Ancient
silver coins are sometimes extremely brittle.
This brittleness is found in coins which are corroded as well
as in coins which show no sign of external corrosion. Unfortunately,
almost any ancient silver coin might suffer from undetectable crystallization. Fortunately, the vast majority of ancient silver coins
are not crystallized or especially fragile. When a crystallized coin is
dropped on a hard surface or handled roughly, it may break. While the
exterior of the coin appears to be normal silver, the interior is white
and does not appear metallic.
Although crystallization is the popular term used to describe this fragile
condition, the term is a misnomer. Granularization or embrittlement are
perhaps better terms (but not customary). Embrittlement of silver
has been studied for a long time. It is the cause of some concern in
museums holding archaeological silver and in archaeology itself.
Embrittlement seems to be linked to inter-crystalline corrosion (see
Ravitch, Lehmann,
Organ, and Werner). Inter-crystalline corrosion can be exacerbated by
the alloying elements present in the silver. Copper and lead are
commonly encountered in brittle silver (Lehmann, Bhowmik, Toda,
Thompson)
but bismuth has also been detected (Rematullah). Discontinuous
preservation of copper at the edges of the silver grains can also lead
to embrittlement. Lead can make silver brittle even without corrosion
(Toda).
References:
Thompson|, F. & A.
Chatterjee. "The age-embrittlement of silver coins" in Studies in
Conservation Vol. 1 No. 3, 1954, pp. 115-126.
Ancient
silver objects are often found to be in extremely brittle condition.
This brittleness can be observed in objects which are corroded as well
as on those which show little or no sign of external corrosion. The
brittleness of apparently uncorroded silver objects represents an
interesting metallographic problem since the silver must have been
ductile at the time the object was manufactured. The embrittlement
implies a drastic change in the metallographic structure. The research
laboratories of the Musie d 'art et d 'histoire in Geneva and the
Metropolitan Museum of Art in New York
City are collaborating on a project to study changes in the
microstructure of silver-rich silver-copper alloys from long exposure
to
ambient temperatures. After preliminary work on a scanning electron
microscope, microhardness tests, and examination of metallurgical cross
sections of silver samples dating from 500 B.C. to A.D. 1000, research
is now centered on copper precipitation from the silver-copper alloy.
The binary-phase diagram for the silver-copper system shows that up to
8% of the copper will remain in solution at the eutectic temperature
but
that the silver can hold only one-tenth percent copper at room
temperature. The precipitation of copper from the super-saturated solid
solution occurs rapidly at temperatures between 150 and 450C, but very
slowly below 100C. C. S. Smith suggested that a small but visible
amount
of copper could precipitate even at room temperature over many
centuries. This type
of precipitation is called "discontinuous" or "cellular." Precipitaion
behavior of modern silver-copper alloys is discussed and compared with
the observed microstructures of ancient silver samples. The
possibilities and limitations of a new method of authentication
by measuring the interlamellar distance between the copper-rich
precipitates is treated.
Kallfass, M., P. Juergen & J. Hermann. "Investigations on the embrittlement of an antique Roman silver bowl" in Prakt. Metallogr (0032-678X) Vol. 22 No. 7, 1985, pp. 317-323.