The question of silver embrittlement is interesting. As Michael McNeil points
out, this is not "crystallization", but the fractured surfaces can look like
dull crystals.
The questions and comments on the arch-metals list made me want to go further.
I looked up "silver AND embrittl*" on BCIN (the Conservation Information
Network) and got 17 references. Three were about silver in photographic film;
"embrittlement" was in the article too. I removed these "false drops" from
the listing below.
I included abstracts when they were in the original records.
Here's a short review:
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).
To sum it up, I quote from Tony Werner: "The complexities involved in treating
brittle silver are illuminated with the aid of metallography, which helps to
reveal intercrystalline corrosion, age or precipitation hardening and large
grain size, any of which may cause embrittlement."
P.S. I haven't seen Kalfass, but I'll go look it up sometime.
Tom Chase, Chase Art Services, Chevy Chase, MD
----------------------------------
BIBLIOGRAPHY FROM BCIN FOLLOWS, most recent one first:
Ravich, Irina G. (Author) Bridgland, Janet (Editor) "Annealing of brittle
archaeological silver: microstructural and technological study" ICOM Committee
for Conservation tenth triennial meeting, Washington, DC, 22-27 August 1993:
preprints Vol. 2 ,1993 pp. 792-795
Paris: International Council of Museums Committee for Conservation
High quality silver artifacts which had belonged to ancient nomads
(Sarmatians)
that became embrittled as a result of burial conditions were studied. The
processes taking place during annealing in the course of conservation of the
artifacts were also studied. Metallography was the main research method of the
study. Intercrystalline corrosion was shown to have resulted in brittleness.
During annealing, partial recovery of ductility occurred due to coagulation of
corrosion products and recrystallization. Annealing conditions were
recommended.
-- AATA
Language Text: English
Location of Document: CAL;ICCROM
AATA Number: 31-718
CAL Accession Number: 28794
ICCROM Shelf: XXI 311/2
BCIN Number: 174543
Lehman, Janusz (Author) Lehmann, Janusz (Editor) "Konserwacja kielicha i
patenty
z xi w., pochodzacych z wykopalisk pod katedra w Poznaniu" (Conservation of an
11th-century silver chalice and paten from excavations under the cathedral at
Poznan) Ochrona obiektow muzealnych: sympozjum konserwatorskie, Warszawa 2-4 X
1984 (Protection of museum's objects: restorer's symposium, Warsaw 2-4 X 1984)
(Biblioteka muzealnictwa i ochrony zabytkow seria b tom lxxx) ,1986 pp.
125-129
Warsaw: Ministerstwo kultury i sztuki
The silver chalice and paten were restored in the 1960s but due to subsequent
deformation required further treatment in 1981. In order to establish the
cause
of the metal's great fragility, spectral and electron microprobe analyses were
carried out as well as metallographic analyses. Both objects had been made
from
silver/copper/lead alloys. In the paten the copper content varied from 15.5%
to
22.3% with lead content as high as 3%. The chalice had been made from an alloy
richer in silver and the copper content did not exceed 10%, the lead not more
than 2%. Both items were considerably corroded. There were a great number of
microcracks, and microcrystalline/ intercrystalline corrosion. Restoration
treatment involved removal of the acrylic resin (not specified) used in the
previous restoration, reassembly of separated fragments, and strengthening of
the chalice and paten structures through use of a polyester resin laminated
with
fine glass fibre fabric.
Language Text: Polish
Location of Document: ICCROM
AATA Number: 28-2689
ICCROM Shelf: Xxi 205
BCIN Number: 139954
Kallfass, Monika; Paul, Juergen; Jehn, Hermann (Author) "Investigations on the
embrittlement of an antique Roman silver bowl" Prakt. Metallogr (0032-678X)
Vol.
22 No. 7 ,1985 pp. 317-323
Language Text: English;German
Location of Document: CAL
AATA Number: 24-1182
CAL Accession Number: 19618
BCIN Number: 115476
Lehmann, Janusz; Nosek, Elzbieta; Mazur, Marek (Author) "Micrometallographical
examination and conservative treatment of early medieval silver chalice and
paten" Icom 7th triennial meeting. Copenhagen, 10-14 September 1984. Preprints
,1984 pp. 84.22.1 1-84.22.13
Paris: Icom
A set of chalice and paten found by archaeological excavations under the
Cathedral at Poznan in 1953, dating to the turn of the 11th and 12th
centuries,
has been investigated metallographically and treated. Investigation included
emission spectrography and metallography, with scanning electron microscopy
and
electron microscopy observations, which revealed many structural and technical
characteristics as well as details concerning mechanisms and effects of
corrosion. Conservation covered dismantling of chalice and paten from warped
supporting structures and reassembly on a new polyester-fiberglass fabric
support.
