7.2. Case study 2

Analysis of figurine Gertrud from the Retable of the High Altar of Tallinn’s St. Nicholas’ Church

Text Author: Assoc. Prof. Dr. Signe Vahur (signe.vahur@ut.ee)

1. Analysed object and aim of the research

In this case study, the sculpture of St. Gertrude of Nivelles as analysis object was selected (see Fig. 2), and the interesting results obtained from the analysis of the chemical composition of the blackish ornament on the golden dress are discussed. More information about the investigation of the other painting materials of the sculpture of Gertrude can be found in the book called „Rode Altarpiece in Close-up” ( available also in Moodle) [1].

This find was a surprise because copper is not in the composition of any known black (or dark brown) pigment. However, both green and blue pigments do contain copper. This indicates the possibility that this layer may have been darkened over time or that a green or blue layer was under this layer, and the blackish layer could have been added later.
To get more information, further analysis was needed with more sophisticated analytical instruments at the University of Tartu laboratories. Small sample pieces from the sculpture were taken to continue the investigation (see Fig. 2).
 

2. Used analytical techniques

While the taken sample pieces were tiny, the instrumental techniques were used in specific order to avoid any waste of the sample. Analytical techniques were used in the following order: 

• An optical microscope was used to determine the layered structure (stratigraphy) of the paint sample. For that, a cross-section block of the sample piece was made. For preparing a paint cross-section block, the paint sample piece was embedded entirely into a liquid polymer (Technovit 2000 LC) in the mould, then the mould was placed in the visible blue-light polymerization chamber (Technotray CU), and after about 30 min a hardened cross-section block was obtained. The cross-section block was polished with a polishing device (MetaServ 250/ Vector Power Head) using different abrasive papers until it was as smooth as glass. The paint pieces and cross-section block were observed under the Leica M165 FC stereomicroscope.

• ATR-FT-IR spectrometer was selected because this technique does not require any sample preparation, tiny samples can be analysed in the broad wavenumber range (4000-2225 cm^-1) and determine the chemical composition of all the paint components (pigment, filler, binder, additives). For the analysis, paint layers were separated manually with a sharp scalpel under the optical microscope. Then the investigation was carried out using Thermo Scientific Nicolet 6700 FT-IR spectrometer with Smart Orbit diamond micro-ATR accessory. In this research, we could not use an ATR-FT-IR microspectrometer (or ATR-FT-IR microscope) to analyse paint layers directly on the paint cross-section because there was an interfering polymer on the paint layers which was not possible to remove. If we had more polished the cross-section block, we would have lost a tiny paint sample piece.

• SEM-EDS technique for the elemental analysis was used to confirm the compositions of the pigment and filler in the paint. For the analysis, an uncoated paint sample was placed onto a sticky carbon conductive tab and measurements were conducted using variable pressure Zeiss EVO MA15 scanning electron microscope (SEM) equipped with Oxford X-MAX energy-dispersive detector (EDS). The analysis was carried out at the Department of Geology, University of Tartu.

3. Results

Analysis with an optical microscope

The cross-section block was examined under the Leica stereomicroscope. Fig. 4. shows well that under the blackish layer there is a blue layer. Based on that, we can assume that with ED-XRF detected copper (Cu) could belong to the blue paint.  

 

Analysis with ATR-FT-IR spectrometer

Analysis with SEM-EDS

For confirmation of the compositions of the blackish material and blue paint under that, SEM-EDS analysis was carried out. Fig. 6 presents two SEM-EDS spectra – the first spectrum was measured from the larger area on the sample piece (blue particles were together with an upper dark brownish-blackish material), and the second spectrum was measured directly from the blue particles. Both SEM-EDS spectra contain intensive Cu peaks, which belong to the composition of the blue paint and confirm the presence of azurite (2CuCO₃·Cu(OH)₂) as the blue pigment. Other elements (Fe, Si, Al, Mg) in the first EDS spectrum (see Fig. 6, spectrum A) probably belong to the dark brownish-blackish material and may belong to the composition of brown ochre (Fe2O3+silicates). A more blackish tone may come from carbon (C), however we cannot rule out the possibility that the blackish colour may come from magnetite (Fe₃O₄). Both SEM-EDS spectra also contain Ca, which may belong to chalk (CaCO₃).

4. Conclusions

This investigation shows well how important it is to carry out analysis with multiple methods. Using different analytical techniques, we discovered that originally the ornaments on the golden dress (containing pure gold with traces of silver) of the sculpture of St. Gertrude of Nivelles were made with beautiful azurite blue tempera paint. The results indicate (detection of Fe, Si, Al, Mg with SEM-EDS) that in some earlier times, the ornaments could have been over-painted with dark brownish-blackish paint (which may contain brown ochre with additives). However, we cannot rule out the possibility of darkening the coating layer that was applied to the surface at some period (or darkening of azurite-containing paint layer), and traces of detected elements may come from other layers or impurities. In any case, there is an interesting puzzle that needs further investigation. Additionally, valuable information about conservation materials (PVAc adhesive or its emulsion was determined) was also obtained, which could be applied during conservation works made in the Soviet Union period.