Interview with Dr Mark Richter, Part Two

Our latest post includes the second part of Rosie Thorp’s interview with Mark Richter on the scientific investigation currently under way on five Spanish portraits in the Stirling Maxwell Collection for comparison with the Lady in a Fur Wrap for our Unwrapping an Icon project. They discuss methods and equipment used for examination. Once again, as in the case of the X-ray process featured last time, the necessity – and mutual benefits – of collaboration are a key feature, as you’ll also see in our video footage here.

Hilary Macartney

Video footage of the scientific examination.

R: Apart from X-rays, what other methods have been used to examine these paintings?

 M: Well, we’ve used a range of other methods to examine the artworks, some of which required the use of samples and cross-sections, as I’ll explain. But others can be used across the surface of the paintings, as they are non-destructive and non-invasive.

X-Ray Fluorescence (XRF) examination has been carried out by Dr Maureen Young and Sarah Hamilton from Historic Environment Scotland (HES). This is a non-destructive method of investigation that provides information on the elements that make up the composition of the artwork. HES have a highly adaptable, portable, handheld XRF machine that’s used to examine all sorts of materials and artefacts, including artworks, as in this case.

Portable XRF equipment ready to examine Alonso Sánchez Coello’s Anne of Austria. © CSG CIC Glasgow Museums Collection

Infrared Reflectography (IRR) is used for the imaging and interpretation of the layered, pictorial stages of paintings. In many cases infrared can ‘see through’ paint layers to the underdrawing – the layout drawing an artist makes before the application of colour – changes in composition and later damages or losses such as filling and retouching. This method involves exposing the paintings to near infrared (IR) radiation. It is one of the most common imaging techniques used in Technical Art History.

R: What’s the typical size of the paint samples you needed to take?

M: The sample size is normally no larger than the head of a pin. These samples cut across the layers of the painting revealing the layer build-up: often the varnish, paint layers and the ground are present.

R: How did you decide which areas to take them from?

 M: Because taking a sample involves permanently removing part of the paint layer, the technique needs to be used sparingly and is usually taken from areas of a painting where minor damage to the paint layers is already present to avoid detection. After carefully studying the paint surface with a stereomicroscope, we are often able to remove tiny paint samples from suitable areas where the original paint layers still seem to be intact. This examination takes time and requires experience. Cross-section samples are extremely useful and can help us to investigate the composition and structure of the paint layers.

R: How are the paint samples prepared for examination?

M: First of all, we’re embedding some of the samples and leaving others loose and unmounted, depending on the type of analysis we’re using.

An embedded sample is one that is ‘embedded’ in a block of resin. To create this, the paint sample is inserted into a small block of synthetic resin. After the resin has hardened, it’s ground down and polished until a high-quality cross-section is prepared with all the paint layers visible. We use an optical microscope to magnify the paint sample 100 to 1,000 times and examine the sample under normal and ultraviolet light. This makes it possible to study not only the composition of the paint layers, but also the individual particles of pigment.

cross section
Example of an embedded paint cross-section under normal (l) and UV illumination (r) at 100x magnification. Taken from the High Altarpiece, Chapel of Maximilian, Cathedral of Freising, c. 1710. Photograph: Mark Richter

An unmounted, loose paint sample is used as it is, with analytical techniques applied directly to it.

unmounted sample
Unmounted paint sample taken from El Greco, Portrait of a Man, c. 1590-1600, Stirling Maxwell Collection, showing flesh painting from his ear at 100x magnification. The varnish layer lies directly over the top flesh paint layer with clearly visible red lake pigment particles. Photograph: Mark Richter

R: So how do you examine the samples and what can they tell us?

M: The embedded samples are being investigated using Fourier Transform Infrared Spectroscopy imaging (FTIR imaging) and fluorescent staining techniques. When we apply these techniques to paint cross-sections, their high lateral resolution enables us to characterise and to locate the various materials in each layer with accuracy. They can help identify different pigment choices as well as several inorganic and organic compounds (pigments such as verdigris or Prussian blue, binding media groups such as proteinaceous binders or drying oils such as nut oil). Also, what we call their ‘micro-destructive process’ is important: this means that the cross-section is still intact after the analysis and therefore available for further investigations.

We’ll use a range of other examination techniques on the loose samples, such as Gas Chromatography Mass Spectrometry (GC-MS). This procedure provides a detailed characterisation of organic materials used: drying oils (e.g. linseed oil), plant resins (e.g. mastic, sandarac, colophony), proteinaceous materials (animal and fish glue, egg), beeswax and plant gums (e.g. gum Arabic or cherry tree gum). These organic binders have the function of binding pigments of all kinds. High Performance Liquid Chromatography (HPLC) is another method we will use to help identify organic lake pigments such as madder and carmine (red lakes make from ground plant roots and crushed cochineal beetles respectively). This technique will be used to analyse a paint sample taken from an area of red glaze on Alonso Sánchez Coello’s Anne of Austria.

R: Do you have a favourite piece of examination equipment?

M: At this moment in time, I’m very excited about the portable SZ61TR Olympus stereomicroscope. It also has a digital camera and advanced image analysis software (Olympus Stream Start 1.8). This has just been purchased by Glasgow University, and has already been indispensable for the detailed surface examination and documentation of the paintings.

mark stereomicroscope
Mark Richer using the stereomicroscope to examine details including craquelure (fine cracks) on Sánchez Coello’s Anne of Austria. © CSG CIC Glasgow Museums Collection

R: Do you have any project highlights so far?

M: I’d say being involved in such a collaborative effort to carry out this scientific examination was a very special experience. I’ve enjoyed the whole process: stereo-microscopic investigations, X-radiography, XRF analysis, IRR and sampling.

It was very exciting to finally have the opportunity to take a much closer and more detailed look at all these works. It always takes a while to absorb and properly comprehend everything you see, though.

Now that the technical examination has been completed, the analysis of the results can begin. We plan to present our findings in early 2019.

For further information on the topics discussed, we recommend the following websites:

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