posted 12-16-2006 08:33 AM            

Non-Destructive Imaging Of Worn-off Hallmarks and Engravings
From Metal Objects of Art Using Scanning Acoustic Microscopy
[ncptt.nps.gov/pdf/2004-06.pdf - link gone from the Internet]

quote:

---

EXECUTIVE SUMMARY

The goal of this project was to determine if worn-off or illegible hallmarks on silver and gold works of art could be imaged using scanning acoustic imaging techniques. The project was quite successful, up to a point, and represents the first time acoustic images have been made of remnant deformation in silver and gold objects of art.

This imaging technique utilizes the residual plastic deformation left in the metal after the hallmarks or engravings have been struck to form a sonogram image representing the differences in the acoustic response of the struck area and the surrounding metal. Theoretically, this should be possible based on the calculated degree of anisotropy and the stress annealing temperature of both silver and gold. In practice, these residual stresses in gold of the historical alloys of 18K-22K (75%-92% gold) could not be imaged while silver, of various experimental alloys, did yield good images.

Numerous combinations of coupling fluids, lens geometries, and sound frequencies were tried in both surface-wave and back-wall imaging modes. In the end, FC-40 (an inert fluorocarbon liquid) used in the surface-wave imaging mode with an input frequency of 20 MHz through an F/1 lens produced the best results. These then are the recommended materials and parameters for imaging worn-off marks on silver and gold when using a scanning acoustic microscope.

Hallmarks and engravings from a total of twenty-six silver, two gold, and one copper alloy (bronze) objects were imaged during the two-year course of the project. Multiple images were made of the marks on these objects so that well over one hundred images were made during the project. The silver objects ranged in age from the 17th to the 20th centuries, the gold objects were modern coupons, and the one bronze object was a coin dating from the 1790's. The silver content of the objects ranged from 80-95% silver while the gold objects were of 18K and 22K. The silver objects examined came from the collections of the Nelson-Atkins Museum of Art, private collections, and from purchases from dealers.

Imaging of worn-off or illegible hallmarks and engravings was successful one hundred percent of the time when residual flow was still present in the metal. Searches of the available literature on hallmarking techniques and consultations with silversmiths and the Superintendent Assayer of the Worshipful Company of Goldsmiths in London revealed that heat was probably used during most of the final cleaning and shaping processes of silver objects after the hallmarks were applied. It now appears that in most cases the heat was sufficient to anneal the metal, thus eliminating the residual plastic flow. In these situations there was nothing to image except metal grain structure. Unfortunately, there are no visual clues on the surface of the metal to indicate whether the requisite plastic flow is still present in the subsurface.

---

.
.
.

quote:

---

CONCLUSIONS

The success rate for acoustic imaging of worn-off hallmarks on the twenty-nine objects in this project has been approximately ten per cent. While this initially appears to be a fairly unsuccessful, the project has succeeded in producing images one hundred per cent of the time where remnant plastic deformation exists. Subsequent imaging processing of the images utilizing a standard software program further clarified about fifty percent of the acquired images. When the deformation no longer exists either through being poorly struck, being annealed out, or completely worn/polished through the zone of deformation, acoustic methods cannot produce an image. Unfortunately, there are no visual clues on the surface of the metal that will permit speculation on the success or failure of the acoustic imaging technique. Each object will have to be imaged individually to determine if there is any residual deformation to be found.

Images of removed or worn-off engravings may be recoverable depending on their method of removal. The inscription would have to have been applied by chasing, i.e. by hammering the lines into the metal as opposed to removal of the metal with a scribe. Then they would have to be "removed" by either filling them with silver solder or by being polished away. If the inscription was "removed" by either hammering or by stoning then the acoustic method cannot be used for their recovery.

Worn hallmarks on objects manufactured from high purity gold cannot be imaged with the acoustic methods as described here. The historical standards for objects made of gold have been 18K or greater. It is suggested that at this purity gold is too malleable to produce plastic flow when struck in the hallmarking process. As the quantity of alloying metal in gold increases (corresponding to a decrease in the purity of the gold) the chances for the acoustic recovery of worn hallmarks and inscriptions should increase but this has not yet been proven experimentally.

Acoustic imaging of archaeological artifacts, e.g. coins, has great potential to be a valuable aid in dating and re-constructing the history of excavated sites. The only caveat being that the objects must have a very smooth surface to permit successful imaging.

---