Paul L. Benson and Robert S. Gilmore
Hallmarks on silver objects can reveal much about the history of the piece. The name of the silversmith, the date of manufacture, the quality of the metal alloy, the registration/assay office or location of manufacture, taxes or duties paid as well as other information can be determined from the study of the hallmarks. To maintain the lustrous metallic surface silver has to be polished and over time the hallmarks may be polished away resulting in the loss of important historical information. By utilizing Scanning Acoustic Microscopy an image of these vanished or illegible marks can be recovered, in certain instances. Several imaging modes are presented, time resolved surface wave imaging of the near surface deformation in the silver, direct reflection images of the surface relief, and direct reflection from the silver back surface through the remnant deformed silver. The contrast in these images is caused by variations in the ultrasonic velocities in the silver, density changes, and acoustic attenuation due to the deformed silver microstructure. This technique is non-destructive, non-contact, and does not require a sample to be taken from the object.
Sterling silver coupons were stamped with various hallmarks by trained silversmiths, the marks then completely polished off in a controlled manner, and the coupons insonified with high frequency sound resulting in the recovery of images of the vanished hallmarks. The same experiments were conducted on 18K and 22K gold coupons with unexpected results.