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John C. Keefe
Alion Science and Technology
Rome, NY
A Brief Introduction to Precious Metals
INTRODUCTION
A precious metal (PM) can be defined as a rare metallic
chemical element of high economic value.[1] Precious
metals, as a group, have a set of physical and chemical
properties that are unrivaled by many other materials. If
the availability of these materials was greater (in both
quantitative and economic terms), there would be far more
application overall. Due to the cost and availability,
however, these materials are limited to applications where
only small amounts are used, such as spark plug tips and
electrical contact plating. Under certain circumstances,
precious metals are used in large quantities for applications
where there is no feasible substitute. In applications and
industries that use a large amount of PM, the capital cost
can be great (tens of millions of dollars), and there is also a
substantial cost associated with managing and securing the
metal assets in a company’s inventory.
The Eight Precious Metals
There are eight precious metals: gold (Au), silver (Ag),
platinum (Pt), iridium (Ir), palladium (Pd), rhodium (Rh),
ruthenium (Ru), and osmium (Os). They are grouped in a
rectangle on the periodic table existing in two periods and
four groups. A subset of this group is called the Platinum
Group Metals (PGM), which includes all but two of the
PMs (Au and Ag).
Mining the Raw Materials
Typically the PGMs are found combined together in rich ore
and are then chemically processed to extract the individual
elements in the group. Also notable are the elements above
the precious metal group on the periodic table: iron (Fe),
cobalt (Co), nickel (Ni) and copper (Cu). All of these metals
have relationships with the eight precious metals, in that
they are found in the primary ore, or they are used as
alloying elements to impart improved properties in many
of the precious metal formulations. The short supply (there
are only a few major mining locations), economic value
and costly mining and extraction methods have raised the
cost of these metals to the high prices that exist today.
Gold and silver have been in use for a very long time,
but the other precious metals have a much shorter history.
The discovery of PGMs occurred significantly later since
they are not typically available in the pure metal nugget
form in which gold and silver are found. Mining locations
for the PGMs are very limited. The key areas of PGM
production are the US (Stillwater, Montana), Canada (most
commonly the Sudbury, Ontario area, as a byproduct of
nickel mining), Russia (Norilsk region, which is first in Pd
production), and the Zimbabwe region in South Africa (first
in Pt and first in PGM general production). In 2004, South
Africa produced a total of five million troy ounces (to)* of
Pt (70% of the world’s output) and eight million troy
ounces of PGMs (50% of the world’s output).[2] Today the
processing of PGMs requires about a ton of rich ore to
produce approximately one troy ounce of PGMs at best.
Some mines only produce PGMs on the level of 5 to 25
grams per ton of processed ore. Whereas PGMs are mined in
fewer locations, the occurrence of gold and silver is much
more common and they are found in larger quantities.
BASIC PROPERTIES OF PRECIOUS METALS
The properties of PMs are typically different than conventional metals in two primary areas: melting point (MP) and
density. The melting point for steel (low alloy iron) is in the
range of 2800°F with a density in the range of 7.8 g/cm3;
compare this to the group to see the differences on the
physical property side. These features, coupled with their
resistance to chemical attack, set PMs apart from most other
Table 1. Basic Properties of Precious Metals.
Name (Symbol)
Atomic
Crystal
Melting
Number
Structure
Point, °F
Density,
g/cm3
Ruthenium (Ru)
Rhodium (Rh)
Palladium (Pd)
Silver (Ag)
Osmium (Os)
Iridium (Ir)
Platinum (Pt)
Gold (Au)
12.45
12.41
12.02
10.49
22.61
22.65
21.45
19.32
44
45
46
47
76
77
78
79
HCP†
FCC‡
FCC
FCC
HCP
FCC
FCC
FCC
4190
3560
2829
1764
5522
4429
3216
1947
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Table 2. Mechanical Properties of Precious Metals [3,4].
