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The Nathaniel Wallich conservation project

Natasha Trenwith
7 April 2014
Paper conservator, Natasha Trenwith, describes the conservation of Nathaniel Wallich’s botanical illustrations.

The project

In 2011 150 drawings by Victorian botanist Nathaniel Wallich were digitised and published on Kew’s website. At the time it was thought these were the only ones in Kew’s collection. It has subsequently become clear that other Wallich drawings existed but had been wrongly attributed to other collections from the 19th century.


Botanical illustration: Magnolia Insignis Wall
Botanical illustration: Magnolia Insignis Wall

These newly-identified illustrations required conservation to preserve them and to get them ready for digitisation. I became the Wallich project conservator a year ago and the conservation side of the project is now coming to completion. The project consists of 412 botanical illustrations, painted by anonymous Indian artists from Wallich’s time spent in India working on the East India Company’s Herbarium.

Surveying the collection

I began the project by surveying the collection. This gave an idea of the condition the illustrations were in and what treatments and requirements they should need. The majority of the illustrations were adhered to a sugar paper backing with four blobs of animal glue in each corner. The sugar paper had become acidic and the cellulosic sugar had started to ferment, causing a detrimental threat to the artwork and therefore needing to be removed. Other treatments included surface cleaning, tear repairs, lining, pigment consolidation and repair, and bespoke archival quality storage.


An example of sugar paper fermentation
An example of sugar paper fermentation

Lead white treatment

While surveying the collection I noticed a number of the illustrations had a black/silvery tint to several of the highlights and especially the white pigments. This is known as the darkening of lead white and this pigment deterioration usually occurs when lead white reacts with sulphur in the environment to form lead sulphide, a brown, black or silvery-black product. It was a difficult decision whether to carry out a conservation treatment to revert the pigment back to white using a hydrogen peroxide and diethyl ether mix, as the treatment does not change the pigment back to lead white but rather transforms it into a more stable lead sulphate.


Photo of a specimen undergoing lead white treatment
A specimen undergoing lead white treatment

So a decision had to be made whether to proceed with the treatment or to keep the pigment as it is in its natural state. I finally came to the decision that, as the illustrations are still heavily used by the public, researchers and botanists, it was important to ensure the illustrations represented the original colours of the specimen as closely as possible and therefore I treated the darkened lead white.

Before and after photographs showing the treatment of lead white darkening.jpg

Before and after photographs showing the treatment of lead white darkening
Before and after photographs showing the treatment of lead white darkening

Copper green deterioration

While surveying the collection I noticed further pigment deterioration on some of the illustrations containing green highlighting. This can be seen as an unusual rust-coloured strike-through onto the back of the illustration and, interestingly, is only present on wove paper. There is a mixture of paper types in the collection. Through research and pigment identification, this kind of deterioration showed identical signs of damage produced by copper-containing pigments such as Verdigris. Artworks suffering from this kind of damage have been known to become so weak that they can fall out of the page, leaving a hole. The deterioration evident in Wallich’s illustrations is at a lesser extent and only a browning of the paper can be seen. An alkali wash using magnesium bicarbonate can inhibit the deterioration, together with storage with a low relative humidity reading.

Illustration Pterocarpus Indicus An example of copper green deterioration..jpg

Pterocarpus Indicus - an example of copper green deterioration
Pterocarpus Indicus - an example of copper green deterioration

The collection of botanical illustrations is now fully conserved and the pigments are in a much more stable condition. Bespoke housing has been made for the collection that will provide sufficient protection limiting any further damage. The illustrations are now awaiting digitisation so that they can be available on Kew’s database as a resource for further research.

- Natasha -

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