The chemistry of conservation

Chemistry probably isn't the first thing that comes to mind when you see skeletons at a museum, but an understanding of chemical reactions is essential to the work of the modern museum conservator.

Bethany Palumbo, Conservator for Life Collections in the Oxford University Museum of Natural History, used her chemical expertise to restore centuries-old whale bones for the Museum's recent reopening. The fruits of her team's hard work are now on display for all to see at the Museum, which reopened on 15 February to a staggering 30,000 visitors in the first week alone.

'Chemistry is a key element of conservation,' says Bethany. 'When I began the whale project in mid-2013, there had been no documented preparation of the skeletons for over a century – some of them have been at the Museum since 1860! We had to examine every inch of each whale and research the chemical composition of their bones and the oils they secrete before deciding how to proceed.'

Cleaning and preserving old bones is an intricate, technical task and each treatment must be tailored to the individual bone. Whale bones are especially challenging, as fatty oils slowly seep out over the years.

'When we began the project, there were thick layers of oxidised natural oils on many of the bones,' says Beth. 'This unsightly residue not only attracts dust and makes specimens look dirty, but it is also acidic in nature so can damage the bone. When we tested the oils, they had an acidity of pH4 – about the same as most acid rain. The density of the oil varied across the specimens, and the skulls tended to have more oil than other areas. Whales have a hollow area in front of their skulls filled with oil to focus sonar signals which seeps into the bones where it can remain for centuries after they die. Areas of bone, still saturated with this acidic oil, were in some cases crumbling with a gritty texture similar to wet sand.'

To remove the oily secretions, Bethany and her team brushed solutions of ammonia and purified water onto the bones. Ammonia is an alkaline chemical that works by a process known as saponification that converts fats into soap. Ammonia breaks fat molecules up into their glycerol and fatty acid elements to produce soluble salts and soap scum, which can simply be wiped or vacuumed from the surface. Concentrations of ammonia varied depending on the areas being treated.

'Particularly oily areas, such as the humpback skull needed to be treated with 10% ammonia, whereas we used only 5% for the other specimens,' explains Beth. 'We were careful when the solution came into contact with the cartilage, as this can also disintegrate with the alkaline ammonia solution. There's always a balance to strike with conservation, the treatment method you choose on should never cause more harm than good.'

As well as damaging the bones, the acidic oil also caused verdigris – the green pigment currently coating the Statue of Liberty – to blossom from the copper wires inside the bones used to support the skeletons. Verdigris can be build up over time when copper reacts with oxygen, and is rapidly accelerated by acids.

'The verdigris on the copper wires was exploding from the drilled holes in the bone, causing the wires to weaken and snap when we tried to remove them,' says Beth. 'We ended up using a soldering iron to heat the wire, softening the surrounding cartilage just enough for us to pull the wires out and then vacuum out any residues.

'We have now replaced the wires with stainless steel, which is strong and resistant to the environmental conditions of the museum. We considered alternative methods of putting the skeletons back together, but it made more sense to use the existing holes in the bones to avoid creating further damage to the skeletons.'

Almost paradoxically, the conditions of the Museum environment are actually rather bad for the bones. Ultraviolet sunlight from the glass ceiling destroys collagen in the bones, weakening their structure, and the fluctuating temperatures cause the bones to expand and shrink, weakening joints. Before the roof was repaired, the fluctuating humidity worsened this problem.

'Technically, the best place for these bones would be a cool, dark room,' explains Beth. 'There is often a trade-off between conservation and education, so we have to do the best we can to make sure the skeletons can cope when out in the open for all to see. The adhesives we select, for example, have to be able to withstand high heats and fluctuating temperatures.'

Finally, when reconstructing the skeletons during the restoration process, the team tried to correct the anatomical features of the whales where possible.

'Dried cartilage will shrink over time, pulling the bones into unnatural positions,' says Beth. 'We corrected this by repositioning bones with new wires where possible, but some areas were just too fragile. Also, since we only had six months to complete the project, we simply didn't have the time to correct everything. There are some parts of the fins and ribcages that remain slightly incorrect, which is frustrating, but the skeletons are still far more anatomically correct than when we started.

'After a thorough restoration project, I think our whales will have a few good decades more on display. I'm extremely proud of what my team achieved in a short space of time, and hope that visitors will continue to enjoy seeing the whales in the Museum for years to come.'

The conservation treatment of the whale bones was documented in real time at the Once in a Whale blog, where you can learn more about the whole process.