Name
Tensile Strength
Elongation in
(MPa)
50 mm (%)
Hardness
(HV)
Young’s Modulus (Static)
at 70°F (GPa)
Poisson’s Ratio
Ruthenium
Rhodium
Palladium
Silver
Osmium
Iridium
Platinum
Gold
220-270
13-100
105-110
25
31-350
600-700
90-95
55-60
414
319
115
74
558
517
171
77
0.25-0.31
0.26
0.39
0.37
0.25-0.28
0.26
0.39
0.42
496
1379-1586
324-414
290
2070-2480
207-241
207-221
3
2
1.5-2.5
3-5
15-18
1-3
4
Table 3. Electrical Resistivity and Thermal Conductivity of PMs
Compared to Copper and Aluminum [5].
Metal
Electrical Resistivity
Thermal Conductivity
Silver
Copper
Gold
Aluminum
Rhodium
Iridium
Ruthenium
Osmium
Palladium
Platinum
(10-6 ohm-cm)
(W/m-K)
1.55
1.70
2.20
2.70
4.30
4.70
7.20
8.12
9.93
10.6
419.0
385.0
301.0
210.0
151.0
147.0
116.0
91.67
71.20
69.10
materials on the periodic table.[3] Table 1 shows some of
the basic properties of the eight precious metals. Several
mechanical properties of the PMs are provided
in Table 2.
Electrical and Thermal Conductivity
Many of the PMs have excellent electrical and thermal
conductivity properties, as shown in Table 3. Silver has the
distinction of having the highest room temperature
conductivity of the PMs, as well as the highest of all
metals. It should be no surprise that copper is the metal
more predominantly used for electrical wire instead of
Ag because of the cost difference. The main drawback to
Cu is that it readily forms an oxide film, which can lower
the electrical properties unless the oxide film is cleaned
from the electrical contacts. This problem is commonly
resolved by plating the contact area of Cu electrical
devices with gold.
Nobility
The precious metals are often called the “Noble Metals”
because they are resistant to most types of environmental
and chemical attack. One of the few chemical solutions to
attack the precious metals (with the exception of iridium)
is aqua regia (Latin for “royal water”). Aqua regia, a
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mixture of one part nitric acid (HNO3) with three parts
of hydrochloric acid (HCl), earned its name because it
dissolves Au and Pt, the royal or noble metals. This
solution is used as an etching solution for metallurgical
analysis of many metals.
APPLICATIONS
As with most materials, the range of applications for PMs
is diverse. However, PM materials are also used in
applications where other materials cannot be used. The
following sections provide an overview of typical PM
applications.
Corrosion Protection
The use of PMs to resist corrosion is long-standing due to
their inherent nobility. Additionally, these metals and their
alloys are used in cathodic protection systems to protect
large systems from the effects of corrosion.
Catalysts
The use of Pt, Pd, Rh and their various alloys as catalysts
in large and small chemical reactors, such as car exhausts,
is widespread. The catalytic surface can be applied by
“washing on” a rich solution of material onto a ceramic
substrate. The surface can also be a robust construction of
woven wire in order to provide a large-scale surface for
chemical production. These applications account for the
primary usage of PMs. Platinum-based catalysts have been
used for nitric acid production for more than 100 years.
High Temperature Applications
Combining high MP temperatures with the additional
feature of low reactivity at elevated temperatures is a key
characteristic of PMs for many applications. Steel melts at
approximately 2800°F, while Pt has a MP of 3200°F.
Vessels made from Pt, Pt-Rh, and Ir are used in the making
of fiberglass and silicon ingots, as well as for the melting
of other high MP, reactive media. One clever application
uses Pt and zirconium oxide to form a powdered metal
that is a highly creep resistant material even when heated
close to its melting point. Zirconia Grain Stabilized (ZGS)
platinum and Pt-Rh alloys have been used in the glass
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industry for many years. The addition of zirconium and its
subsequent oxidation during metal spraying create a grain
structure that limits grain growth and increases the high
temperature creep strength.
Thermocouple Devices
Platinum and platinum-rhodium wire pairs for thermocouples are the most effective at measuring temperature.
Platinum thermocouples exhibit the widest range of
temperature measurement, accuracy and linearity, which
is required for critical applications. Currently, the producers
of wire are able to make wire diameters small enough that
the cost of PM material is kept to a minimum.
High Temperature Heating Coils
A heating coil can obviously only go as high as the melting
point of the material used to construct the device. Additionally, in many operations repeated oxidation cycles can reduce
the life of the heating coil. The use of PGM alloys satisfies
both the issue of high service temperature and the problem
of long-term oxidation attack.