Language Text: English
Location of Document: ICCROM;CAL
AATA Number: 21-2216
CAL Accession Number: 12502
ICCROM Shelf: Xxi 170/2
BCIN Number: 123276
Craine, Clifford (Author) "The examination and treatment of a 6th Century
silver
openwork lamp from the Byzantine Church of the Holy Sion" Preprints of papers
presented at the eighth annual meeting of the American Institute for
Conservation of Historic and Artistic Works, San Francisco, California, 22-25
May 1980 ,May 1980 pp. 9-19
Washington: American Institute for Conservation
Treatment of a medieval silver object which was crushed and fragmented.
Discusses condition before treatment, method and materials of fabrication,
problems of conservation and the treatment carried out which included
cleaning,
annealing, and assembly. V.G. Comment by Advasio, 17-1206. Binders in pigment
printing, 17-1209. Identification of dyes on old textiles, 17-1294. Present
day
views on the yellowing of the haircoat of furs, 17-1303. Red cochineal dye
(carminic acid); its role in nature, 17-1336. my use of acrylic transfers as
an
alternative collage-making method. AATA 17-1343.
Language Text: English
Location of Document: CAL;ICCROM
AATA Number: 17-1533
CAL Accession Number: 6745
ICCROM Shelf: Xxi 113
BCIN Number: 84509
Schweizer, Francois; Meyers, P. (Author) "A new approach to the authenticity
of
ancient silver objects: the discontinuous precipitation of copper from a
silver-copper alloy" Proceedings of the 18th international symposium on
archaeometry and archaeological prospection, Bonn, 14-17 March 1978;
Archaeo-Physika, Band 10, Rheinisches Landesmuseum Bonn ,1979 pp. 287-298
Koln: Rheinland-Verlag GmbH
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 Musée 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.
Language Text: English
Location of Document: CCI;CAL;ICCROM
AATA Number: 23-2876
CAL Accession Number: CALMSC
CCI Shelf: AATA 23-2876
ICCROM Shelf: Vii c 15
BCIN Number: 111207
Schweizer, Francois; Meyers, Pieter (Author) "Structural changes in ancient
silver alloys: the discontinuous precipitation of copper" ICOM Committee for
Conservation. 5th Triennial Meeting, Zagreb, 1 - 8 Oct. 1978. Preprints ,1978
pp. 16
Paris: International Council of Museums
The age embrittlement of ancient silver could be due to discontinuous
precipitation of copper from the supersaturated silver-rich silver-copper
solid
solution during the long sojourn at ambient temperatures. Microstructures of a
number of ancient silver samples (500 B.C. to 600 A.D.) have been examined
with
the optical microscope and by microhardness measurements. The microstructures
are compared with the precipitation behavior of modern, artificially aged
silver-copper alloys. Based on kinetic data the growth-rate of the
precipitating
cell at 25 C is estimated to approximate 10 m/year. The temperature-dependence
of the interlameller distance of the segregated copper in the precipitated
cell
could be used to distinguish between ancient objects (aged at ambient
temperatures) and artificially aged modern objects (aged at elevated
temperatures).
Language Text: English
Location of Document: ICCROM;CAL
AATA Number: 16-908
CAL Accession Number: 3049
ICCROM Shelf: Icom zagreb 1978/23/5
BCIN Number: 13008
Rehmatullah (Author) "Conservation of metals in central archaeological
laboratory" Pakistan archaeology No. 10-22 ,1974 pp. 303-308
Discusses, metal conservation practices of the central archaeological
laboratory
of the department of archaeology and musuems in Lahore, Pakistan, with special
reference to conservation problems posed by ancient silver in Pakistan (traces
of impurities, copper, bismuth and lead - lead to silver embrittlement) bronze
objects, including a bronze bull in the Taxila Museum (Pakistan) and two
copper
plates excavated from the Mainamati Museum (Bangladesh). -- ICCROM
Language Text: English
Location of Document: ICCROM
ICCROM Shelf: Per. Pakistan 1
BCIN Number: 160867
Bhowhik, S.K. (Author) "The conservation and technique of silver bangles
discovered at Rojdi, India" Studies in conservation Vol. 13 ,1968 pp. 150-5
The treatment of two silver bangles found at Rojdi and dated c. 2000 B.C. is
described. The bangles were separated by removing the hard incrustations with
a
20% sodium hexametaphosphate solution of 0-88 ammonia and a 20% aqueous
solution
of ammonium thiosulphate. Silver sulphide was removed in a 20% formic acid
bath.
