Gunshot Residue Examinations
Clothing
submitted to the laboratory will be examined to determine if a
pattern of gunshot residues are present. There are basically three steps taken by the
firearms examiner to process an item for the presence of gunshot residue. 
The first step is to visually and
microscopically examine the evidence. The presence of any
gunshot residues found around the bullet hole as well as the
shape and appearance of the hole will be documented.
The next two steps involve chemically
processing the exhibit for gunshot residues.
The first chemical test
conducted is called the Modified Griess
Test. The Modified Griess Test is performed first on the
exhibit because it will not interfere with later tests for lead
residues.
The
Modified Griess Test is a test to detect the presence of nitrite
residues. As described earlier, nitrite residues are a
by-product of the combustion of smokeless gunpowder. When
a firearm is discharged nitrite particles are expelled from the
muzzle of a firearm and can be imbedded in or deposited on the
surface of a target. The Modified Griess Test is the
primary test used by firearms examiners to determine a
muzzle-to-garment distance.
The Modified Griess Test is performed by
first treating a piece of desensitized photographic paper with a chemical
mixture of sulfanilic acid in distilled water and alpha-naphthol in
methanol. Desensitized photographic paper is obtained by exposing
the paper to a hypo solution. The photographic paper will no
longer be light-sensitive but will be reactive to the presence of
nitrite residues.
The exhibit being processed is placed face down
against a piece of treated photo paper, with the bullet hole
centered on the paper.
The back of the exhibit being examined is then
steam ironed* with a dilute acetic acid solution in the iron
instead of water.
The acetic acid vapors will penetrate the
exhibit and a reaction takes place between any nitrite residues
on the exhibit and the chemicals contained in the photographic
paper. The resulting reaction will appear as orange specks
on the piece photographic paper.
A
close-up of the results can be seen below.
The last
test conducted on the exhibit is called the Sodium Rhodizonate Test. This chemical test
is designed to determine if lead residues are present
on the exhibit.
The Sodium
Rhodizonate Test
is performed by spraying the exhibit with a weak solution of a mixture of Sodium
Rhodizonate and distilled water. This solution has a dark
yellowish/orange color.
 
The exhibit is then sprayed with a buffer
solution which causes the background color to disappear.
The Sodium Rhodizionate reacts with any lead that may be present and
turns the lead a very bright pink. The pink color is only an indication
of the presence of lead residue and to confirm the presence of lead
residue the area can be treated with a diluted Hydrochloric Acid
solution.
If the pink turns to a blue then the presence of lead is confirmed.
 
The above images
show a car's headrest with a sooty deposit around a bullet entrance
hole. The right
image shows the headrest after processing with
Sodium Rhodizonate and a buffer solution.
Here's a
brief case history on the headrest above. A subject was holding a gun to the head of his estranged girlfriend who was
sitting in the driver's seat of her car. As police approached her
car the subject fired several shots through the windshield of the car at
the officers. He then turned to shoot his ex-girlfriend as a
police officer returns fire through the windshield. The subject
fires a shot off, missing the victim's head, shooting through the
headrest above. The victim now has opened the car's door and
started to roll out of the drivers seat. The suspect manages to
get off one more shot, striking the victim in the "lower back
side". The assailant is struck several times by shots fired
from the officer and later dies. The pattern of residue found on
the headrest certainly shows the intent by the suspect to kill his
ex-girlfriend. She later fully recovered from her injuries.
Her car had sustained fatal injuries from multiple gunshot
wounds.
The steps
taken to conduct the tests described above have been shortened
slightly. Only the main procedures have been described.
Additional quality control procedures are normally taken but have been
omitted to simplify the descriptions.
Once a pattern
of gunshot residues is found on the evidence it's time to test the submitted firearm
to determine at what distance it will deposit a similar pattern
of gunshot residues. Click the "Next" button below
to learn about Gunshot Residue Distance Standards.
* Steam Ironing of
the back of the garment is a variation of the test that I described in
a paper published in the Association of Firearm and Tool Mark
Examiners (AFTE) Journal in May of 1987. The test in its
original form called for soaking pieces of cheesecloth with dilute acetic
acid, placing the cheesecloth on the back of the questioned garment
and then ironing the back of the cheesecloth. Myself and others have used the "steam iron" method since that time with great
success.
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