Illuminating Objects: Observation, Connoisseurship and a bit of Detective Work

Illuminating Objects intern Natasha Gertler tells us about how her research is going. 

As previously touched upon in my last blog post (and frankly, contrary to my natural instinct to apply analysis to any given problem) identifying decorative stones, such as those in the Courtauld’s ornate frame, is actually more an art than a science.

Whereas geologists can use destructive techniques to analyse and accurately identify rocks and minerals from the field – smashing and slicing them to view under a microscope – this of course is not an option when considering valuable objects from a museum collection. Even most non-invasive analytical techniques are inappropriate as the majority of these aim to establish information about the chemical composition and arrangement of a sample but many materials, both natural and manmade, have essentially the same chemical make-up. For example, both quartz and glass are composed of silica (SiO₂) and so cannot be distinguished by such analysis.

Therefore, identifying decorative stones is normally achieved through a mixture of observation, connoisseurship and comparison to stones of known identity. Awareness of such stones, their appearance, uses and provenance, is hence essential for identification. However, this is rather a niche discipline as it lies somewhere between geology and art history but is never fully taught in either field, but instead gained through experience and exposure. There are only a handful of such experts in the UK.

Dr. Ruth Siddall, Senior Lecturer in Earth Sciences at UCL, along with her student, Nadine Gabriel, kindly came to the Courtauld to lay eyes on the frame and carry out a bit of detective work. Armed with a microscope and a wealth of decorative stone knowledge, Ruth corrected our prior suspicions of the inclusion of man-made imitations in the frame, reassuring us that all the segments of the frame were indeed rocks and minerals sourced from around the 17th century.

Ruth Siddall and Nadine Gabriel observing the frame

Microscopic camera taking images of the frame

Natasha Gertler and Nadine Gabriel looking at the microscopic images of the frame

Ruth’s certainty arose by comparing the stones in the frame to the extensive Corsi collection. Housed in the Oxford University Museum of Natural History, it is composed of 1000 polished stone slabs of uniform shape and size (145 x 73 x 40 mm). By referring to the online catalogue, she identified the presence of various Sicilian jaspers in the frame.

In particular, Ruth identified what we initially thought was an imitation of tiger’s eye to in fact be a distinct variety of Sicilian jasper characterised by its yellow and black banded appearance. Shown below are images of the Sicilian jaspers in the frame (left) alongside the corresponding samples from the Corsi collection (right).

Corsi 764 © Oxford University Museum of Natural History

Corsi 745 © Oxford University Museum of Natural History

Corsi 776 © Oxford University Museum of Natural History

Ruth Siddall using the Corsi collection online catalogue for comparisonWith such an excellent resource on our doorstep, naturally our next step on the quest for identity was to contact Monica Price, Head of Earth Collections at the Oxford University Museum of Natural History and author of The Corsi Collection website.

Read all about my collaboration with Monica Price and trip to the Oxford University Museum of Natural History in my next blog post.

P.s. As the major contributor of information to the London Pavement Geology website, Ruth also pointed out to us the decorative stones used just on the doorstep of the Courtauld. These are Grey Oland Limestone and Red Oland Limestone arranged in a checkered pattern. Admittedly, I have never noticed these details, so next time you come and visit us make sure to spot the ancient fossils under your feet!

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