Some hazards are obvious, like swords and crossbows. Other hazards are less visible, as the hazardous element is incorporated, deliberately or not, into the object. More everyday items like a book or a film cannister may seem innocuous, but they could have more to them than meets the eye. From poisonous pesticides to arsenic in green pigments, to cellulose nitrate film which can spontaneously combust, hidden hazards can be found in any collection, archives included.
In this case, a very dapper doorkeeper’s hat in our collection had warnings of potentially containing mercury written on the box and on the catalogue, written by a predecessor conservator around five years ago. Mercury is a very toxic heavy metal used in the form mercuric nitrate, in the millinery industry from the 17th century– 1940s to stiffen animal fur fibres. It was so common that symptoms from prolonged contact with mercury is even nicknamed ‘Mad Hatter’s Disease.’ While there was no documentation to show that it had been analysed for mercury, knowledge of its use in the millinery industry is likely why a previous conservator erred on the side of caution and put these warnings on the box.
To start our investigation, we looked at the hat itself. Unhelpfully, the hat had no manufacturing labels or other information on the box. The date given on our catalogue was 1977, however this appeared to be the date that the hat and other related items were donated to the Archives, so we could not confirm that was the date the hat was made. So, analysis was the next step. Knowing that the heritage science team at The National Archives are well equipped with analytical equipment, I reached out to Dr Lora Angelova, Head of Conservation Research, to ask if we could make use of their x-ray fluorescence (XRF) analyser. Having only studied the principle of XRF analysis but never used it, I was very keen to gain first-hand understanding of the practical applications of this equipment.
Put simply, XRF is a non-invasive technique which fires an x-ray beam to excite the atoms in the sample material, which once they begin to stabilise again release the excess energy as fluorescence. The amount of energy that is displaced/replaced is different for different elements, so recording these peaks allows us to see an elemental fingerprint of the sample material. So, if this hat did contain mercury, we would see a peak on the software characteristic for Hg. You can see in the image below the graph that the analyser produces with these ‘peaks’ for different elements. The National Archives team were as intrigued as we were, so, Katerina, Natalie and I packed up the hat and went off to Kew!
The machine in the picture below is the XRF analyser. It is held on a stand to be kept steady and is positioned as close to the surface as possible. We took readings from all over the hat to gain a full understanding of what elements the hat contains. The results have since come back, and we are delighted (though also a smidge disappointed?) that the hat is mercury free. One fun finding was the green-and-gold ribbon decorating the hat show high levels of Copper (Cu), Gold, (Au) and Silver (Ag), indicating that it is a metal-wrapped thread. The green colouring is copper corrosion, so the ribbon would have been a pure gold colour when it was made.
A huge positive from this project is that we can remove the warnings from the outside of the box, and we can review the closed record status of the hat with this added information in mind. We are incredibly fortunate to have been given the opportunity to use highly expert analytical equipment to inform our investigation – we thank our colleagues again at the National Archives for their generosity, time, and expertise.
 Hazards are categorised as materials that have the potential to cause injury, illness or even death, if improperly handled, or can cause damage to other collections or equipment.
 It is important to label all known hazards clearly so that public services may properly inform a researcher if the item is requested, and so we can protect ourselves with the necessary PPE and procedures when handling.
 This would have been too late a date for Mercury to have been used, but we could not exclude the possibility the hat was made earlier than the year it was given to the archives, as it looks like it has been worn.