Removal of concretions (1997)

When a vessel sinks the ferrous materials that make up its structure or contents or armaments begin to change chemically. During this process any objects close to these materials are enveloped and bonded together into large masses known as concretions.

From the earliest days of the excavation two large concretions were very evident in the north and south of the site. Both clearly contained many artifacts including ships' rigging, tool handles, muskets and, dominating the northern concretion, two large cauldrons. Late in the summer of 1996, using a tool chest of large hammers, masonry chisels, pry bars and a small, pneumatic hammer several exposed muskets and one cauldron were successfully removed from the concretion in the north.

Removing these particularly interesting artifacts protected them from potential damage during the ongoing excavation and the danger of ice damage posed by the on come of winter. The process of their removal however risked damage to other artifacts hidden in the body of the concretion.

Early in the 1997 excavation two concreted muskets on the west side of the site were freed by less aggressive means. Loose and partly concreted materials surrounding the objects were removed by dredge and judicious use of a small pry bar. This left the muskets and main body of the concretion elevated on three slender pedestals that extended to the sterile, clay layer below. A gentle sideways push sufficed to sever these connections and freed the muskets for lifting. Subsequent examination of the concretion revealed a number of artifacts including a wooden cup or bowl embedded in the matrix, strong encouragement for the recovery of the remaining concretions as a whole or at least in large pieces.

The recognition that these concretions were directly supported by the sterile clay layer or above it on stalactite-like formations was the key to their removal in manageable chunks. As the excavation proceeded up to and under each concretion the pneumatic hammer would be used to score along a line across the top much like a mason scores a brick before cutting it. The location of the scored line would be determined by the location of visible artifacts within the concretion and the size of the resulting piece. The remaining material beneath the concretion would be removed, any supporting structure severed and the concretion would break along the selected line under its own weight. The actual sizes of the individual pieces varied but a rough average dimension would be 1 metre long by 70 cm. wide by 40 cm. thick weighing 75 kilograms.

Once loose the piece would be prepared for lifting. Five centimeter wide nylon straps were slung under and around the concretion and attached to lift bags which, when inflated from a divers air bottle, provided approximately 25 kilograms of buoyant, lifting force each. Most of the pieces required 3 lift bags but were easily maneuvered out of the excavation grid by one diver and brought to rest on the bottom under and beside the dive tender Red Bay. Utilizing the on board lifting boom, block and tackle and the muscles provided by the volunteers and crew, the pieces were slowly raised until clear of the water. Our Zodiac, fitted with a platform constructed from pieces of the ladders used to raise the hull timbers earlier, was then swung underneath and the concretion fragment was lowered onto the platform. After being secured to the platform and draped with sea water soaked towels, the concretions were transported to shore and the basins at the laboratory.

Using this technique the large concretions were freed and raised in ten main pieces. The remaining cauldron was recovered intact and complete with some of the bricks that presumably formed part of the hearth, a piece weighing roughly 125 kilograms.

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