Spark Erosion Resistance Applications
In the new generation of cars and trucks, replacing spark
plugs could be a thing of the past. The development
and application of Pt alloys, Ir alloys, and pure Ir (some
combinations of PMs are patented) make plugs that
last for more than 100,000 miles. To accomplish this, some
manufacturers use ball bearing fabrication equipment to
make small Pt alloy spheres that are then resistance welded
onto the plug to form the electrode pair. For the more
critical applications on aircraft, short pieces of Ir rod stock
material are centerlessly ground§ to an exacting size and
form and then installed in the spark plug. Additionally,
electrical contacts with an extended operational life
capability have been made from Pt and Pd strip stock
for various devices by high speed stamping of small
crowned circular blanks.
Fuel Cell Applications
Fuel cells are similar to conventional batteries in that they
are silent with no moving parts and generate electrical
power by an electrochemical reaction within the cell. The
best part about a fuel cell, unlike a battery, is that it
requires no recharging and it will run continuously as long
as the fuel supply lasts. The electrical output from the
fuel cell is made by combining hydrogen (the fuel) and
oxygen (from air) over a catalyst such as platinum.
Biocompatibility
Not all materials have the ability to be implanted within or
used in contact with the human body without causing an
adverse reaction or poisoning. Those that can be are termed
biocompatible. The medical devices produced from PMs
include: stents, marker bands for angioplasty devices,
pacemaker wire, heart muscle screw fixations, endoscopy
tips and special surgical tools. The material used for such
applications is mainly Pt (or alloys of Pt), and in dental
applications the use of Au and Pd is common.
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Radio-opacity
X-rays do not easily pass through Pt, Au and Ir because
of their atomic absorption coefficients, as well as their high
densities, and thus these materials typically show up as a
white area on film or scanning device. This property,
referred to as radio-opacity, coupled with their biocompatibility characteristics allows for the location of these
materials to be determined when used within the human
body. The identified location of these materials is called the
“marker band” because it marks the location to allow the
surgeon to correctly position the angioplasty device within
the body during surgical procedures.
Pharmaceutical Use
Platinum-based drugs, such as cisplatin, have been in use
for 30 years to treat cancer. The treatment of testicular
cancer with cisplatin has been widely accepted as a standard
course of treatment. A new drug, stataplatin, is under the
last stages of FDA approval for treatment of a form of
prostate cancer. This new drug will offer an alternative to
those patients that did not respond well to chemotherapy.
Gold has also been used for the treatment of prostate
cancer, whereby small gold “seeds” are irradiated
and injected into the cancer site to kill the cancer cells
by the slow release of radiation.
Labware, Equipment & Related Devices
Platinum and gold have an excellent resistance to attack
from many substances, and as such they are used as
crucibles, electrodes, inoculating loops, ignition boats and
many other forms of labware. Since these materials are
noble, the testing method is not skewed by contamination
from the test equipment. Basic forms of material (wire, tube,
sheet, and strip) can be fabricated into countless products as
required for industrial use. Joaquim Bishop, a former lab
assistant at the University of Pennsylvania, established the
first Pt works in the US in 1842 for the purpose of refining
Pt and the production of Pt labware. His early success in
this work earned him a prestigious Franklin Institute silver
medal in 1845 for “skill and ingenuity in the manufacture of
Pt scientific instruments.” Many of the complex Pt alloy
fabrications have been used in the production of glass
melting “bushings” for the fiberglass industry.[6]
Photographic Applications
Photography’s impact on society has been vastly important
over the last 150-plus years since its discovery. During the
formative period of discovery, precious metals played a key
role in making the art a reality. At one time, the Eastman
Kodak Company was the single largest user of silver in the
world. Many films and photo papers used silver compounds
as the light-sensitive emulsion. Platinum and palladium
compounds were used to make black and white printing
paper, which was and still is considered by many to be the
best paper for reproduction of the complex tonality on black
and white negatives. These prints are the most archived of
any produced because of their resistance to environmental
attack. Famed photographer Ansel Adams used gold chloride
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toner to add permanence to his work and advocated this to
all photographers who wanted to extend their work’s life.
