Iziko Slave Lodge
Slavery in South Africa

Conserving the pulley from the Sao Jose


Visible on the sea floor in the area of the Sao Jose wreck the pulley was a priority to excavate and retrieve.  The current and movement of the sediments on the sea bed had scoured the exposed surface of the Pulley flat.

Despite being recognizable as a pulley, it was concreted to a nearby rock.  

Pulley in situ


Once the pulley had been recovered and brought to the laboratory, it was immediately placed in successive baths of fresh water.  Not only was this important to start the desalination of the pulley, but because the pulley is made of wood it was important to keep it wet.

Pulley immediately after recovery

It is important to remember that wood in structure is in essence a bundle of straws that allowed movement of the tree’s sap.  Wood that has been immersed in water (fresh waters / rivers / lake / bog or salt water / estuary / coastal regions / open sea) for a long enough time becomes completely saturated with water.  The longer the wood remains in this environment, the more damage to the internal microstructures of the wood – all those straws. The wood is known as waterlogged wood.

Removal from the sea causes drastic changes in objects, and can start degradation processes.  In the case of wood, as the water evaporates from the wood the atmospheric pressure starts to rupture the internal structures even more – causing the wood to shrink and deform when it air dries.

So it is essential for waterlogged wood to be kept wet when it is removed from a wet environment. 

While the Pulley has been in the sea, the wood and metal have been absorbing salt and chlorides from the water around it. It is important to remove these chlorides, as they promote corrosion of the metal and damage the wood of the pulley.

To prevent the pulley being damaged, it was immediately placed in fresh water when it was recovered.  This actually was very helpful as it started the process of diffusion, a gentle passive way of removing the chlorides and salts from the pulley. The chlorides slowly migrating from the pulley to the surrounding water until the water had the same level of chlorides as the pulley: this process can take several months depending on the size and shape of the object.

Looking at the Pulley when it was immediately removed from the sea, its condition was deemed fragile.  It was completely surrounded by a thick layer of concretion.  This indicated that there was possibly some metal associated with the wooden pulley that was visible.


Pulley underside in water on a bed of bubble wrap

The concretion is the result of metal corrosion.  Metal ions were being released into the environment, and these start to react with the sounding material; sand, shells and other items on the sea floor.  As the metal corrodes / reduces – more and more ions are released into the environment, the concretion builds up over time. The longer the object with the metal is there, the less recognizable it is, until finally the whole object’s shape is blurred by the thick layers of concretion.   So the concretion is a combination of metal and sand as well as any other items, this makes the concretions extremely hard – this is why they are called concretion, and need special care to remove.

So just looking at the pulley as it came from the sea, we can only see the wood, but there must be metal too because of the concretion. As we cannot see inside we must use X-ray photography to document the pulley.  


The pulley was documented not only with photographs and written assessment, but also using X-Rays.  This very powerful technique shows not only the wood, but also the metal remaining. 

The pulley was encased in concretion on the sea floor, that had been formed by the corrosion of iron and other metals associated with the pulley.  To find out how much of the pulley had been preserved and to see the condition of the metal associated with it a series of X-ray photographs were taken using the Lodox X-ray machine that is operated by the Division of Biomedical Engineering of the University of Cape Town.

Lodox X-ray photograph of the pulley


X-rays were taken from three angles; side on, overhead, and at a 45 degree angle.  These three angles gave the viewer a wide variety of information regarding the condition of the wood and metal associated with the pulley.  Specifically how much metal is associated with the pulley and some details of materials trapped inside the concretion.

Looking at the X-rays the extent of the damage to the waterlogged wood could be noted as well as the depletion of the metals associated with the Pulley. 

Once these X-rays had been carefully analyzed the Condition of the Pulley was revised to very fragile. The X-rays showed the construction of the pulley, with the housing, wheel and the metal dowel and strapping. This was very useful information for the archaeologists. 

At this point the wood of the pulley wheel was identified as Lignum Vitea – a very hard wood that had been traditionally used for pulley wheels.

