This is heritage science…Darwin’s Daguerreotypes

Throughout British Science Week 2016 we’ll be blogging on the theme ‘this is heritage science’. Our examples come from NHSF’s 20 member organisations and show how science and technology are applied to heritage to improve our understanding and enjoyment of it.

We start with Dr Naomi Luxford of English Heritage and her work with early Daguerreotypes of Charles Darwin. Her investigations seek to determine if the original cover glasses of the Daguerreotypes (which have on them a hand written label of the sitter’s name) can be used in the display of the Darwin Daguerreotypes. It will assess if the corrosion of the glass can be controlled by low relative humidity, or if it poses too great a risk to the image plate. The aim is that visitors to Down House will be able to clearly see the images of Darwin’s children, alongside the original travelling cases and labels of the Daguerreotypes.

Down House, the home of Charles Darwin, displays six Daguerreotypes of the Darwin children, including the only known image of Darwin with another person, his son William. They were taken at Claudet’s studio in 1842 and so are early examples of Daguerreotypes, a type of photography which was invented in 1839 by Daguerre. Deterioration of the original cover glasses had obscured the images leading to conservation treatment. The original cover glasses were analysed, which identified an unstable glass composition. Corrosion of the glass can lead to deterioration of the silver Daguerreotype image plate, as droplets of sodium formate (the glass corrosion product) fall onto the surface and silver tarnish forms around the droplet.

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William Darwin Daguerreotype before conservation treatment. The image is obscured by the droplets formed due to the glass corrosion.

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William Darwin Daguerreotype after conservation treatment with the cover glass replaced by a modern piece of glass

Heritage science is being used to determine the level of moisture in the air (relative humidity, or RH) at which the salt deliquesces from a solid salt crystal to a droplet. The original glass covers have been placed in an environmental chamber at a lower RH (28%), controlled using glycerol. Water has gradually been added to the glycerol to increase the RH levels (currently at 41% RH) and a USB microscope used to capture images of the salt crystals on the glass. As the RH increases these will become droplets again, similar to the room environment at 55% RH. The experiments will help determine the critical RH level to prevent the droplets forming and obscuring the images on display.

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Droplets on the cover glass in room conditions (55% RH)
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Salt on the cover glasses (between 28 and 41% RH so far)

A second set of experiments is testing different methods for sealing the edges of the Daguerreotype packets to prevent pollution ingress. The Daguerreotype packet is formed of the Daguerreotype image plate (a thin layer of silver nanoparticles on a copper sheet), a brass mount and the cover glass. The silver on the image plate can tarnish, affecting the appearance of the image as well as the contrast between different areas of the picture. The Daguerreotype packets are normally held together with paper tape and placed inside the original travelling case. The travelling case is a wooden box, covered in leather and lined with wool velvet. These protect the Daguerreotypes from light, pollution and abrasion, as the silver nanoparticles of the image are easily rubbed away. Different sealing materials are being tested alongside the paper tape to prevent pollutants causing silver tarnish or glass corrosion. The sealing methods being tested are a mix of improved sealing, pollution absorbing materials and sacrificial layers. Inside the test frames area number of filter papers normally used inside diffusion tubes to monitor the levels of the pollutants. Analysis of the filter papers will determine how much pollutant was in the environmental chamber and how much got inside each packet. If successful the alternative sealing methods could be used on the Daguerreotypes to improve protection against pollutants.

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Diffusion tube filter papers inside test frames. The outside edge of the test packets have been covered with different barriers over P90 paper tape. (Top row from left) copper metal foil, corrosion intercept, activated charcoal cloth, (bottom row from left) aluminium tape, P90 paper tape, no sealant on edges.

To find out more contact English Heritage customers@english-heritage.org.uk.

 

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