Coinage, Collector’s Items and Jewelry
Precious metals have been used throughout history
for currency and jewelry, as well as other objects that have
become collector’s items. Our ability to examine past
cultures is due, in part, to the nobleness of these materials
since they are able to survive hundreds or even thousands
of years of concealment and burial and still be viable as
a historical record. Jewelry accounts for the second largest
demand of PMs, with China, Japan and the US accounting
for 85% of the total demand. The history of coinage is
filled with the use of metals, especially gold and silver.
FABRICATION TECHNIQUES FOR PRECIOUS METALS
As with most pure metals, the properties of PMs are
improved by alloy additions. This is true of the precious
metals where the alloying elements are other PMs or similar
metals. This is important especially in machining, where
pure metals cannot be cut as precisely as metals that
are slightly alloyed due to their ductility and less than
clean shearing nature.
Table 4. List of Fabrication Methods Used for PMs.
Machining
• All methods, including:
• Electrical discharge machining (EDM)
• Wire EDM (especially for Ir crucible making)
Grinding
• All methods
Cold working • Rolling
• Drawing (wire, rod and tube)
• Spinning
• Stamping
• Forming
• All methods, including:
Welding
• Metal inert gas (MIG)
• Tungsten inert gas (TIG)
• Electron beam
• Laser
Hot working
• Forging
• Drawing
• Rolling
• Swagging
Casting methods • Gas fired (low MP materials)
• Induction
• Vacuum induction
• Electron beam (EB) vacuum melting
• Argon carbon arc vacuum melting
QUICK FACTS ABOUT PRECIOUS METALS [1, 3, 7-8]
Gold
As a material, gold has the longest and most storied history of all the precious metals. The chemical symbol,
Au, is derived from the Latin word aurum, meaning “gold”. It is the most malleable of metals and therefore can
be worked with simple tools to form complex shapes. Gold’s low MP has made it one of the first metals that
could be readily cast. The visual allure of gold and the economic value acquired with its possession have driven
the mining of this PM throughout history. It is used for numerous commercial and industrial purposes. Its excellent
corrosion resistance and thermal and electrical properties have made it a top design choice for many devices. The ability
to plate gold in very small thickness still allows for more extensive application of the material. The cost of gold is
constantly varying; currently it is approximately $668/to.
Silver
Silver has the best room temperature electrical and thermal conductivity of all metals. The largest silver ore
deposits in the US were discovered in 1859 at the Comstock Lode in Virginia City, Nevada, and in northern
Idaho at the world-famous Sunshine Mine – the richest silver mine in American history, which has had more
than 350 million ounces of production over the past century. The principal sources of Ag are Cu, coppernickel, Au, lead and lead-zinc ores obtained from Canada, Mexico (historically Batopilas), Peru, Australia and the US.
Silver has found many applications primarily because of its lower melting point and ease of fabrication. From coinage and
tableware to its extensive use in the photography industry, silver is part of our lives each day. Silver is the most available
and least costly of the precious metals at $14/to.
Platinum
Platinum was discovered in South America first by Antonio de Ulloa in 1735 and again by Charles Wood in
1741. Platinum occurs naturally and is accompanied by small quantities of the other PGMs. When pure,
platinum is a beautiful silvery-white metal that is malleable. The metal is extensively used in jewelry, wire,
vessels for laboratory use, and in many valuable industrial products including thermocouples, medical devices
and anti-cancer drugs. It is also used for electrical contacts, corrosion-resistant devices, and in dentistry. Platinum-cobalt
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alloys have powerful magnetic properties. One such alloy, made of Pt-23.3 wt.% Co, offers a maximum magnetic field
strength almost twice that of Alnico V, a strong permanent magnet material. Platinum resistance wires are used in the
construction of high-temperature electric furnaces. It can be drawn into wire, then knit into fabric referred to as gauze,
and fabricated into a large catalytic surface. Platinum is an excellent catalyst, having long been used in the process for
producing sulfuric acid (H2SO4) and nitric acid (HNO3). There is much current interest in the use of Pt as a catalyst in
fuel cells and in anti-pollution devices for automobiles. The present cost for Pt is approximately $1200/to.
Iridium
Iridium is the rarest precious metal and the densest material known. It was discovered in 1803 in the insoluble
residue from platinum reduction with aqua regia. It earned its name from the highly colored iridium salts.
