Elements; August 2008; v. 4; no. 4;
p. 227-232; DOI: 10.2113/GSELEMENTS.4.4.227
© 2008 Mineralogical Society of America
The Platinum-Group Elements: "Admirably Adapted" for Science and Industry
James M. Brenan*
* Department of Geology, Earth Science Centre, University of Toronto
22
Russell St, Toronto, Ontario, M5S 3B1, Canada
E-mail:
j.brenan{at}utoronto.ca
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ABSTRACT
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The platinum-group elements (PGE) tend to exist in the metallic state or
bond with sulfur or other Group Va and VIa ligands, and often occur as trace
accessory minerals in rocks. Combined with three isotopic systems that contain
the PGE, these elements afford a unique view of early solar system evolution,
planet formation and differentiation, and biogeochemical cycling. Initial
purification of the PGE was accomplished in the late 1700s, at which time
their unique properties, including high melting point, chemical inertness, and
ability to catalyze chemical reactions, became apparent. This led to enormous
industrial demand, most notably for fuel production and engine emission
control, which combined with scarcity in crustal rocks, has made the PGE a
highly valued commodity.
KEYWORDS: platinum-group elements, siderophile, chalcophile, alloy, sulfide, catalysis
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INTRODUCTION
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"Platinum is a most valuable metal; as it is not oxidisable, nor
fusible under common circumstances, and only with difficulty combinable with
sulphur, and not acted upon by common acids, it is admirably adapted for the
uses of the philosophical chemist." Elements of Chemical
Philosophy, BY HUMPHRY DAVY
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INTRODUCTION
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On 23 January 1817, Sir Humphry Davy delivered a lecture to the Royal
Society of London describing the results of his recent experiments on the
combustion of gas mixtures. Davy reported "the discovery of a new and
curious series of phenomena," in which mixtures of highly explosive coal
gas and air were found to cause a heated wire of platinum to "become
ignited nearly to whiteness." Once the glowing of the wire was
extinguished, it was found that the gas mixture was no longer inflammable.
Davy reported that "I tried to produce these phenomena with various
metals; but I have succeeded only with platinum and palladium; with copper,
silver, iron, gold and zinc, the effect is not produced." What Davy had
discovered was heterogeneous catalytic oxidation. Davy immediately recognized
the practical application of his discovery and modified the safe-lamp he had
designed for coal miners to include a Pt wire around the lamp flame
(FIG. 1), thus
limiting the amount of combustible gas available to the flame and reducing the
chance of explosion.
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FIGURE 1 Detail of the Davy Safe-Lamp (A) employed in British coal mines,
with the Pt wire modification (J and H) used to catalyze methane
oxidation (after Davy 1817).
REPRINTED WITH PERMISSION OF THE ROYAL SOCIETY
OF LONDON
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Synthetic crystals of platinum-rhodium alloy grown by high temperature
vapor deposition (center crystal is  1 mm in diameter)
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Fast forward to the year 2007, and the impact of Davy's original discovery
becomes apparent: 9 million ounces of Pt and Pd (worth US$7 billion)
were produced for the heterogeneous catalytic oxidation of automobile exhaust
gas. This is in addition to the amounts produced for jewelry and other
industrial processes (mostly catalysis related) involving one or more of the
platinum-group metals (PGE: Os, Ir, Ru, Rh, Pt, Pd), totaling some 7 million
ounces, worth US$10
billion1. How does an
issue of Elements devoted to the PGE fit into this picture? Most
significantly, many of the chemical properties of the PGE that are of
industrial interest also result in distinct geochemical behavior, affording a
unique view of early solar system evolution and planet formation and
differentiation. The PGE, along with rhenium and gold, are grouped together as
the highly siderophile elements, which are defined by their extreme
partitioning into the metallic, relative to the oxide, phase. The PGE are
highly refractory, as gauged by their high melting and condensation
temperatures (FIG.