Although it was very exciting to see how the pulley was constructed with both metal and wood, this made it very fragile and complex to conserve. Conservation is meant to stop any further decay of the objects materials:  the wood AND the metal.  Metal is inorganic and Wood is organic, they need two different specialized treatments to conserve them.  This posed difficult challenges that needed to be met so that the pulley could be put on exhibition.


Once the challenges were identified, the treatment was separated into three parts:

Part I: Removal of the concretion

Part II: Electrolysis of the Metal / Treatment of the Waterlogged Wood

Part III: Freeze Drying and Pacification of the Iron

Part I: Removing the Concretion

Using the X-rays as a guide, the concretion was removed slowly with a variety of seemingly harsh instruments: various sized hammers, chisels, dental picks and wooden sticks.  During this process the pulley had to be kept wet.


The pulley during initial stages of concretion removal

The pulley needed to be kept wet because of the waterlogged wood. Waterlogged wood has internal damage to the wood microstructures and allowing it to dry would lead to irreversible shrinkage and deformation.

During the process of removing the concretion, it became apparent that the pulley had possibly been subjected to an oil spill, as the concretion and wood were saturated with a black substance that could have been oil.  This could have been as a result of the Apollo Sea Oil Spill in 1994.  During the process of conserving the pulley the black substance diffused out naturally, so no special treatment was required.

The Pulley showing the extent of the black substance imbedded into the material and further concretion removal

Throughout the process of removing the concretion, rocks and other associated material; the wood was kept wet, and the pulley was documented photographically.

Pulley During concretion removal

Pulley when the majority of the concretion had been removed

Pulley with metal strapping and dowel area exposed

Once the metal strapping and dowel area had been exposed, even more care was required as the metal was soft and damaged. Very little force would result in significant losses.

Following the X-rays, damages in the wood could be seen, but were not apparent to the naked eye. As a result the wood was supported as much as possible to put the minimum stress and strain on it.

Part II

Treating the Waterlogged Wood AND the Metal

Treating the waterlogged wood

Between the internal microstructure damage and the visible cracks and voids shown in the X-ray, it was clear that the wooden housing of the pulley needed to be supported internally. At this point a bulking agent needed to be chosen to assist with this.

Knowing that the pulley had potentially been exposed to an oil spill was important because it was clear there could be a great deal of sulfur in the concretion and waterlogged wood. 

Usually the treatment to conserve the wood would be to replace the water with another material.  From the late 1970’s to the turn of the century, Poly Ethylene Glycol (PEG) wax was used.  This was a very popular treatment.  In the early years of the 21st Century however it was found that elements in the waterlogged wood were reacting with the PEG and sulphuric acid was being produced.  In the case of the Wasa – the Imperial Flagship of the Swedish Crown, that sank on its maiden voyage – after being sprayed with PEG for over a decade, nearly a tonne of sulphuric acid was calculated to be within its PEG treated timbers.  Once this was discovered, a rescue project was started and today research continues to find a replacement for PEG as a bulking agent for water-logged wood.


Another bulking agent used before PEG was sugar.  It had been in use as a wood preservative since the 1920’s.  Research into this method was undertaken in the 1980’s and 1990’s but it fell out of fashion in favour of PEG.

Cellulose, a major component of wood is in fact a wood sugar, very similar in structure to sugar that we use in our daily lives.  Sugar does not dissociate into its component parts when it dissolves, that’s why you will find a sticky residue at the bottom of your cup if you take sugar.   This is an excellent quality that allows internal voids and microstructures to be supported by the sugar that had been dissolved in the bulking solution.

Degree of Degradation

Several tests were done to ascertain just how degraded the wood was. This involved testing the structure of the wood with a pin test carried out at regular intervals across the surface of the wood. The degradation was a factor of how deep the pin went into the surface.  Also the wood is weighted in water and out of water. These two weights are compared to the volume and density of the wood, and that also showed the degree of degradation of the wood. 

