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Archaeological Papers

Odyssey In Depth


Principal Marine Archaeologist Neil Cunningham Dobson

Dobson photo documents the tusk in the ship lab

The tusk being molded

Crew Talk - How I Made a Mold of an Elephant Tusk

As the Principal Marine Archaeologist for Odyssey Marine Exploration, when aboard the Odyssey Explorer my duties include directing archaeological investigations, excavations and recoveries from deep-ocean sites discovered by our expert team of scientists, researchers and technicians. I also document and record artifacts and conduct basic conservation and storage until the artifacts can be brought ashore and transported to a conservation laboratory.

Each day at sea brings new adventures and equally exciting challenges. What I enjoy most of all is having the opportunity to tell the story of the ships we discover and through the study of the artifacts, to learn about the lives of those who worked and traveled aboard the vessel before it foundered at sea. I am very passionate about my archaeology and love “bringing the past alive” by sharing our intriguing adventures with the general public. One very memorable experience I'd like to share with you today…

Since 2005, Odyssey has conducted a program of offshore archaeological surveys in the Western Approaches to southwestern England and in the western English Channel using side-scan sonar, magnetometry and Remotely Operated Vehicle (ROV) reconnaissance. In 2006 we conducted a survey and preliminary excavation of a wreck site (35F) nearly 400 feet deep in the Western Approaches. The site had been profoundly damaged with hull remains, cannon and cargo severely broken up, scattered and destroyed.

A dominant (and most interesting) characteristic of this site's scattered cargo were the nine elephant tusks visible on the surface of the southern half of the site. Importing elephant tusks was a highly profitable commercial pursuit for European traders from the early 16th century onwards. Tusks were an exotic, rare and expensive medium for the artistic and functional manufacture of combs, knife handles, sewing articles, keys for clavichords, syringes and a myriad of other useful and decorative objects. Ivory from African elephants was considered to be of far finer quality than the Asian elephant because of its superior hardness, pale blonde transparency and ability to be more finely polished. African elephant tusks can measure up to 2m (6.5 ft) in length, with diameters of 9-11cm and weigh up to 90kg (24 lbs), while tusks of Asian elephants are smaller and lighter.

Today, the Convention on International Trade in Endangered Species of Wild Fauna and Flora bans international trade in ivory. As such, landing an ivory tusk ashore would not be legal. The challenge for me then was to record a sample tusk on the seabed, recover it to the ship where it would be fully documented, after which it would serve as the model for producing a silicone mold. Once completed the authentic tusk would be returned to the seabed.

Archaeologists have at their disposal a myriad of scientific methods to record, document, sample, and identify an artifact and the material from which it is made. When an artifact may not survive conservation treatments, or as in the case of the ivory tusk, it must be returned to the seabed, then one method is to make a mold of the object from which a plaster cast can then be produced. This way we have an exact replica of the object which can later be used for further study, research and exhibit.

Now, while, I've made plaster molds and dental putty molds of small artifacts, I've never had the opportunity to use silicone rubber, nor on such a large object as an elephant's tusk! Yet, silicone rubber is the most suitable molding material, and was the material of choice for this project. This was indeed going to be a challenge.

A return visit to the wreck site in the summer of 2008 enabled me to select an ivory tusk for recovery. The chosen tusk was at a distance from the other tusks and any ship structure, concretions or artifacts. The tusk selected appeared to be in reasonable condition. It was skillfully recovered using the ROV manipulator arms, yet not without some unexpected drama. During the dive, an approaching storm made surface conditions for the ROV very tricky and created poor visibility. If we had lost power or sustained damage in the mounting seas and swell we could have lost the tusk and perhaps even our mighty ROV ZEUS.

However, we managed to safely recover both the ROV and the tusk onto the deck of the Odyssey Explorer. My first job was to document and record the tusk which was covered with various types of marine growth. There were large areas of pitting on the rind (outer tusk layer) and it was chipped in various places. Two-thirds of the tusk was hollow and there was a long crack visible on the interior. Measurements revealed that the tusk was 145cm (57 in) long; the open end had a diameter of 40cm (16 in). The tusk weighed 24kg (53 lbs). Indeed a large object for my first silicone molding task.

So how do you go about molding a replica of an elephant's tusk? Crucial to this process is the fabrication of a container that will hold the tusk and permit it to be molded in two sections. Being a curved object, my first task was to design a container of the appropriate size and shape. The best person for this job was Gary Peterson, ROV Supervisor and fabricator extraordinaire. Using scrap boards of Formica-covered plywood and some creative saw cuts we were able to build a curved box that would do the job.

