Conserved powder flask of thin leaves of wood that would originally been covered with decorative metalwork. The nozzle at the top was cast from a hollow concretion

Conserved powder flask of thin leaves of wood that would originally been covered with decorative metalwork. The nozzle at the top was cast from a hollow concretion

As mentioned in Immediate Actions, waterlogged wood is much more fragile than its weight and appearance suggests. Wood from the sea must first go through the desalination process, and also it has recently been found that it is necessary to remove as much of any iron residue as possible and to assess whether there is a high sulphur content. There is a risk that first came to light with the conserved Swedish warship Wasa that in the presence of iron sulphur can be catalysed to form sulphuric acid that in turn can threaten the integrity of the timber.

Sword original and replica

Sword in concretion and replica

Once the salt has been removed and the iron/sulphur risk assessed and if necessary dealt with, the decision has to be made as to which conservation process will be used. This will depend on the condition of the wood and the size of the timbers. The preferred method is pretreatment with two grades of polyethylene glycol (PEG) wax. followed by freeze drying. The PEG gives some support to the weakened wood structure, but it also protects the wood when the timbers are frozen by absorbing the expansion of the water as it turns to ice. In the freeze dryer the temperature is brought down to minus 30oC and a vacuum is applied. Under these conditions the ice in the wood sublimates (turns directly from solid to vapour) and the water vapour is condensed away from the wood. This avoids the damage that would result from the water evaporating from its liquid state and timbers and wooden objects survive in perfect condition.

For large objects freeze drying is not always an option, and in these circumstances the wood must be stabilised by a much longer process that involves the replacement of much of the water in the wood by PEG.

We have recovered some very complex objects, notably sword handles with their iron tangs intact and bound with copper wire decoration, and a miniature compass in a box made from wood with lead lining and copper fittings – with of course a magnetised iron pointer. These objects could not be treated with PEG and so they were subjected to a novel technique developed at the University of St Andrews called supercritical drying. This process involves first replacing the water in the wood with methanol, and then exposing the wood to supercritical carbon dioxide – that is carbon dioxide at a high pressure that has properties that are neither those of a liquid or a gas. When the CO2 is introduced to the chamber and pressurised to supercritical levels it replaces the methanol in the wood; when all the methanol has been removed the pressure is reduced at which point the supercritical CO2 returns to its natural state and is lost to the wood without any damage to the wood or the associated metals.

Concretions and iron

In some concretions the original object still survives and some concretions may preserve other objects that were close to the major object when the concretion formed. One of the two cannons recovered in 2008 was a treasure trove of objects. The cannon had part of its wooden carriage still attached to it, the tompion that plugged the barrel was still in place and when removed the interior of the barrel was still dry – the gun was loaded with explosive charge and a cannon ball and ready to fire! There was a coil of rope around the cascobel (the knob at the rear end of the barrel) and another around the barrel towards the muzzle end. These were the remains of the ropes that would have held the gun in place while the ship pitched and tossed through the waves. Over the back end of the gun was a lead apron held in place by a light cord, this would have kept the touchhole dry and prevented water getting through to the charge. An arquebus (an early form of musket) was bound up in the concretion as were a powder flask, a shoe, a helmet and a breastplate. Nearly the complete kit of one of the soldiers.

Concretions at The Tower

Cannon in concretions at The Tower of London

Breaking open a concretion looks like a very aggressive procedure. The tools are cold chisels and hammers which in the right hands can progressively fracture the brittle concretion at key points so that it can be removed to expose the objects underneath it. Once the freed objects have been recorded the process of desalination begins. The cannon barrels have the salt removed by electrolysis – almost the reverse of the process that got the salt there in the first place. The cannon is made the cathode in an electric circuit, the anode being stainless steel that is ideally wrapped around (but not touching) the barrel, the whole being contained in a bath of a suitable electrolyte such as sodium hydroxide. A low voltage current is passed through the circuit, causing the chlorides to be released from the iron. It can take many months or even years to release all the chloride, but when the concentration is very low (typically less than 50 parts per million) when the bath solution is tested the barrels can be removed from the bath and dried before having their surface treated with a corrosion inhibitor.

Less substantial objects and those of other materials are treated with more care but the aim remains the same, to remove the saline sea water, where appropriate to remove chlorides that have formed with metals, then to dry and apply protective coatings as appropriate.

The overall objective is to be able to handle the finds and gain what information is possible from them, to record it and to put the collection into safe storage and make it available for scholarly research, exhibition and education.