2), and were therefore relatively concentrated in the feedstock
for the terrestrial planets (Palme
2008 this issue). However, because of an aversion to bonding with
oxygen, the PGE are now scarce in the crusts of differentiated planets and,
instead, are concentrated in the core-forming Fe-Ni alloy. As discussed in the
articles by Palme (2008) and
Lorand et al. (2008 this
issue), this tendency to exist in the metallic state (termed siderophile) or
bond with sulfur and other Group Va and VIa ligands (termed chalcophile) has
thus provided geochemists with a sensitive tracer of differentiation processes
that involved metal or sulfide phases. As an invaluable bonus, three
radioisotope decay systems include the PGE as parent
(107Pd-107Ag), daughter
(187Re-187Os), or both
(190Pt-186Os). These systems have not only provided
chronological information, but differences in parent-daughter fractionation
have produced distinct isotopic tracers of a variety of geochemical processes
(Carlson et al. 2008 this
issue).
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FIGURE 2 A portion of the periodic table containing the highly siderophile elements,
which includes the platinum-group elements, rhenium, and gold. Elements are
depicted with their atomic number, crystal structure, and melting point.
COURTESY OF H. PALME
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Owing to their scarcity, but enormous industrial demand, there is
significant interest in understanding the rare occurrence of mineable PGE
concentrations in the highly depleted crust. In this context, Mungall and
Naldrett (2008 this issue)
emphasize the chalcophile behavior of the PGE and the massive extraction
capacity of magmatic sulfide liquids.
As a resident of London in the 1800s, Humphry Davy would have most
certainly been subject to the poor air quality that typified this time. Today,
it's not the burning of coal, but automobile exhaust that degrades the air
quality of many urban centers. Autocatalysts have significantly reduced this
problem (see below), but the PGE they contain (and those from other industrial
sources) are released to the environment, and some of these PGE are
bioavailable. As Rauch and Morrison
(2008 this issue) describe,
this is a matter of some concern, as the risks of PGE emissions to human
health and the environment have not been accurately assessed. There is a clear
call for more intense research in this area.
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DISCOVERY AND PURIFICATION OF THE PGE
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In 1741, the metallurgist Charles Wood returned to England with samples of
platinum acquired in Jamaica but which likely came from the placer deposits of
the Choco district of
Colombia2. This
material was given to Dr. William Brownrigg, whose description was read to the
Royal Society of London by William Watson
(Watson and Brownrigg 1750).
In this description, the samples are referred to as "platina,"
which is derived from the Spanish for "little silver" and alludes
to their appearance (FIG.
3). William Lewis performed an exhaustive battery of tests on this
newly found material and concluded that "it follows, that platina is
not, as some believe, gold naturally debased by the admixture of some other
metallic body, but a metal of the peculiar kind, essentially different from
all others" (Lewis
1757).
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FIGURE 3 Samples of "platina" recovered from the placer deposits in the
Choco district of Colombia. Similar material was described by William Lewis to
the Royal Society of London in 1754 and served as the feedstock for the first
purification of Pt, Pd, Ir, and Os by Smithson Tennant and William Wollaston.
The field of view is 5 cm wide (each nugget is about 3-5 mm in length). Sample
is from the mineralogy collection of the Museum National d'Histoire Naturelle
(#4.283; Picture D. Brabant, MNHN). PHOTO COURTESY OF
JEAN-PIERRE LORAND
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One important discovery made by Lewis is that platina is soluble in aqua
regia, "excepting a little blackish matter"
(Lewis 1754), which suggested
the presence of impurities in the raw sample. Subsequent work on this residue
led to the discoveries of iridium and osmium by Smithson Tennant, reported to
the Royal Society of London on 21 June 1804. In the case of iridium, Tennant
writes, "I should incline to call this metal Iridium [from
iris, Greek for rainbow], from the striking variety of colours which
it gives, while dissolving in marine acid [HCl]"
(Tennant 1804). Tennant notes
that during one of the osmium purification steps, "a pungent and
peculiar smell is immediately perceived." He discovered that this smell
"arises from the extrication of a very volatile metallic oxide,"
providing the basis for the name Osmium, which is derived from
osme, Greek for smell. That smell, of course, originates from the
highly toxic osmium tetroxide.
Three days later, Tennant's colleague William Wollaston reported the
discovery of rhodium (Rh), which was purified from the solution remaining
after precipitating platinum by treatment with sal ammoniac
(NH4Cl). He indicated that this new element should be
"distinguished by the name Rhodium [from rhodon, Greek
for rose], from the rose-colour of a dilute solution of the salts containing
it" (Wollaston 1804).