Iridium is more difficult to mechanically work than any other face-centered cubic (FCC) metal. This has
been attributed to a reduction in ductility caused by trace element impurities that are picked up during the
mining and refining process. These impurities cause a modification of the grain boundary behavior. Due to its high tensile
strength at elevated temperatures and high MP, it has been used for crucibles in crystal growing. Hot working, which is
mechanical working of a metal above its recrystallization temperature, is one of the few ways to reasonably work the
material. To achieve precise dimensional cutting, either grinding or wire EDM must be used. Iridium possesses quite
remarkable properties. It is the most resistant of all metals to corrosion; it is insoluble in mineral acids including aqua regia.
The earliest uses were for making durable, wear-resistant tips for fountain pen nibs and compass bearings. Resistance to
spark erosion has made this element popular for spark plug applications.
Palladium
Palladium was recognized as an element in 1803. Deposits are found in the former USSR, South and North
America, Ethiopia and Australia. It is a steel-white metal; it does not tarnish in air and it has the lowest density
and melting point of the PGMs. Palladium is attacked by nitric and sulfuric acids. At room temperature the
metal has the unique property of absorbing up to 900 times its own volume of hydrogen. Hydrogen readily
diffuses through heated Pd and development of this property provides a means of purifying the gas. Its alloys are used in
the jewelry trade. White gold is often produced by addition of Pd. Like Au, Pd can be beaten into leaf form as thin as
1/250,000 of an inch. The metal and its alloys are used in dentistry, watch making, surgical instruments, catalytic
converters and electrical contacts.
Rhodium
The name for rhodium comes from the Greek word, rhodon, meaning rose and was selected for the many
“rose colored” solutions that are distinctive of rhodium’s presence. It was discovered in the early 1800s
in a crude Pt ore obtained from South America. Rhodium occurs naturally with other PGMs in river sands
of the Urals and in North and South America. It has a high reflectance and is hard and durable. Sputtering
targets of rhodium are used to make the reflective surface for automobile mirrors and other optical instruments. As
a bulk metal, it is mostly used as an alloying agent to harden Pt and Pd. Such alloys are used as furnace windings,
thermocouple elements and to make bushings for glass fiber production. An addition of Rh increases both the operation
temperature and the mechanical properties of the material. It is a useful electrical contact material due to its low
electrical resistance and low and stable contact resistance, as well as its high resistance to corrosion. Rhodium is also
used for a range of catalyst applications and as alloys and coatings for jewelry. Rhodium has the current distinction
of being the most costly precious metal at over $6000/to.
Ruthenium
Ruthenium is found naturally occurring along with other PGM members. It was discovered in 1827 when
examining the residues left after dissolving crude platinum from the Ural mountains (Russia) in aqua
regia. Ruthenium is a hard, white metal mainly used as an alloying agent for platinum. An addition of 0.1%
ruthenium to titanium immensely improves the corrosion resistance. It is a versatile catalyst and can be
used to promote the splitting of hydrogen sulfide. Pure ruthenium is a very difficult material to work.
Osmium
Used almost exclusively as an alloying agent, osmium has the distinction of having the highest melting point
of the precious metals. Certain forms (tetroxides) are highly toxic. Osmium tetroxide is used in forensic science
as a stain for fingerprints, microscope samples and DNA materials. Osmium was discovered in 1803, and
it occurs in Pt-bearing river sands of the Urals, North and South America, South Africa and elsewhere.
The metal is lustrous, bluish-white, and extremely hard and brittle even at high temperatures. As a pure metal it has
relatively few industrial uses, but can be used to produce very hard alloys with other PGMs.
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The method for production of PMs into a desired form
is similar to other metals. Platinum is very tough on
tooling for turning, drilling and milling, even when carbide
tools are used. The use of polycrystalline diamond (PCD)
tools, however, can significantly extend tool life when
machining PMs. Typically welding of Pt in air is done with
oxy-hydrogen torches, not oxy-acetylene, in order to deliver
enough heat to the weld area to accomplish the task.
Precious metals have very high MPs, and therefore when
welding or performing any other process where the MP is a
factor, it is important to be able to focus the energy in a
local area to minimize the heat affected zone (HAZ).
