BEST OF
AEQVITI
Aes Formatum
Aes Rude
The Age of Gallienus
Alexander Tetradrachms
Ancient Coin Collecting 101
Ancient Coin Prices 101
Ancient Coin Dates
Ancient Coin Lesson Plans
Ancient Coins & Modern Fakes
Ancient Counterfeits
Ancient Glass
Ancient Metal Arrowheads
Ancient Oil Lamps
Ancient Pottery
Ancient Weapons
Ancient Wages and Prices
Ancient Weights and Scales
Anonymous Follis
Anonymous Class A Folles
Antioch Officinae
Aphlaston
Armenian Numismatics Page
Augustus - Facing Portrait
Brockage
Bronze Disease
Byzantine
Byzantine Denominations
A Cabinet of Greek Coins
Caesarean and Actian Eras
Campgates of Constantine
Carausius
A Case of Counterfeits
Byzantine Christian Themes
Clashed Dies
Codewords
Coins of Pontius Pilate
Conditions of Manufacture
Corinth Coins and Cults
Countermarked in Late Antiquity
Danubian Celts
Damnatio Coinage
Damnatio Memoriae
Denomination
Denarii of Otho
Diameter 101
Die Alignment 101
Dictionary of Roman Coins
Doug Smith's Ancient Coins
Draco
Edict on Prices
ERIC
ERIC - Rarity Tables
Etruscan Alphabet
The Evolving Ancient Coin Market
EQVITI
Fel Temp Reparatio
Fertility Pregnancy and Childbirth
Fibula
Flavian
Fourree
Friend or Foe
The Gallic Empire
Gallienus Zoo
Greek Alphabet
Greek Coins
Greek Dates
Greek Coin Denominations
Greek Mythology Link
Greek Numismatic Dictionary
Hellenistic Names & their Meanings
Hasmoneans
Hasmonean Dynasty
Helvetica's ID Help Page
The Hexastyle Temple of Caligula
Historia Numorum
Holy Land Antiquities
Horse Harnesses
Illustrated Ancient Coin Glossary
Important Collection Auctions
Islamic Rulers and Dynasties
Julian II: The Beard and the Bull
Julius Caesar - The Funeral Speech
Koson
Kushan Coins
Later Roman Coinage
Latin Plurals
Latin Pronunciation
Legend
Library of Ancient Coinage
Life in Ancient Rome
List of Kings of Judea
Medusa Coins
Maps of the Ancient World
Military Belts
Military Belts
Mint Marks
Monogram
Museum Collections Available Online
Nabataea
Nabataean Alphabet
Nabataean Numerals
The [Not] Cuirassed Elephant
Not in RIC
Numismatic Bulgarian
Numismatic Excellence Award
Numismatic French
Numismatic German
Numismatic Italian
Numismatic Spanish
Parthian Coins
Patina 101
Paleo-Hebrew Alphabet
Paleo-Hebrew Script Styles
People in the Bible Who Issued Coins
Imperial Mints of Philip the Arab
Phoenician Alphabet
Pi-Style Athens Tetradrachms
Pricing and Grading Roman Coins
Reading Judean Coins
Reading Ottoman Coins
Representations of Alexander the Great
Roman Coin Attribution 101
Roman Coin Legends and Inscriptions
Roman Keys
Roman Locks
Roman Militaria
Roman Military Belts
Roman Mints
Roman Names
Roman Padlocks
romancoin.info
Rome and China
Sasanian
Sasanian Dates
Sasanian Mints
Satyrs and Nymphs
Scarabs
Serdi Celts
Serrated
Siglos
The Sign that Changed the World
Silver Content of Parthian Drachms
Star of Bethlehem Coins
Statuary Coins
Sylloge Nummorum Graecorum
Syracusian Folles
Taras Drachms with Owl Left
The Temple Tax
The Temple Tax Hoard
Test Cut
Travels of Paul
Tribute Penny
Tribute Penny Debate Continued (2015)
Tribute Penny Debate Revisited (2006)
Tyrian Shekels
Uncleaned Ancient Coins 101
Vabalathus
Venus Cloacina
What I Like About Ancient Coins
Who was Trajan Decius
Widow's Mite
XXI
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.