The following year, Wollaston
(1805) reported the discovery
of palladium, which was purified from the aqua regia solution used to dissolve
raw platina. In his report to the Royal Society of London, Wollaston proposed
the name palladium, "from the planet that had been discovered nearly at
the same time by Dr. Olbers," referring to the discovery of the Pallas
asteroid (originally considered to be a planet) by the German astronomer
Heinrich Olbers on March 28, 1802. Records from his monetary accounts
(Chaldecott 1979) reveal that
Wollaston garnered significant income from a method he developed for platinum
purification from raw platina, which he only made public in the Bakerian
Lecture to the Royal Society (Wollaston
1829) shortly before his death in 1828. This income helped to
furnish the endowment for the Wollaston Medal awarded by the Geological
Society of London (FIG.
4). The medal was originally cast in gold, but from 1846 to 1860
it was cast in palladium extracted from Brazilian gold donated by noted
assayer and metallurgist P. N. Johnson.
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FIGURE 4 Front and back faces of the Wollaston Medal presented to Sir William Logan
(first director of the Geological Survey of Canada) in 1856. The front of the
medal depicts William Wollaston, the benefactor of the award. The medal is
cast in palladium, which Wollaston
discovered in 1805. The Wollaston Medal is the highest award
bestowed by the Geological Society of London. NATURAL
RESOURCES CANADA IMAGE NRCAN-4374, USED WITH
PERMISSION OF NATURAL RESOURCES CANADA.
HER MAJESTY THE QUEEN IN RIGHT OF
CANADA
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Ruthenium (Ru) was the last of the PGE to be discovered, and was isolated
from the insoluble residue after aqua regia digestion of platinum ore from the
Urals. The initial claim of discovery was by the German chemist Gottfried
Osann in 1827, although the definitive purification is attributed to the
Russian chemist Karl Klaus, reported in 1844. Ruthenium is derived from
ruthenia, Latin for Russia, named for Klaus's homeland, and in honor
of Osann's original work (Griffith
1967).
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INDUSTRIAL DEMAND FOR THE PGE
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The most significant industrial demand for the PGE (see
Figure 1 of
Rauch and Morrison 2008)
arises from their important role in the operation of modern automobiles, which
is to catalyze key reactions involved in fuel production and engine emission
control. The special role of the PGE in this context arises from two key
properties. First, as a consequence of the very high oxygen potential defined
by their metal-oxidation reactions, the platinum-group metals are stable in
air even at relatively high temperature (see
Figure 1 of
Palme 2008). Second, the PGE
have the remarkable ability to chemisorb simple gaseous molecules, like
O2 and CO, so as to result in significant coverage of the metal
surface, but adherence is not so strong that surface reaction rates are
retarded (Bond 1991).
Consequently, the rates of certain chemical reactions are vastly accelerated
by the presence of the PGE metal surface, although the metal itself is not a
reactant or product, and reactions can be allowed to take place at higher
temperatures without degradation of the metal surface. Optimal engine
performance requires fuel that can be highly compressed before controlled
ignition by the spark plug, and the measure of fuel performance in this regard
is the octane rating. At petroleum refineries, the octane rating of fuel is
improved by using Pt-Ir or Pt-Re catalysts in the reforming process to convert
aliphatic hydrocarbons to aromatic hydrocarbons, such as in the
dehydrogenation of n-heptane (octane rating = 0) to toluene (octane rating =
114). Octane ratings can also be enhanced using platinum catalysts for the
isomerisation of C4-C6 alkanes
(Parkins 1991). Combustion of
fuel in car engines produces CO2 and H2O, as well as
lesser amounts of CO, volatile organic compounds, and nitrogen oxides (NO and
NO2, together known as NOx). The latter three are considered
harmful and can be reduced at the tailpipe using two important
emission-control devices that incorporate the PGE as essential components.
Just downstream from the fuel combustion chamber is an oxygen sensor, which
consists of a ceramic solid electrolyte (usually impurity-doped
ZrO2) plated with platinum electrodes. This device monitors the
oxygen content of the combustion gas and is part of a feedback loop to adjust
the fuel/air ratio in the combustion chamber for peak performance and to
optimize the conversion of unburnt fuel, CO, and NOx in the catalytic
converter. The catalytic converter is located downstream from the oxygen
sensor and consists of a honeycomb-structured ceramic core, coated with a
porous "washcoat" of alumina, then the catalyst, which consists of
some combination of Pt, Pd, and Rh (Bond
1991). The converter catalyzes three reactions that result in the
production of harmless products:
- 2NOx =
N2 + xO2 (NOx reduction)
- 2CO + O2 = 2CO2 (CO Oxidation)
- 2CxHy + (2x+y/2)O2
2xCO2
+ yH2O (oxidation of unburned fuel)
A combination of physical and chemical degradation of the catalyst is
thought to be responsible for the release of PGE aerosols to the roadside
environment and beyond (see Figure
2 of Rauch and Morrison
2008).