Essential to the process was preparing the object to be molded while also ensuring that the container was completely sealed so that no silicone could escape. The tusk was washed in fresh water and any loose surface material removed. The hollow interior was plugged with modeling clay as were all gaps and corners of the inside of the container to prevent any leakage of the silicone.

The mold would be completed in two sections. First, the lower half would be molded in silicone and then allowed to cure. The top would then be molded with a filling hole and vent so that when the two molds were joined together and sealed the plaster could be poured in to make the replica tusk. As this was a large object to mold, metal pins were screwed to the inside of the mold to use as locating pins to ensure that the two halves of the mold would join properly. It worked out that we would need about four to six 5-gallon buckets of silicone. This was calculated approximately by the volume of the silicone rubber material required to fill our mold allowing for about 20% extra to account for any losses through spillage.

Next the tusk was coated with a homemade release created by mixing Vaseline and paint thinner. This would prevent the silicone from sticking permanently to the surface of the tusk. It needed to be thin enough so that it would not fill in any of the surface detail on the tusk. The inside of the mold container was smeared with Vaseline as this would be the outside surface of the mold and would ensure that the cured silicone mold could be easily removed.

The tusk was placed carefully in the curved container sitting on strong supports. Gary and I then carefully and thoroughly mixed the two part silicone compound making sure that no air was introduced into the mix. Minimizing bubbles was important so that there were no flaws on the silicone mold. Any air bubbles on the surface of the mold would be burst using a nail or small screwdriver.

So far, our mixing and pouring went to plan, yet we had to keep in mind that the working life of the silicone rubber material is about 15-20 minutes before it starts to cure. Our options were limited if we lost this batch of silicone as there are no silicone supply shops out at sea. Once filled, the mold was then left to cure for 24 hours. Now came the really cool part. Gary and I carefully removed the tusk and checked the mold. It was fantastic and even better than we had expected. The silicone had precisely replicated the surface of the tusk. The detail was amazing! With this done, it was time to mold the top half of the tusk. Once again the tusk was coated in our release agent and the top surface of the silicone mold smeared with Vaseline so that the two parts could be easily separated. Two small chimneys were made of coin storage pots, their bases cut and placed on the top of the tusk so that we would have a filling vent for pouring the plaster cast.

Using the same methods for the bottom of the mold, we mixed the silicone rubber compound and poured it into the top half, allowing it to cure for 24 hours. Fortunately, we had calculated for just enough silicone. Our completed silicone mold was 60 inches long, 18 inches wide and about 18-20 inches high. The top half had also molded perfectly and separated with no problems. The tusk was removed and the release agent washed off. The tusk was then prepared for placement back onto the seabed.

However our challenge was not yet over. We now had to make a plaster replica tusk. Since this was a large mold, it required two people to carry it, while mixing and pouring the plaster also required extra hands. I enlisted the help of Project Manager Mark Martin and Datalogger Dave Kamm. It was their job to mix the plaster, and this was no ordinary plaster; it was very fine and quick setting. The proportion of water to plaster was crucial as the plaster cured very quickly.

Gary and I then prepared the mold for the plaster casting. Both halves were coated in the release agent and matched up perfectly for a solid join and seal. Weather conditions good, the mold now safely secured was brought out onto the aft deck to free up the ARC Van for the plaster mixing. A funnel was placed in the filling chimney and the buckets of plaster brought out for Gary and I to pour into the mold. Again careful and gentle mixing of the plaster was essential to prevent air bubbles from forming; interestingly, with the engine room below us, I believe the vibration of the ship actually aided in ‘dissolving' all of the air bubbles in the plaster mix.

Once the plaster was carefully poured into the mold, we allowed for a good few hours for the quick-setting plaster to cure and harden, after which the mold was taken back to the ARC Van for removal. The final moment came when Gary and I removed the silicone mold to see if we had a perfect replica of the tusk cast. I was truly excited, and a tad nervous having never made a mold or cast of anything as big or historically significant as an elephant tusk. We knew the mold was good but were not assured the plaster would remain intact when we took it out of the mold. Fortunately, the plaster tusk was flawless in every detail. It was a precise replica of a tusk that could now be copied and sent to various expert and academic institutions and museums for further study. Preserving the silicone mold will also permit additional replica tusks to be cast. This challenge was a great success and highlighted the potential use of casting artifacts that cannot be permanently removed from the ocean floor.

Further study revealed that the tusk was that of an African elephant and dated to the mid-17th century. This site represents the Westernmost cargo ever discovered containing elephant tusks and has yielded amongst the largest examples discovered underwater to date.

This momentous project now successfully accomplished, I'm naturally quite pleased and very happy the tusk was not attached to the entire elephant—as that indeed would have been a challenge to mold!

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