The giant sequoia: preserving a slice of natural history
How do you conserve an ancient wedge of giant tree? With solvent, buckets of conservation-grade resin and a lot of patience.
Giant sequoias are among the largest organisms in the world. It took 12 weeks for three conservators to clean and conserve this enormous slice.
The specimen has been in the Museum since 1893, after the tree was felled in California.
After more than a century on public display as part of the Museum's botany collections, it needed cleaning and protecting for future generations. Conservator Chelsea McKibbin took on the task.
Preparing the specimen
Chelsea's team looked closely at the sequoia to monitor and document any changes to its condition. Some parts of the tree slice, including the bark, had become fragile and unstable after so many years on display.
This instability meant it was vital that conservators repair and enhance the tree, to preserve it for millions of future visitors.
When Chelsea's team examined the sequoia slice, they found it had gathered a layer of dust an inch thick on its top sections.
Underneath the dust were older layers of varnish and wax from historic restorations. The slice had also been exposed to moisture over the years, and the varnish had become cloudy, which obscured the natural surface of the tree.
A double clean
Chelsea and her team used two types of cleaning method: dry cleaning to remove dust, and solvent cleaning to remove the old, discoloured coating.
Chelsea says, 'The tree was covered in dust, which we cleaned off with low-suction vacuums and soft brushes. While we were doing that, we mapped out all the cracks and damage on its surface so we can monitor its condition.'
The team then applied a special solvent gel to the surface that is on display.
Chelsea explains, 'The solvent reactivated the old varnish, which was then drawn into the gel. The gel works like a sponge, soaking up the discoloured varnish.
'Once saturated, it was removed. The varnish was so thick we had to reapply fresh solvent gel several times.'
A long history revealed
The careful treatment process exposed the natural surface of the wood, enhancing its appearance by revealing the sequoia's growth rings.
Tree rings are created under the bark as the organism grows taller and thicker. Experts can tell how old a tree is by its number of growth rings.
This sequoia has a long history. The tree was 1,300 years old and 101 metres tall when it was felled. Its incredible lifespan is evident in its many rings, which were revealed as conservators stripped away old varnish and returned the tree to its original state.
The next thousand years
Once the sequoia segment was completely clean, new conservation-grade resin was applied for protection. This particular resin is stable and reversible, two important qualities required for treatment materials.
Chelsea says, 'The resin is used in paintings conservation as a varnish because it will remain clear. It won't discolour as it ages or react to environmental changes
'It is also reversible, so we can remove it at a later date if needed, although it is hoped that the tree is now protected for generations to come.'
A thinner solution of the same resin was used on the bark of the segment. The bark was so fragile, conservators found it was regularly losing tiny particles from its surface.
To prevent further loss, the resin was painstakingly applied with a pipette, as Chelsea feared brushes would cause damage.
A new timeline
Once the segment was ready to go back on display, a new timeline showing the age of the tree was created to sit alongside it.
In its 1,300 years the sequoia lived through empires, wars, plagues, scientific discoveries and inventions.
As well as scientific achievements and historic world events, the timeline includes human population figures for each century. These numbers are a reminder that human population growth has had a huge impact on tree population.
When the sequoia was felled in 1893, nearly 70% of the world's land area was covered in trees and forest.
After 100 years, this dropped to below 40%, with an estimated 15 billion trees being cut down every year.
Continuing deforestation may accelerate climate change because forests play an important role in absorbing carbon dioxide from our atmosphere.
From summer 2017, the newly restored sequoia will sit in the eaves of Hintze Hall.
It is part of a redevelopment of the space, which will explore humanity's relationship with the natural world through new specimens and exhibits.