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HEALTH AND WELL-BEING FROM THE PGE
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From improving our smiles to curing life-threatening diseases, PGE are used
in a number of therapies to enhance our health and well-being. For example,
prosthetic teeth (bridges and crowns) are typically made of gold alloyed with
Pt and/or Pd, which are added to improve strength and stiffness so as to
endure the rigors of biting and chewing. Owing to their biocompatibility, high
electrical conductance, and resistance to corrosion, Pt and Pt-Os alloy are
essential components of implant devices used to treat irregular heartbeat and
in the replacement of defective heart valves. Surgeons also make use of Pt as
X-ray-opaque guidewires when performing angioplasty. Although osmium tetroxide
is highly toxic to living tissue, biomedical researchers have long employed
solutions of this compound to fix and stain fatty tissue for electron
microscopy. Perhaps one of the most important PGE-based therapeutic materials
is the anticancer drug cis-diamminedichoroplatinum(II) or
cis-[Pt(NH3)2Cl2], also known as cisplatin.
Although the cisplatin molecule had been known for some time, the discovery of
its cancer-fighting properties was by chance. Barnett Rosenberg, a
biophysicist at Michigan State University, had set out to examine the effects
of electric fields on bacterial growth. Cultures of E. coli were
immersed in a buffer solution containing ammonium chloride, and Pt was used as
the electrode due to its perceived chemical inertness. The specific choice of
AC electric field frequency and current were to eliminate electrolysis
effects. As described in their original letter to the journal Nature
(Rosenberg et al. 1965), it
was subsequently realized that "both are mistaken ideas which led, via
serendipity, to the effects described in this communication." Rosenberg
and coworkers observed that the bacteria in cultures subject to the electric
field exhibit filamentous growth
(FIG.
5A), in contrast to their typical "sausage"
shape (FIG.
5B). The effect was not due to growth, but rather to
inhibition of cell division, and after further investigation, it was
determined that the causative agent was not the electric field, but hydrolysis
products of the platinum electrode. Subsequent work showed that several Pt(II)
and Pt(IV) compounds exhibit antitumor activity, effectively inhibiting a type
of sarcoma and leukemia in mice (Rosenberg
et al. 1969). In 1971, cisplatin underwent clinical trials and was
determined to be effective against tumors in human subjects, and in 1978, it
was approved for treatment of ovarian and testicular cancers. A number of
cisplatin analogs have since been developed that show less toxicity and equal
or greater antitumor activity.
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FIGURE 5 Phase contrast photomicrographs of E. coli cultured in growth
medium containing (A) 8 ppm of platinum as the neutral species
[PtCl4-(NH3)2]0 and (B)
less than 6 ppm of platinum as the doubly negative species
[PtCl6]2-. Magnified 600x. The image in (A) shows
the development of filamentous forms, in contrast to the normal sausage shape
shown in (B), indicating the inhibiting effect of
[PtCl4-(NH3)2]0 on cell division,
but not on growth. From Rosenberg et al.
(1967). COPYRIGHT
1967 CBY THE AMERICAN SOCIETY FOR
BIOCHEMISTRY AND MOLECULAR BIOLOGY. USED
WITH PERMISSION OF AMERICAN SOCIETY FOR
BIOCHEMISTRY AND MOLECULAR BIOLOGY IN THE
FORMAT MAGAZINE VIA COPYRIGHT CLEARANCE
CENTER
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FIGURE 6 Single crystal of the mineral sperrylite (PtAs2) from Noril'sk
(Russia). The sample is from the mineralogy collection of the Museum National
d'Histoire Naturelle (#193.27; picture D. Brabant; MNHN). PHOTO COURTESY
OF JEAN-PIERRE LORAND
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THE IMPORTANCE OF PGE ACCESSORY MINERALS
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One of the most fundamental, but perhaps least understood, aspects of PGE
geochemistry is their distribution in rocks. Knowledge of this has an
important bearing on the behavior of these elements during melting and
solidification, in turn affecting PGE isotopic systems, as well as mineral
beneficiation and ore recovery. Both experiments and studies of natural
samples suggest that although trace quantities of Ir, Ru, and Rh can reside in
the structures of silicate and oxide minerals (i.e. olivine and chromite), by
far the dominant hosts for all the PGE are base-metal sulfides (pyrrhotite,
chalcopyrite, and pentlandite) and PGE-bearing "accessory"
minerals (PGM; Lorand et al.
