Metallurgical Examinations in Criminal Cases
| Author | Gustav W. Pirk |
| Published date | 01 October 1940 |
| Date | 01 October 1940 |
| DOI | http://doi.org/10.1177/0032258X4001300411 |
| Subject Matter | Article |
Metallurgical Examinations in
Criminal Cases*
By
GUSTAV
W. PIRK
Consulting Metallurgist, Scientific
Crime
Detection Laboratory, Bureau of
Police,
New
York
pIECES
of metal frequently play an important part in the
solution of a crime. At times a physical examination or a
chemical analysis will suffice,
but
very often a study of the fine
internal structure of the metal is also of considerable impor-
tance. Although in a given sample a chemical analysis will dis-
close the percentage of each element present, it tells nothing
about the way these elements are combined or arranged within
the specimen.
It
is the form, size, and arrangement of the
various microstructural constituents which are of particular
significance to the forensic metallurgist.
By way of illustration suppose that two pieces of ordinary
brass are presented for laboratory examination to determine
whether or not they are identical.
The
results of a physical
examination and a chemical analysis reveal that they are
similar in external form and appearance, and that both contain
70 per cent. copper and 30 per cent. zinc.
The
tendency at
this point would be to declare the two pieces of metal identical.
However, until a microscopic examination has been made the
question of identity cannot be considered completely answered.
Therefore, a sample is taken, polished, etched, and studied
under the microscope. Observation is made of such features as
the size and shape of the grains composing the sample, and
also of such abnormalities as non-uniformity in grain size,
impurities (which may be present as separate constituents),
porosity, and mechanical defects
(e.g.,
cracks of microscopic
•
Reprinted
from The Journal
of
Criminal
Law
and Criminology by courtesy of
the
author
and
of
Mr.
Fred
E.
Inbau,
Editor.
435
436
THE
POLICE
JOURNAL
magnitude). Perhaps the microscopic examination may show
that in one of the samples the grains are large; well-formed,
uniform as to size and distribution, and equi-axed (i.e., have
approximately the same dimensions in all directions); while
in the other sample the grains are found to be elongated into
fibres all running in the same direction, and are neither well-
formed nor equi-axed.
Obviously, then, these two samples of brass are different,
because the arrangement of the grains which form their micro-
structure differ.
If
they were from the same piece of metal
both would be similar in their microstructural details.
To
the
metallographer the well-formed grains which are uniform as to
size, geometrical outline, and distribution, indicate soft or
annealed brass; while on the other hand, the grains which are
elongated into fibres indicate that the metal has been cold-
worked, that is, it has had its shape altered by hammering,
rolling, or drawing at room
temperature-low
compared to
annealing temperatures
(850°
F. to
1,000°
F.), which would
cause the well-formed grains observed in the first sample.
(For arather simple example of how grain structure charac-
teristics may serve as the basis for a comparison between two
pieces of metal see Figure I.)
Microscopic examinations are especiallyvaluable when the
available pieces are small, for then the physical properties such
as tensile strength, ductility, etc., cannot be determined.
In
addition, the tensile test is destructive, as
it
is necessary to pull
the test piece apart in order to measure this property, whereas
in a microscopic examination the sample is not destroyed
but
is carefully prepared and studied in its normal and unaltered
condition. This statement, however, should not be interpreted
as meaning that the tensile or other similar destructive tests
have no place in criminal investigation; it merely is intended
to indicate the greater advantages of a microscopic examination,
especially when only a limited amount of material is available
or when other tests prove inadequate.
VALUE OF THE SPECTROGRAPH
Although this paper is primarily concerned with the
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