Examination of Firearms Review: 2007 to 2010 - Interpol

obliterated marks on medium carbon steel. The 2009 study have revealed that
Fry’s reagent, comprising of cupric chloride (CuCl2), hydrochloric acid (HCl) and
water, provided the necessary contrast and was concluded to be the most
sensitive of the tested reagents. The same reagent was recommended by earlier
workers for revealing strain lines in steel surfaces. Earlier, another reagent
containing copper sulphate (CuSO4), water, concentrated ammonium hydroxide
(NH3OH) and concentrated HCl was proved to be more sensitive for restoring
erased marks on low carbon steel (128).
Great minds probably think alike, since similar results, regarding the effectiveness
of the Fry’s reagent, were also reported by Wightman and Matthew (130). These
authors have also developed an etching paste, for use on steel surfaces (131).
Their proposed paste is based on a mixture of alumina powder and the Fry’s
reagent. The paste proved to be as effective as liquid in most cases, and often
gave even better results. The paste is thixotropic, making it much easier to use,
particularly with irregular shapes or on site. Coverage is good as vibration allows
the paste to flow and give an even cover. Apart from cases where erasure has
occurred by metal burrs filling the indentation, the paste gave as good a recovery
as the liquid etch, and often it appeared to give better recovery. According to these
authors, the ease of using the paste gives it distinct advantages over the liquid
reagent. It seems, however, to this Reviewer (NL) that an apparent drawback of
such paste is that it is not translucent, so the restored marks are not visible during
the process, unlike when using the liquid reagent.
Kamila and Colleagues (132) demonstrated the application of visual and
microscopic, non-chemical, methods for the successful restoration of an
obliterated serial number on a .38 calibre revolver submitted for examination. The
conclusion rightly drowns by these authors is that visual and microscopic
examinations, in tandem with other methods or independently, should always be
applied in such cases.
5.2 Aluminium alloy surfaces
A recent work by Bong and Kuppuswamy (133) assessed the etching technique
for the restoration of obliterated engraved marks on high-strength aluminium alloy
(AA7010) surfaces. The aluminium surfaces were engraved mechanically with
identification marks before the marks were erased by removal of the metal to
different levels up to and below the depth of engraving. Five different
metallographic reagents were then tested on the obliterated surfaces by etching.
According to these authors, the most effective methods for the restoration of the
obliterated marks were: (1) immersion in 10% aqueous phosphoric acid, and (2)
alternate swabbing of 60% hydrochloric acid (HCl) and 40% sodium hydroxide
(NaOH). Both procedures could also show the marks obliterated by overengraving
and centre-punching. Notably, alternate swabbing of HCl and NaOH
proved to be the common reagent for restoration on pure aluminium surfaces as
well as on its alloys, providing support for the findings of previous studies. These
findings are relevant because of the increasing use of high-strength aluminium
alloys in car and firearm manufacturing.
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