The problem areas with PM fabrication come into play
when formability, work hardening or melting point have a
part in the manufacturing process. Iridium most notably
has problems with formability and even though it is a FCC
material, it does not behave as such. This could be
improved by hot working, but since Ir has a MP greater
than 4400°F, the range of hot work temperatures can
quickly compromise the equipment if the process is not
monitored carefully. Table 4 provides a list of fabrication
methods that can be used for PMs.
that deals with all things PGM. This is not a technical
review, but rather a readable publication of what is current
in the PM world.
Platinum Metal Review, www.platinummetals
review.com - This is a quarterly online journal that
provides the latest research information on PGM advances
in all technical areas from metal extraction methods to
specific applications. It is a highly technical publication
that contains application information as well as the exact
science of the product or process described.
The PGM Database, www.platinummetalsreview.com/
jmpgm/index.jsp - This is the definitive materials
properties website with very extensive information on
PGM properties in all of their alloy variations. It is a good
resource for the materials scientist and engineer. General
information is included here too, but it is amongst some
very technical data.
International Platinum Association, www.platinum
info.net - This is a nonprofit association of the leading
mining, production, and fabrication companies in the
global PGM industry.
FUN FACTS
The density of gold and platinum is almost twice that of
lead (Pb, density = 11.34 g/cm3), which has been typically
viewed as being a very dense, heavy material by the general
public. In most of the old western movies, and more
recently in the George Clooney movie, Three Kings (1999),
the “bad guys” are shown loading up stacks of gold bars.
This is a bit far-fetched since many of the bars at the sizes
shown would weigh approximately 80 to 90 pounds. A
saddlebag of these plus the rider would be too much for the
horse, or a human, to carry. Since most people don’t have
the opportunity to actually lift a gold bar, the myth is
perpetuated by Hollywood.
* In the PM world a different weight measurement system is used.
The standard unit is the troy ounce. The troy ounce (to) is different
than our common ounce, in that, the troy ounce has a mass of
31.1035 grams versus 28.350 grams for the standard (avoirdupois)
ounce (32.1507 troy ounces = 1 kilogram). Even when handling
large amounts of precious metal, the quantity is still expressed
in troy ounces.
† Hexagonal Close-Packed
‡ Face-Centered Cubic
§ Centerlessly grinding is a method of grinding where the workpiece,
which is supported by a workblade instead of a spindle, is placed
between a regulating wheel and a grinding wheel.
[1] “Precious Metal,” Wikipedia, http://en.wikipedia.org/
[2] Implats, www.implats.co.za
[3] The PGM Database (Platinum Metals Review), www.platinum
metalsreview.com/jmpgm/index.jsp
[4] A.R. Robertson, “Precious Metals,” Metals Handbook, Tenth
Edition, Volume 2: Properties and Selection: Nonferrous Alloys and
Special-Purpose Materials, ASM International, 1990, pp. 688-698.
[5] “The Online Materials Information Resource,” MatWeb,
http://www.matweb.com/
[6] Technical & Industrial Platinum, J. Bishop & Company Platinum
Works, 1931.
[7] “Periodic Table of Elements, Corrosion Source, www.corrosion
source.com/handbook/periodic/
[8] International Platinum Association, www.platinuminfo.net
RELATED WEBSITES
For more information on precious metals, check out the
following websites:
Johnson Matthey (JM), www.matthey.com - The main
website of one of only two integrated PM producers in
the world. JM mines, processes, sells, fabricates, engineers
and basically leads the efforts to apply and promote
precious metals.
Platinum Today, www.platinum.matthey.com - A
website by Johnson Matthey, this is an online magazine
NOTES AND REFERENCES
Mr. John C. Keefe is a Senior Engineer with Alion Science and Technology. He holds a BS in Industrial Engineering from Purdue
University and an MS in Industrial Engineering from Lehigh University. Previously he worked as the Manager of Manufacturing
Engineering at Johnson Matthey (Precious Metals Division) in West Chester, PA, where he supervised engineering and fabrication of
a wide range of products made from precious metals and their alloys. His career focus has been on manufacturing processes, machine
tool selection and engineering education. He has been a college instructor for more than 20 years.
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