2008). Although present in very low modal abundances, the PGM can
dominate the whole-rock budget of the PGE they concentrate, especially in the
case of refractory mantle samples. This is directly analogous to the role of
zircon, apatite, monazite, etc., in controlling the behavior of trace
lithophile elements in felsic igneous rocks. There are more than 100 distinct
PGM species: some are comprised of PGE only, some occur in combination with
Fe, Cu, or Ni, and some are bonded to Group Va and VIa ligands, such as S, As,
Sb, and Te (Cabri 2002).
Knowledge of intensive variables during PGM formation can be obtained with the
appropriate phase-equilibrium and partitioning information, for which there is
a growing literature (Makovicky
2002; Andrews and Brenan
2002; Bockrath et al.
2004). In sulfide-saturated igneous rocks, the PGM are likely to
form either by exsolution from primary base-metal sulfides as a consequence of
cooling, or by enrichment of PGM-forming components during the final stages of
differentiation of magmatic sulfide liquid (e.g.
Barnes et al. 2006). Although
rare, spectacular, relatively large (centimeter-sized), euhedral crystals can
be found in some differentiated massive-sulfide bodies
(FIG.6).
A common but somewhat enigmatic petrographic association for the PGM in
sulfur-poor ultramafic igneous rocks is their occurrence as inclusions within
chromite. Although a wide range of compositions have been documented for
chromite-hosted inclusions (Legendre and
Augé 1986), typical PGM are isoferroplatinum
(Pt3Fe), osmiridium (hcp Os-Ir alloy), and
laurite-erlichmanite (RuS2-OsS2 solid solution;
FIG. 7). Inclusions
are generally submillimeter in size but may also occur at the nanometer scale,
as inferred using in situ analysis by laser ablation ICP-MS (e.g.
Ballhaus and Sylvester 2000).
Although the process of PGM formation is not well understood, the presence of
primary magmatic minerals included with some PGM suggests both phases formed
at high temperature and were entrapped at the time of chromite
crystallization. Alternatively, some chromite-hosted PGM could be the product
of deposition from a PGE-laden hydromagmatic fluid or the result of
desulfidation of pre-existing base-metal sulfides.
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FIGURE 7 Backscattered-electron image depicting an aggregate of platinum-group
minerals included in a chromite grain from the Mayarí-Baracoa ophiolite
belt of eastern Cuba. The inclusion consists of distinct grains of irarsite
(IrAsS) and laurite (RuS2). The laurite contains a core of Ru-rich
erlichmanite (OsS2) surrounded by a layer of Os-poor laurite
grading to Os-rich laurite at the rim. USED WITH PERMISSION FROM
SPRINGER SCIENCE+BUSINESS MEDIA:
GERVILLA ET AL.
(2005;
FIG.
4G)
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FIGURE 8 The quaternary system Ru-Os-Ir-Pt depicting the composition of
platinum-group element alloys from world-wide placer deposits (after
Cabri et al. 1996). The
minerals in this system are either hexagonal (osmium, ruthenium,
rutheniridosmine) or cubic (platinum, iridium). The majority of platinum-group
minerals found in placer deposits are Ru-Ir-Os-Pt and Pt-Fe alloys. Rhodium
and palladium are typically minor constituents of these phases. The yellow
shaded region corresponds to the miscibility gap in the quaternary system, as
defined empirically by the alloy compositions, and is consistent with
experimental data in the Ru-Ir, Os-Ir and Os-Pt binary systems. Figure from
Exploration and Mining Geology, volume 5, no. 2; REPRODUCED WITH
PERMISSION OF THE CANADIAN INSTITUTE OF
MINING, METALLURGY AND PETROLEUM
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Weathering and erosion of PGM-bearing rocks produces PGM placers, which are
dominated by alloys. Placer alloys are typically either Pt-Fe or Ru-Ir-Os-Pt;
the latter exhibit a compositional gap consistent with the limits of
miscibility known from phase-equilibrium experiments
(FIG. 8;
Cabri et al. 1996). Placer PGM
have compositions similar to those found as inclusions in chromite, consistent
with studies that have located placer sources in sulfur-poor ultramafic rocks
(Hattori and Cabri 1992;
Cabri et al. 1996). Of
historical significance is that samples of platinum alloy from which the other
PGE were originally purified were from placer deposits located in the Chocos
district of Colombia and the Ural Mountains of Russia.