Introducing the Bulking Agent

Once the degree of degradation was ascertained, and the bulking agent was decided upon. The following steps are usually taken:  The waterlogged wood is moved from desalination to a low concentration of the bulking agent- 5-10% and left for 2-3 weeks. During this time diffusion is forcing the bulking agent into the waterlogged wood.  The waterlogged wood is transferred to a solution of 20-30% bulking agent and left for several more weeks for the bulking solution to diffuse into the wood.  Depending on the perceived degradation of the wood, the length of time in the solution and the number of steps increasing the concentrations of the bulking agent is varied.

The purpose of these steps is to ensure the bulking agent moves into the wood and saturates it.

Treating the Metal

Metal from a marine environment is highly susceptible to corrosion once it has been recovered. As mentioned previously the chlorides and salts impregnate the iron structure.  Electrolysis is the best way of removing the chlorides from within the iron structure and helping to stabilize the iron matrix.

While the iron is in the sea, electrons flow away from the iron object into the environment carried by an electrolyte, the surrounding sea.  When we recover the iron we want to reverse this process.  This is called electrolytic consolidation.  Three positive events come from using electrolysis to treat marine archaeological iron:  electrons that were lost are moved back into the iron nominally strengthening it.  Chlorides that it absorbed from the sea water are removed and finally the concretion that surrounds the iron object is loosened. 

The Pulley undergoing electrolytic consolidation in sugar solution electrolyte

Electrolysis reverses the movement of electrons away from the iron, by using a power source; the electrons are forced back into the iron.  The power provided by the power source enables the iron to accept the electrons and change from reduced iron to oxidized iron.

This process can’t happen without four elements in place: a power source, an anode (the iron object), a cathode (a mild steel) and a conductive electrolyte. 

When an object is just iron and not a combination of iron and wood, then a sodium hydroxide electrolyte is used. This has high conductivity and a high pH that is not conducive to corrosion.

WOOD and IRON treatment together

As the pulley’s wood and the iron were both fragile, a solution was needed to treat them both. Because of the possible high sulphur content in the wood, sugar was chosen as a bulking agent.  Sugar solutions were tested for conductivity and pH. It was found that table sugar had the best combination of pH and conductivity. So the electrolyte for the electrolysis was a sugar solution instead of a solution of sodium hydroxide.  

So at the same time the electrolysis was being carried out the bulking of the waterlogged wood was happening too. Over the weeks and months the electrolyte solution was increased in concentration of sugar, allowing for the diffusion of sugar into the wood, as well as creating a pathway for the electrons to move back into the iron.

Part III

Freeze drying

By combining the treatments for wood and iron, the treatment of the pulley was speeded up.

Once the electrolysis had been completed the final stages of the wood treatment had to be carried out.  The drying process:  the removal of the water from the pulley.

Pulley ready for freezing

Normally freeze drying is used to do this.  Freeze drying is the gentlest way of removing the water.  Once the pulley had finished the electrolytic treatment, it was frozen.  It was then weighted and placed in the freeze drier. The freeze drier sublimates the frozen water from ice to air.  A vacuum pump pulls the air from the drying chamber, and the super cooled water vapour condenses in the condensing chamber.  In this way the water is removed slowly from the wood, so that just the sugar remains to support the wood.

This process can take several weeks; during the process the wood is weighted.  Once it shows a consistent weight, the treatment is finished.

Finally the iron portions of the pulley that had been stabilized with electrolysis needed to be protected with tannic acid.  A mild solution of tannic acid was brushed onto the surfaces of the metal, protecting it from further corrosion.

Pulley after tannic acid treatment

During treatment and after

During treatment and after

Once the treatment was completed a report was generated showing the changes to the pulley from before and after treatment.  Because the wood was impregnated with sugar and the metal had been electrolytically treated, it is important to keep the pulley in an environment that is controlled and stable between 45-50% relative humidity.  At this level both the wood and the metal are protected from further damage. They need a constant and controlled environment to remain in a stable condition.