Written by Colin R. Gagg
Evidence, in any criminal case,
is usually sent for analysis by government-recognized forensic science laboratories.
These laboratories are usually staffed by scientists and various specialists
(such as handwriting experts)
who, although generally quite
experienced and skilled in their respective fields, seldom have
experience in manufacturing and general engineering. Occasionally, investigating police officers may instruct independent experts in
relevant engineering fields.
However, it is more common for the independent
expert to be appointed (given instruction in the matter) in order to challenge or expand upon the evidence given to the court
by scientists. In addition, insurance companies
may also have an interest in the outcome
of a case. So, they too may wish to
have material evidence examined by an independent expert and, if necessary, to challenge the interpretation given to the court.
The criminal cases that follow are selected from the authors’ experience in this field
and, not surprisingly, have a strong metallurgical bias.
A Mercedes 300D had allegedly hit a white people
carrier when losing control on a greasy road surface (Figure 1A). The third-party vehicle had suffered
only minor damage, whereas the Mercedes had the appearance of having collided
with a solid upstanding such as a bollard or fire hydrant.
The vehicle had been recovered and transported to a storage
facility where a local
independent engineer inspected it. His report concluded that the extent of
damage sustained had made repair of the vehicle an uneconomic proposition—the vehicle should be written off, with
the owner being compensated for its full market
However, when inspected by an assessor, the general interior
condition of the vehicle—steering wheel shiny,
seats worn, peddle rubbers worn, seat belts frayed,
leather stitching coming
undone, etc.—along with only 46,000 miles recorded on its odometer,
aroused his suspicion. An examination
by a forensic engineer then followed, where regular vertical score marks at a 30-cm pitch (Figure
1B), and yellow paint (no sign of white paint) on damaged
areas (Figure 1C), was found.
a JCB (back-hoe) was noticed
at the rear of the compound. Measurement of its bucket tooth
pitch (Figure 1D) was found to be 30
cm, and furthermore, the JCB was yellow! It
transpired that both parties were working with the recovery storage
company and deliberately inflicting damage to old vehicles in order to
fraudulently recover an inflated
insurance value. Furthermore, there were at least four other old vehicles in the facility,
perhaps waiting their turn
to “sustain damage”.
Nonferrous metals are of considerable value, and many small enterprises make a living by collecting scrapped
items at source
and selling them to scrap merchants who sort and re-grade
them before sending them to refineries. At the very bottom of this pyramid are individuals who ask no questions
about the source of their scrap and who make their living by selling it to the highest
bidder. This case concerns one such
individual who made a serious
An electroplating firm ordered 9 tons of copper cathode from a copper
refinery (Figure 2A). This is a pure
form of copper that is produced only
by electrolysis; the cathodes are somewhat irregular, 5 to 10 mm thick, with a smooth surface on one side and
numerous tiny nodules standing proud on the other,
as illustrated in Figure 2B. This
sample is a piece of cathode of similar size to those stolen and is much larger than the guillotined pieces that
it was to have been cut into for the electroplating process.
It is included here to
show the characteristic nodules
on the surface as well as along
the edge at the
right-hand side. A freshly produced cathode is a bright salmon pink color when it leaves the refinery but oxidizes to a dull brown after a few weeks.
For use as anode in barrel electroplating, the cathode has
to be reduced to small pieces about 25 mm2, so the electroplating firm needed to send its metal to another firm that had a powerful
guillotine. The electroplating firm arranged to have this done on a Monday but, because the consignment arrived late on Friday
afternoon, the truck was driven inside the plating firm’s yard and the gates locked. It was not visible from the outside. On Monday morning when the gates were opened up the truck was gone,
and with it 9 tons of copper worth more than £42,500 ($56,000) in terms of today’s money.
The nonferrous metal trade has a system of notifying all
dealers and metal merchants of a
theft and the nature of the material stolen. Some four to five days after the
theft, a small van drove to a scrap dealer 250 miles away from where the truck was taken. The driver offered to sell
a quarter ton of scrap copper, saying
he had more if the price was good enough. The scrap merchant realized that in
the load of mixed copper—a few old hot water cylinders, pipes, wire and so on—were
pieces of cathode, so he purchased that load and offered to buy all that the driver could deliver. He also
notified the police. Within an hour the driver was back with another
quarter ton, most of
which was cathode. He was arrested, and the police accompanied him back to a
small yard where they found the stolen truck containing just over 8 tons of
The man arrested claimed that he was just a small-scale
scrap dealer, who had bought all the metal in small lots from local housing
estates and small industrial units. It hardly needed a metallurgist to identify
it as ex-refinery cathode, which is totally different from swarf or clippings
arising from manufacturing processes, or discarded items from plumbing systems
and electrical wiring. The thief was obviously unaware that the only uses for
cathode copper are in electroplating and for re-melting and alloying with other metals.
In no way could the metal in his possession have been collected from housing estates and light
industry. It was particularly
incriminating that the total amount was 9 tons and was just starting to develop the brown oxide coating. It could only have emanated from the one refinery still operating in the UK, and its color was
consistent with it having been produced some three to four weeks before it was
recovered. Further inquiries revealed that the man arrested had been living
with his family in a caravan not far from the refinery at the time of the
theft. The suspect was charged with theft and subsequently served a custodial
A woman abruptly left her group of friends in a bar, allegedly because she was being pestered by a man. She walked
home and locked
her door, securing the chain,
but the following day she was found raped and brutally murdered. The chain was hanging
broken from the door handle.
