New system could provide detailed images — even of soft tissue — from a lightweight, portable device.
While one group of MIT scientists is investigating ways to inspect and maintain the USS Constitution in the future, Professor Heather Lechtman is also using advanced technology to look at its past-specifically, to see if any of the ship's copper fasteners could have been made by Paul Revere.
Historians know that Revere (1735-1818), a skilled silversmith and metalworker as well as a revolutionary, made many of the fasteners used during construction of the Constitution in 1797. In fact, after devising a way to roll sheet copper at the age of 65, his copper sheets were used to resheath the bottom of Old Ironsides' hull. However, the ship has undergone many repairs and replacements of parts in the last two centuries, so today's caretakers don't know if any of his original fasteners remain in place. The solid rods, which range from 12 inches to more than two feet long and about three-quarters of an inch to two inches in diameter, are used throughout the ship to hold together planks in the hull and decks.
Since written records don't provide enough information about the fasteners, scientific analysis must be used instead. Naval historians and conservators working on the ship in its current extended drydocking have sent some of the fasteners to Ms. Lechtman, professor of archaeology and ancient technology and director of the Center for Materials Research in Archaeology and Ethnology at MIT. She and Thomas Tartaron, a graduate student in archaeology at Boston University (a member of the CMRAE consortium), are investigating the rods' chemical composition and the method of manufacture to try to pinpoint their age. When the project began in the summer of 1993, Sara Dubowsky, then an incoming freshman, also worked on the project. (She is the daughter of Professor Steven Dubowsky, who is heading a project to develop robotic inspection equipment for the Constitution).
Copper and copper alloys can't be carbon-dated the way steel (an alloy of iron and carbon) can, Professor Lechtman explained. What she hopes to do instead is to analyze fasteners from the Constitution and other historic ships, look for patterns in their chemistries and microstructures, and use these technological characteristics to suggest the chronological order in which they were made. "We might be able to see distinct differences between ones made earlier and later, so we could construct a relative chronology rather than an absolute chronology," she said.
To learn about a fastener's chemical composition, Professor Lechtman drills a bit of metal weighing about 200 milligrams from the side of a rod and subjects the material to two techniques: neutron activation analysis (NAA) and inductively coupled plasma mass spectrometry (ICP-MS), which both determine the minor and trace element components of the copper. NAA does this by measuring the gamma radiation emitted by radioactive isotopes of elements whose nuclei are activated by bombarding thermal neutrons, while ICP-MS measures the stable isotopic masses of the elements when the sample is introduced into a mass spectrometer in the form of a plasma.
Complicating interpretation of these analyses is the fact that the metal in the fasteners may originate from other ships' copper from a variety of sources. In an early example of recycling, the US Navy gathered together copper scrap from shipyards all over the country and melted them down for reuse in the early 1800s.
"We have to be very careful in interpreting patterns, because we don't know where the metal came from," Professor Lechtman said.
She and Mr. Tartaron are also doing metallographic studies of the macrostructrue and microstructure of the rods in an effort to correlate their dates of manufacture with the techniques that were used in fabrication. This research can reveal, for example, whether a fastener was made by casting molten metal in a mold or whether the copper was worked as a solid with hammers and other tools.
The metallographic studies are being carried out on longitudinal sections cut from both the tip and the head of each fastener. Some of those heads are cracked from the stress of being worked and later pounded into the hull or deck. Other rods are thinned and corroded, although that doesn't necessarily mean that they are the oldest; it could mean that they were simply exposed to seawater more often, Professor Lechtman noted.
For comparison, she is examining fasteners from the USS Constellation, launched in 1853 and now docked in Baltimore; HMS Trincomalee, built in India in 1817 and now docked in Cleveland, England; and HMF (His Majesty's Frigate) Unicorn, a ship built in 1824 in Chatham, England that never saw service as a warship (it is now preserved as a historical ship in Dundee, Scotland).
Professor Lechtman's work began in the summer of 1993 and is expected to last until August 1995. It is being funded by the US Navy.
A version of this article appeared in the November 16, 1994 issue of MIT Tech Talk (Volume 39, Number 12).