Of exotic chewing gum and curious pigments

A tale of scientific discoveries and team work

Written by Lucia Burgio, Senior Scientist, Victoria and Albert Museum

As part of NHSF’s contribution to British Science Week 2021 we’re sharing examples of heritage science from a range of different organisations. This blog post features work at the V&A, with help from the Natural History Museum and the National Gallery.

What do you do when a rare museum object suddenly springs a surprise on you? Easy: you investigate, and then call in the cavalry.

This is exactly what happened at the Victoria and Albert Museum with a seventeenth-century South American table cabinet, the first object of its kind on display in a UK public collection (Fig 1).

Fig. 1 – The V&A barniz de Pasto table cabinet (W.5-2015). Copyright: The Victoria & Albert Museum

Why was this table cabinet unique? Because it was made using materials and techniques centred around mopa mopa, an indigenous resin from the Andes. The traditional method of preparation of this resin involves chewing it as if it were bubble gum, stretching it and applying it on the surface of objects. The result is a lacquer-like finish, glossy, beautiful and very, very durable.

The scientific investigation at the V&A produced the first bombshell: the white pigment used everywhere on the cabinet was calomel, mercury(I) chloride. In the past calomel was well known as an alleged medicinal remedy for all sorts of illnesses, from syphilis to constipation. But as an art material? Certainly not. What a discovery! We re-christened it ‘mercury white’.

The second bombshell fell when we X-rayed the object: a grim reaper suddenly appeared on the lid (Fig 2) – this must have been painted on the object first, and then covered in the second half of the 20th century with a less frightening decorative scheme.

Fig. 2: The inside of the lid (top) and the corresponding X-radiography image (bottom). Copyright Victoria & Albert Museum

Enter the cavalry: I picked up the phone and called our friends at the Natural History Museum, across the road.

Full disclosure: the largest cultural heritage institutions in the UK have their own dedicated team of scientists, who can rely on many pieces of in-house scientific equipment. But no single institution has every possible type of scientist and kit, so we rely on one another to lend a hand (or type of expertise, or equipment) when the need arises.

And so it was that the mopa-mopa cabinet went for an outing and crossed the road to undergo a micro-CT scan at the NHM. The results were jaw-dropping: the hidden, original scheme was revealed. Our NHM colleagues also verified the crystallinity of the calomel on one of our samples, using their micro-X-ray diffraction equipment.

It was then time to call other colleagues, this time at the National Gallery, and get their help and equipment to map the distribution of mercury white within the hidden scheme. Lo and behold, the grim reaper, with its bits and bobs, had indeed been painted with mercury white too (Fig 3).

Fig. 3: XRF mercury map of the inner surface of the lid. Courtesy of the National Gallery London

Moral of the story: when there is a good relationship between different heritage institution, and capacity can be found to help each other out, the results can be very rewarding. Together we can unlock the secrets of the objects in our collections, understand more about their materiality, history and context, and have the tools to care for them and preserve them for the enjoyment of present and future generations.

Further reading

  1. https://www.vam.ac.uk/articles/box-of-mysteries 
  2. https://www.vam.ac.uk/blog/caring-for-our-collections/hidden-surprises 
  3. https://www.vam.ac.uk/blog/caring-for-our-collections/mercury-white-a-rare-artists-pigment
  4. https://www.vam.ac.uk/blog/caring-for-our-collections/the-discovery-of-mercury-white-on-a-barniz-de-pasto-cabinet
  5. Burgio L., Melchar D., Strekopytov S., Peggie D.A., Melchiorre Di Crescenzo M., Keneghan B., Najorka J., Goral T., Garbout A., Clark B.L.; Identification, characterisation and mapping of calomel as ‘mercury white’ a previously undocumented pigment from South America, and its use on a barniz de Pasto cabinet at the Victoria and Albert Museum, (2018) Microchemical Journal, 143, pp. 220-227. https://doi.org/10.1016/j.microc.2018.08.010

 

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