The man who had been pestering her was known to her
friends and lived in the same district. He was questioned by police and claimed
that he had walked home soon after the woman left and had watched a late-night
movie on television.
The police searched his premises and found a small crowbar
under sacking at the side of his house with four links of brass chain twisted tightly round the hooked end, shown close up in Figure
3A. Figure 3B shows part of the broken
chain that the woman had used to secure her door alongside the four links twisted round the end of the crowbar.
Forensic examination revealed that the chain was composed of
welded links made from brass wire. The
diameter of the wire and the pitch of un-deformed links away from the
fractures of both exhibits were practically identical. The chemical
compositions of one link from the
chain on the door and one from the crowbar were not identical
but were both within the specified range for 70/30 copper–zinc alloy.
The man was arrested and charged with murder. DNA
tests later confirmed that he had raped the woman. At trial, the defense
sought to claim that the scientific tests had not established beyond all doubt that the links on the crowbar were indeed from the same length of chain
used to secure the door. At this point an
independent engineer was called. He found that the links had been electrically welded on the
same type of machine, but the damning
evidence was that the torsional fracture surfaces where the wires had been twisted to destruction were perfect matches
to the crowbar and the chain still hanging from the door.
A number of public houses in a large city had experienced a
spate of thefts of aluminum beer barrels. Whole batches of empty barrels placed
in their yards awaiting collection by the brewery had disappeared in the early
hours of the morning. In a suburb of the same city, a large warehouse and yard
were occupied by small firms in the metal recycling
trade, among which were
vehicle dismantlers, one of whom had a small metal melting unit used for
separating assemblies that contained aluminum
alloy parts. As a vehicle
was taken apart, components containing alloy were placed onto a sloping hearth under oil burners so that the aluminum alloy melted and ran down into a
bath while the iron and steel were left on the hearth to be raked out from time
to time. The molten alloy was cast into ingots and sold to a metal refiner.
The police suspected this warehouse
yard might have been the destination of the stolen beer barrels, so they kept
watch in a number of ways, including helicopter surveillance, but were never
able to observe any barrels on vehicles entering the site. (It was later learned that the reason for this was that the barrels were carried in
closed vans and the sheet-metal gates of the yard were always kept locked “to
prevent entry by nosey parkers and casual passers-by”.)
Although most of the firms on the site were car dismantlers, one of them ran a one-man business
stripping old electrical cable to recover lead and copper. He owned the lease of the site and lived a prosperous
lifestyle, despite the small throughput of his business. Early one morning, the
police raided the site and found the beer barrel melting
process in full swing,
with a closed van still partly filled with barrels backed up
to the door and five men, including the cable stripper, apparently engaged in operating the furnace and casting 50-kg ingots.
All five were arrested,
ordered to strip and change their
boots, and the burners were shut down
by the cable stripper. However, the
cable stripper claimed he had nothing to do with the melting
process but had merely attended to open the yard and carry on with some cable stripping in a
different part of the building while the others were working. He admitted that
he did this from time to time but had
no idea where the barrels came from and said this
job was usually done at night because the furnace was in use by the
other firms for car parts during
normal daytime hours.
His association with the beer barrel melting became clear
after forensic examination of metal particles found on the soles of the boots
and on the clothes he was wearing when arrested.
All the suspects’
clothes were brushed, and a number of tiny particles of metal were collected.
In addition, there were numerous particles in the heels and soles of their boots,
such as illustrated in Figure 4. Examination under the SEM revealed
that all were rounded particles
characteristic of splashes and droplets such as those emitted by molten metal running into open molds. Every one of the 20
samples taken from the boots and clothing of all
five men were found to be aluminum alloy containing low percentages of magnesium and silicon.
This composition spread would include the alloy used for beer barrels, but not
the ones commonly used for automobile castings, which usually contain
greater amounts of silicon. Most significantly, no particle recovered from the cable
stripper’s clothing or boots was copper or lead—but, even if there had been any
from this source, they would have
taken the form of mechanical slivers or clippings, not solidified droplets.
The clothing exhibited numerous holes formed by hot
particles landing on them, particularly below the trouser knees. There were two
holes joined together in the side pocket
of a nylon jacket worn by the cable stripper.
These had been formed by one, or possibly
two, splashes of hot metal striking the
fabric and melting through the nylon. The lower
leg of a pair of corduroy trousers had a series of holes, formed where
small, hot particles had landed. Although most of the above observations are
essentially scientific, the engineering dimension becomes
important in identifying the particles and clothing
damage as typical of that expected
when workers are standing close to foundry operations. Few scientists have ever
observed the splashing that occurs and the distance droplets may carry when a stream of molten metal poured
from a furnace first strikes a launder and then runs into an open ingot mold, or the fine spray of
droplets that are thrown up as it solidifies when the metal has picked up
hydrogen due to being melted under reducing
conditions. In this instance, the
damage to the clothing worn by the cable stripper clearly established that he
must have been standing very close to the stream of molten metal as the ingots
were being cast.
Colin R. Gagg, M.Sc., holds an honors degree in
engineering technology and a master’s degree in management and
technology of manufacturing. He is a chartered engineer and professional
member of the Institute of Mechanical Engineering. His practical
experience includes two years at the Structures Laboratories of Imperial
College of Science, Technology and Medicine and four years at the
Engineering Department of the University of Toronto. He currently holds
the post of research projects officer at The Open University and, for
the past 10 years, he has been a member of the Forensic Engineering and