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FUTURE PROSPECTS
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Improved chemical separation and analytical techniques have provided the
means to measure the elemental and isotopic composition of the PGE in
laboratory and natural samples with unprecedented precision. The wealth of
data acquired by these methods has expanded our knowledge of these elements
and posed new questions regarding their behavior. As noted by Lorand et al.
(2008), estimates of the
primitive-mantle concentrations of the PGE are not in complete accord with
levels in primitive meteorites, and our understanding of this discrepancy will
require fuller knowledge of PGE fractionation processes during mantle
differentiation. The distribution and stability of scarce platinum-group
minerals may be critical to this assessment. In the context of core formation,
Palme (2008) has emphasized
that metal-silicate partitioning does not adequately account for the
upper-mantle abundances of PGE, supporting a heterogeneous accretion model for
Earth evolution. Complete assessment of this model awaits more experiments on
the effect of high temperature on metal solubility, and agreement as to the
interpretation of metal heterogeneity in experimental run-products. Although
PGE ore deposits such as those in the Bushveld Complex in South Africa have
been exploited for many years, the debate continues regarding the primary
metal-concentration mechanisms. Whereas immiscible sulfide liquids have been
shown to be important, both experiments and detailed field observations
provide support for the role of hydrothermal solutions in dissolving and
transporting the PGE (Mungall and Naldrett
2008). The PGE isotope systems have provided a wealth of insights
regarding a range of geochemical processes. Uncertainties still exist,
however, concerning the origin of Os isotope anomalies in oceanic basalts
(Carlson et al. 2008), in terms
of core input or the production of isotopically distinct reservoirs by
low-pressure processes. As mentioned previously, the extraction and industrial
use of the PGE have accelerated their dispersal in the natural environment.
Rauch and Morrison (2008)
emphasize that the ecological impact of the PGE is not well known, and there
is clear need for further research in this area.
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FOR MORE INFORMATION
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The articles in this issue of Elements have been written with a
specific focus on topics relevant to Earth scientists, but the scope of
research related to the PGE is significantly broader. Useful texts that
provide synopses of the chemical properties of the PGE are by Cotton
(1997) and a compendium of
papers edited by Hartley
(1991). The reader is referred
to Platinum Metals Review, published by Johnson Matthey Ltd., for a
range of technical and historical information on the PGE. A detailed account
of the history of the PGE, including important discoveries and scientific
work, can be found in the book History of Platinum and Its Allied
Metals by Donald McDonald and Leslie B. Hunt
(1982). Enjoy!
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ACKNOWLEDGMENTS
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I am grateful to Jean-Pierre Lorand for providing images of samples from
the collection of the Museum National d'Histoire Naturelle (Paris), and to
Michel St. Martin (Natural Resources Canada Library) for the image of Sir
William Logan's Wollaston Medal. Jennifer Kelly (librarian for the North of
England Institute of Mining and Mechanical Engineers) confirmed the use of the
Pt wire modification to the Davy Safe-Lamp. Louis Cabri, Rick Carlson, and
Jean-Pierre Lorand provided critical readings of the manuscript. Thomas Clark
and Pierrette Tremblay are thanked for their skillful editing of all the
manuscripts in this issue.
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FOOTNOTES
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1 2007 PGE production statistics obtained from Johnson Matthey market data
tables
(www.platinum.matthey.com/publications/).
Total PGE values are calculated from metal prices averaged for the 12-month
period. 
2 The pre-Hispanic natives of this region had used material from the Choco
deposits to produce exquisite jewelry and objects of practical use (fishing
hooks, sewing needles, awls, tweezers). These people had remarkable
metallurgical ability, making use of liquid-phase sintering of Au-Pt mixtures
to produce malleable alloys which were fabricated into intricate objects
(Scott and Bray 1980).
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