MOOC: Instrumental analysis of cultural heritage objects

7. Multimethod analysis

Cultural heritage materials (paints, varnishes, textiles, dyes, paper components, archaeological materials, etc.) are complex multicomponent mixtures (see section 1.1. Cultural heritage as analysis object for explanations and examples), and for the analysis, different analytical techniques is possible to use.

Golden rule: The best results will be obtained when combining different analytical techniques!

1. Selection of analytical techniques

The selection of analytical techniques depends on the following aspects:

Research questions
Analysed objects and their materials (paintings, polychrome objects, manuscripts, dyed textiles, archaeological finds etc.)
Value and age of the artefacts (e.g. usually, it is not allowed to take sample pieces from very valuable (expensive), famous artists and old objects)
Is only non-invasive analysis acceptable, or is it possible to take a small sample piece from the object
The amount and size of the analysed sample

2. Preliminary analysis of the objects

Before the analysis with complex analytical techniques in the laboratories, a preliminary investigation of the historical background, condition of the objects, and their materials should be made. Based on that, formulation of the research questions and compilation of the research plan for the chemical analysis of the materials can be done.

If possible, at first, in situ non-destructive analysis directly on the object with imaging, portable or movable devices (e.g. pXRF, FT-IR, Raman, USB digital optical microscope, etc.) should be carried out. These give preliminary information about the materials, and these results may be (in some cases) more than enough for conservators .

If portable techniques do not give enough information about the chemical composition of the materials, then analysis with the devices in the laboratories should be continued.

3. Analysis with the analytical techniques in the laboratories

With some analytical techniques (e.g. ATR-FT-IR, Raman), non-destructive analysis can be performed directly on the object without sampling. In most cases, tiny sample pieces are taken from the object and examined in the laboratory with more complex devices. More than one analytical technique is usually reasonable to apply for analysing these samples, and these should be used in the correct order.

Fig. 1 presents the scheme where the analytical techniques are grouped into steps – from non-destructive techniques that require no sample preparation to techniques that require sample preparation and specific data analysis.

This scheme in Fig. 1 and an explanation of the steps in Table 1 will be helpful for the analysis of different cultural heritage materials.

analytical_techniques_steps — Fig. 1. The scheme of the steps of analytical techniques in the laboratory to analyse sample pieces taken from the object.

Table 1. Explanation of the steps (in Fig. 1) of analytical techniques to analyse sample pieces taken from the object.

STEP 1	Involves microscopic examination of the samples that give valuable information about sample colour, size, structure, additional additives on the sample (for example, glue, plant remains, fibres, dust, etc.) and also stratigraphy of the paint sample (for that small sample piece for preparing cross-section block is needed). Optical microscopy is one of the main tools for determining textile fibres.
STEP 2	Involves two vibrational spectroscopy techniques – FT-IR and Raman spectroscopy. FT-IR, especially ATR-FT-IR, and Raman spectroscopy are widely used non-destructive techniques that give information about inorganic, organic, and polymeric materials. ATR-FT-IR and Raman can perform analysis directly on the object or analyse tiny samples. For the analysis, ATR-FT-IR is easier to use than Raman (e.g. problems with fluorescence, need to select suitable laser wavelength, etc.). After the analysis with ATR-FT-IR or Raman, investigated samples can be reused for other analyses (for example, SEM-EDS, py-GC-MS, GC-MS, MALDI-, ESI-, APCI-MS, etc.). This is very beneficial in the case of tiny samples. Often for the FT-IR and Raman analysis, a confirmation analysis is needed, e.g. with SEM-EDS (gives elemental composition of pigments, fillers, etc.) or py-GC-MS (identification of binder, varnish, glue, etc.) is carried out.
STEP 3	Involves elemental and X-ray methods. One of the main tools for the analysis of tiny samples is SEM-EDS. Besides elemental composition, it gives an image of sample topography with very high magnification (max 100 000 x). This is especially valuable for determining the elemental composition of fibres or tiny particles in the paints, archaeological samples, construction materials, etc. If we are interested in determining elements at the trace level, the (LA)-ICP-MS is more suitable for that than SEM-EDS. This technique requires a small sample, but after the analysis, it cannot be reused, compared to SEM-EDS (if the sample piece is placed on the carbon sticker, it can be reused, for example, with ATR-FT-IR microspectrometer). If the available sample amount is larger and quantitative elemental composition is needed, WD-XRF is a suitable technique (SEM-EDS is semiquantitative). XRD (which gives information about crystalline phases in the mixtures) also requires a larger sample amount and is widely used for the quantitative analysis of objects of mineral composition.
STEP 4	Involves chromatographic and mass spectrometric techniques. These are high-level techniques that require skilled operators and require that samples (even if small) are taken. After the analysis, the samples cannot be reused (if the sample is dissolved or pyrolysed, etc.). One widely used method for tiny paint, varnish or polymeric materials is py-GC-MS. This technique does not require complicated sample preparation (it pyrolyses the sample at a high temperature) and gives valuable information about the composition of organic and polymeric materials. The limitation is that this is a very fragmenting method, and often pyrograms of the mixtures are challenging to interpret. Also, GC-MS without pyrolysis is widely used for the analysis of paint binders (oils, waxes, proteins, etc.), varnishes, etc., however that technique requires dissolving the sample and often also derivatisation (so for that, enough sample is needed). For the textile dye analysis, HPLC with different detectors is widely used. If only textile dyes are analysed (for example, it is unnecessary to know the type of textile fibre), then we can skip STEP 1 to STEP 3 and perform HPLC analysis immediately. More and more HRMS techniques with MALDI, ESI, and APCI ion sources are used for the analysis of the composition of paint binders, varnishes, organic pigments, etc. These techniques give much valuable information about organic constituents in the samples (numerous peaks in the mass spectra) and usually require very little sample preparation (in comparison with HPLC and GC analysis). However, the interpretation of these mass spectra is often problematic due to the absence of reference mass spectra of pure components. Nevertheless, these techniques are very capable and versatile, and method development in this area is ongoing.

4. Case studies

In this section, a selection of case studies has been described. These case studies demonstrate well the applicability of different analytical techniques for analysing different cultural heritage objects and how these methods could be combined.

The list of case studies: