Sea salt crystals damage archaeological objects from a shipwreck as they dry out. It’s important to remove the salt – this is called desalination. Wooden objects are desalinated by soaking them in tanks of fresh water.
But do you have a big enough tank? Invincible’s cutwater (found at the front of the ship) is over nine meters long and weighs 5.8 tonnes.
The science bit
Organic materials like wood are desalinated in tap water. As the objects soak, the salt is drawn out through a process called osmosis and the conservator keeps changing the water until no salt is left.
The objects are then soaked in a bath of poly ethylene glycol to preserve them – PEG is a bit like plastic. PEG is drawn into the object, again by osmosis, and the conservator keeps adding PEG to the bath until the objects are fully saturated with it. Once it is dried out the object is ready to be put on display in a museum.
What about metal objects?
Most metal found on Invincible is cast iron, which is hard to desalinate. Metal objects are soaked in tanks of sodium hydroxide and electric currents are run through the liquid – this causes electrolysis and attracts the salts, making a salty scum on the surface. A conservator will regularly scoop the scum out.
It can take years and be very expensive to conserve objects like the iron swivel guns found on board. Archaeologists must always think ‘can we conserve this?’ when they lift objects from the seabed.