Polyvinyl acetate was used for consolidation, and a coating of polymethyl
methacrylate was applied. The silver was found to contain copper and lead. The
causes of embrittlement and the manufacturing technique are discussed. -- AATA
AATA Number: 7-1287
BCIN Number: 99102
Organ, Robert M. (Author) "The reclamation of the wholly-mineralized silver in
the ur lyre" Application of science in examination of works of art.
Proceedings
of the seminar: September 7-16, 1965, conducted by the Research Laboratory,
Museum of Fine Arts, Boston, Massachusetts ,1967 pp. 126-144
Boston: Museum of Fine Arts
Describes the investigations and methods of treatment performed on a lyre
found
during the excavations at Ur in 1927 by C.L. Wooley. The methods of treating
the
ancient silver musical instrument were electrolytic reduction and the
restoration of bending strength to brittle metal.
Language Text: English
Location of Document: ICCROM
ICCROM Shelf: Xxi 11/2;A 6 73
BCIN Number: 24911
Toda, T. (Author) "Embrittlements of copper-3.8% of silver alloys by the lead
content" (Effrittement des alliages de cuivre à 3.8% d'argent dû au plomb
qu'ils
contiennent) Nippon Kinzoku Gakkaishi Vol. 30 No. 2 ,1966 pp. 195-200
The effects of lead on the hot-working properties of the copper3. 8% silver
alloy were investigated by drop-hammer tests, impact tests, detection of the
lead phase by an electron-probe microanalyser, and observations of the
high-temperature alloy structure. Below the melting point of the lead phase,
the
difference of the elastic and plastic properties between the lead phase and
the
matrix caused a stress concentiation at the lead phase under external stress,
and led to embrittlement. At temperature above the melting point of the lead
phase, "burning" occurs, i.e. the liquid lead "wets" the grain boun daries,
thus
causing grain-boundary embrittlement in the alloy. -- AATA
Language Text: Japanese
AATA Number: 7-573
BCIN Number: 100184
Werner, Anthony E.A. (Author) "Two problems in the conservation of
antiquities:
corroded lead and brittle silver" Application of science in examination of
works
of art. Proceedings of the seminar: september 7-16, 1965, conducted by the
Research Laboratory, Museum of Fine Arts, Boston, Massachusetts ,1965 pp.
96-104
Boston: Museum of Fine Arts
The author suggests various treatment processes useful for conservation of
badly
corroded lead objects, elaborating on methods devised by R.M. Organ for the
treatment of lead and silver corrosion. The complexities involved in treating
brittle silver are illuminated with the aid of metallography, which helps to
reveal intercrystalline corrosion, age or precipitation hardening and large
grain size, any of which may cause embrittlement.
Language Text: English
Location of Document: ICCROM;CAL
AATA Number: 15-860
CAL Accession Number: 3615
ICCROM Shelf: Xxi 11/2
BCIN Number: 24908
Organ, R.M. (Author) "The treatment of the St. Ninian's hanging bowl complex"
Studies in conservation Vol. 4 No. 2 ,1959 pp. 41-50
Treatment of the two metal bowls found in the island of St. Ninian, Shetland,
is
described. (1) The hanging bowl was made of silver alloy, partly gilded, and
heavily corroded. Removal of the corrosion products would have made the bowl
too
weak to support itself. For this reason it was decided to line it with a layer
of transparent cold-setting epoxy resin, which would form a permanent support
for the metal. (2) The second bowl, also in silver alloy, was also restored.
Here, chemical cleaning was assisted by ultrasonic vibration.
Language Text: English
Location of Document: ICCROM;CAL
AATA Number: 2-2072
CAL Accession Number: 20668
ICCROM Shelf: Per. iic 1
BCIN Number: 122616
Thompson, F.C.; Chatterjee, A.K. (Author) "The age-embrittlement of silver
coins" Studies in conservation Vol. 1 No. 3 ,1954 pp. 115-126
It has been shown that all the brittle coins contained appreciable quantities
of
lead, due presumably to inefficient cupellation, a process which has been
practiced from early times. Owing to the reduced solid-solubility of lead in
silver with falling temperature, quenched samples of plumbiferous silver
undergo
rapid ageing effects.
Language Text: English
Location of Document: CAL;ICCROM
AATA Number: 2-1840
CAL Accession Number: CALMSC
ICCROM Shelf: Per. iic 1
BCIN Number: 106577
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