Your Silver May Tell a Story

By Paul L. Benson and Robert S. Gilmore

The hallmarks on objects made from silver or gold can reveal much about the history of the piece. The name of the master smith, the town where he worked, the date of manufacture, taxes paid on the piece, and the quality of the metal can all be determined by reading the hallmarks.

The history of an object, and to some extent, its value, is intrinsic in the ability to read and decipher the various marks found on them. This information is of great importance to curators, historians, collectors, and even social anthropologists who can trace the movements of individual smiths and sometimes entire families through their hallmarks. Without these marks the objects can be appreciated only like archeological objects taken out of context, i.e., they are still appreciated for their beauty but they have no history.

The use of hallmarks on precious metal objects has a long and sometimes troubled history. The first recognized use of hallmarks was on Byzantine silver objects dating from the sixth AD, although their meaning has not been clearly defined. In 1260, France became the first European country to use hallmarks as an indicator of silver followed by England in 1300. In England, the importance attached to these hallmarks was substantial as seen in the penalties attached to altering or incorporating a master's legitimate hallmark into the work of another smith: heavy fines, imprisonment, loss of body parts, transport, and death.

Silver's susceptibility to tarnishing requires a thin layer of metal must be removed each time it is polished to return the metal to its bright lustrous finish. Through time then, enough metal may be removed that the hallmarks can become illegible or may disappear altogether, resulting in the loss of important historic information.

Fortunately, when the hallmarks are stamped into the metal, the metal deforms. When the surface image is worked away, information may still be saved in the residual deformation of the metal beneath the surface.

Scientists can use sound waves to image residual deformations within a metal. An acoustic beam is focused on the surface to be studied and the reflected sound waves are analyzed. Changes in the amplitude of the sound waves are measured and imaged on a CRT screen. This method is non-contact and no samples are taken from the metal. Therefore, even though the hallmark may be completely polishedoff, it may still be possible to recover an image of it.

Imaging has been accomplished using the scanning acoustic microscope (SAM) at the Schenectady, New York, facilities of General Electric Research and Development. SAM uses a technology first proposed in 1929 but not fully developed for subsurface imaging until the early 1970's.

The imaging process uses short electrical pulses applied to a transducer to create the acoustic beam at ultrasonic frequencies. Then the acoustic beam is focused either on the surface or beneath the surface of the object. An image is acquired by raster scanning the ultrasonic beam across the object and acquiring echo amplitudes along the scan lines. The echo pulse is captured by the transducer and is converted into an electrical pulse with the amplitude of the pulse modulating the brightness on a computer screen to create the image line-by-line.

High frequencies are used to create the images. Because air is too thin to transmit the acoustic beam at these frequencies, a liquid carrier must be used to carry the acoustic beam between the object and the transducer. This means that the silver object must be placed in the liquid carrier. A perfluoro hydrocarbon medium was chosen instead of the usual water medium because of its superior acoustic properties in our application. This liquid is non-corrosive to both silver and gold and is environmentally (ozone) friendly.

Images 1-3 demonstrate the quality of the image recovery possible with this method. A sterling silver coupon was hallmarked by a trained silversmith and then the marks were polished-off just to the point where they were no longer visible.

By processing the surface waves produced by the acoustic pulses on the silver an image was obtained of the invisible (polished-off) hallmarks. The word individual letters in the word "sterling" could not be resolved due to their small size and the overlapping deformation created when the silver was struck.

Experimentation on a small number of silver objects from various time periods has generally produced good results but further tests need to be performed on a larger sampling of objects to determine the limits of the imaging process. Co-author Benson is very appreciative to both the NCPTT and Robert Gilmore of General Electric for their generous funding of this project.