layer is extremely thin.

Colour photograph of a Hawaiian painted cloth from The Hunterian collection, reference GLAHM E.667. (copyright University of Glasgow)
A Hawaiian painted cloth from The Hunterian collection, GLAHM E.667

Colour photograph of a detail of the cloth, GLAHM E.667, with a box showing the red-painted area under investigation (copyright University of Glasgow)
Detail of GLAHM E.667; the box shows the red-painted area under investigation

Colour photograph showing a magnified image of a cross-section through the barkcloth. This shows that two paint layers have been used, a lighter and a darker red. The top darker red layer is extremely thin. (copyright University of Glasgow)
Magnified image of a cross-section through the barkcloth, showing two layers of red paint

Polishing a complete cross-section including the beaten bark poses problems that a cross-section of paint layers alone would not. The beaten bark is easily damaged if polished and so here a layer of resin still remains. The sample shown still has some resin on top of the sample which slightly impairs the quality of the image. Further developmental work is needed to improve the microscopic imagery from painted barkcloth cross-sections. This may include sandwiching them in a softer embedding material that can be cut revealing the layers without the need for polishing which can so easily damage the bark.

With a high quality embedded cross-section, further information can be obtained from the sample to shed light on the materials used. This could be done by using a variety of analytical instruments such as scanning-electron microscopy with energy dispersive X-ray (SEM/EDX) spectroscopy, Fourier transform infrared spectroscopy mapping. These methodologies can help identify the materials used to create the painted barkcloths.

Fourier transform infrared spectroscopy (FTIR) was used to investigate the fibres used to create the barkcloth and to differentiate the plant species.

FTIR is a vibrational spectroscopy technique that provides information about the molecular composition of the material being examined. It measures the vibrational energy levels associated with the chemical bonds in a given material. As the spectrum for each material is unique – like a fingerprint – the technique can be used to identify and characterise materials. It is a non-destructive technique and it is not necessary to remove samples of the material for analysis.

The molecules of organic materials are composed of carbon atoms bonded to hydrogen, nitrogen, oxygen and sulphur atoms. The bonds between carbon and these other atoms vibrates in the presence of infrared radiation. FTIR emits infrared (IR) frequencies from the near-IR region: 14,000–4000cm−1, the mid-IR region 4000–400cm−1 and the far-IR region 400–10cm−1. The absorbed infrared radiation frequencies and associated bond movements are characteristic of the molecular structure. This enables a sample’s molecular structure to be identified or, more commonly for historical materials, for the resulting spectrum to be compared with known reference materials to see if they are similar or different. Success with this technique depends on the skill of the analyst and the use of known standards to make accurate identification.

In attenuated total reflectance mode (ATR-FTIR), an external arm with an ATR crystal (an internal reflection element) is used to view an object as a whole if it is flat enough to fit on the instrument examination area. ATR crystals enable the analysis of samples that are too opaque for transmission and too strongly absorbing for reflectance. Usually pressure of some sort is employed to hold the sample in place against the crystal to ensure good contact.

X-ray fluorescence (XRF) was used to identify non-organic materials, principally pigments, on the barkcloths.

This spectroscopic technique analyses the elemental composition of inorganic materials using X-ray radiation. Electrons in atoms can absorb energy from radiation sources to become what scientists term “excited”. These excited electrons eventually release their energy in a characteristic way which can be detected and analysed.

XRF bombards samples with X-rays, causing electrons to be displaced from their atomic orbital positions. Electrons from the outer shells of the atom drop down to fill the vacant position and to return the atom to a more stable, ground state. When this occurs energy is produced, some of which is in the form of X-rays.

The energy released is characteristic of a specific element including, importantly, metals, and this allows it to be identified. The intensity of the energy is proportional to the relative amount of each element present.

Quite large objects can be analysed with the instrument, and a portable version enables in-situ analysis, as used here.

High-performance liquid chromatography (HPLC) was used to identify the dyes on barkcloths.

HPLC is a chromatographic technique that separates organic mixtures, like dyes and organic paint pigments, so that individual components can be analysed. All forms of chromatography work on the same principle: a mobile phase (a liquid or a gas) flows through a stationary phase (a solid), carrying the components of the mixture with it. Different components travel at different rates and thus separate over time. The time taken for each compound to pass through the column from injection until it is detected is measured. This is called the retention time.

HPLC is used with a variety of detection methods: photodiode array (PDA), ultra-violet (UV), visible (vis), refractive index (RI) and mass spectroscopy (MS). Identification of the molecules and derivatives is based on retention time, molecular weight and spectral matching. Here a UV diode array detector was used to measures the wavelength and intensity of uv-visible light absorbed by each chromatographed compound. These were compared to an in-house spectrum library.

The technique is specialised, in sample preparation, instrument use and interpretation of the results. It is destructive as the sample is dissolved in solvent – therefore it should be done using a sample that has previously been analysed non-destructively, in order to maximise sample information and minimise sample taking.