Last year conservators cleaned the Craigleith fossil tree which stands in the east garden of the Natural History Museum (London, UK).
This 330 million year old fossil trunk is one of seven excavated from the Craigleith Quarry, near Edinburgh. An even larger example is on display in the Edinburgh Botanical Gardens. The Pitys withamii trunk was originally thought to be a type of conifer but is now known to be from an extinct group of plants known as the seed ferns (pteridosperms). The 12 tonne trunk is only the lower portion of what would have been a much larger tree. The crown would have branched, and it probably bore large fronds. The original wood has been petrified as calcium- and iron-carbonates.
The trunk sections were acquired by the museum in 1873 and have been on display in the gardens of the NHM for 140 years. The tree was knocked over and broken into several pieces by a bomb, dropped during an air raid on the evening of November 7th, 1940. The trunk was moved to its current position in 1975 to make room for the building of the NHM’s palaeontology wing.
Over the decades the tree had become covered in moss, lichen, and algae (and pigeon guano). Plants can cause a lot of damage (biodeterioration) to stone, as their roots grow into the surface and cause flaking and cracks. Plants also retain moisture on the surface, accelerating chemical reactions and also causing physical damage when the water freezes and expands in winter. There is a certain amount of controversy regarding the removal of biological growth from buildings and monuments (Pinna 2014), but after assessment of the fossil tree, the Conservators and Curators made the decision to undertake cleaning- The algae was actively causing delamination when it cyclically dried and contracted. The moss was creating “soil” pockets, both from direct deterioration of the specimen and from windblown sediment which was furthermore trapping moisture and unknown catalysts on the surface. The lichen may not have been causing direct damage to the surface, but it was preventing accurate assessment of the physical stability of the fossil tree and any signs of deterioration. In addition the plant matter was aesthetically unappealing and damaging to the Museum’s reputation for their duty of care of specimens.
Conservators undertook cleaning tests in October 2018 to establish the safest and most effective method of reducing or removing the biological growth present across the surface of the specimen. The most suitable method for removing pigeon guano proved to be swabbing with deionised water. A soft overall brush was necessary to remove delaminating algae, whilst soft plastic and wooden picks were needed for moss and lichen. More stubborn algae required swabbing with ethanol.
In July and August 2019 Conservation Centre staff worked at the top of scaffolding, to carefully remove the vegetation from the entire tree. Paraloid B72 in acetone was used to stabilise some of the historical concrete fills. Conservators also took photographs to assess the current condition of the tree, and allow future deterioration to be monitored.
Pinna, D. 2014. Biofilms and lichens on stone monuments: do they damage or protect? Frontiers in Microbiology 5: 133.
Many thanks to Sanjay Bhudia and James Nicholson for their logistical support during this project, Nicola Harrison for assisting the initial trials, Paul Kenrick for details on the history of the tree, and Cheryl Lynn for undertaking the cleaning.