A “Toolbox” for Forensic Engineers

366 Forensic Materials Engineering: Case Studies
Figure 12.1 Edge of a counterfeit coin above a legally minted coin, showing
irregular milling marks and poorly formed lettering.
examines such details in their loose change? The face patterns are excellent
although many do not match known patterns and dates. Metals are heavy
compared with paper so the more manufacturing processes the forger has to
use to make good copies, the less the “profit.” The tendency has thus been
to cast the coin by an investment casting technique, using the “lost wax”
process in which many wax patterns copying the genuine coin are set in a
refractory mold and cast at the same time from the correct alloy. The tiny
area on each coin where the pattern was joined with others onto the “tree”
inside the mold is easily removed by polishing to match the rest of the rim.
However, some very good counterfeit “silver” (actually cupro-nickel
alloy) coins were produced by casting when decimal coins were first introduced
in the U.K. Coins were physically larger than they are now and the
value of currency and the cost of living were such that there existed a stronger
motive for counterfeiting the 50p coin; the pound at that time was still a
paper note. The first 10p coin weighed 11 g and the 50p coin weighed just
over 13 g. It did not require any great proficiency in mathematics to work
out that, with some means of melting and casting, £1–30 worth of 10p coins
could be converted to £5–50 worth of 50p pieces. Molds containing upward
of 30 or so impressions of 50p pieces were mounted in a small centrifugal
casting machine, the 10p coins melted and then forced into the investment.
It was a job that could be done in a few minutes when the boss wasn’t around.
There was no record of any purchase of the cupro-nickel alloy. When solidified
the individual castings were broken off the sprue and the tiny area where
they had been connected was polished. Such castings of course are not quite