Chemical treatment in the development of latent finger impressions on
paper, cardboard, and newly finished or unpainted wood may involve a
slightly more complicated technique than that in which powders are
utilized, but the results justify the additional effort.
It is very strongly recommended that powders not be applied to
articles of the above types. This recommendation is made for several
reasons. First, powders cannot be removed from paper and possibly may
interfere with some types of document examinations. In this
connection, they are likely to prevent restoration of the specimen to
a legible condition. Powders will not develop as many latent
impressions as chemicals on paper or cardboard. In some cases they
will obscure latent impressions subsequently developed chemically.
Neither scientific training nor complete knowledge of the chemical
processes involved is necessary for one to become proficient in the
use of chemical developers, two of which will be discussed more fully.
These two, iodine and silver nitrate, are the most commonly used,
inasmuch as they are relatively inexpensive, readily procurable,
effective, and easy to apply.
All specimens which are treated should be handled with tweezers or
When iodine crystals are subjected to a slight amount of heat they
vaporize rapidly, producing violet fumes. These fumes are absorbed by
fatty or oily matter with which they come in contact. If the specimen
treated bears latent impressions which contain oil or fat, the print
is developed or made visible by the absorption of the iodine fumes and
the ridges of the print appear yellowish-brown against the background.
Iodine prints are not permanent and begin to fade once the fuming is
stopped. It is necessary, therefore, for the operator to have a camera
ready to photograph the prints immediately.
Control of the fumes is achieved by using the crystals in an iodine
gun or fuming cabinet. The iodine gun may be assembled by the
individual examiner, by a druggist, or it may be purchased through a
fingerprint supply house.
Material for making the iodine gun, as well as iodine crystals, may be
procured from a chemical supply house or through a druggist. The gun
itself consists essentially of two parts. One tube (the end of the gun
through which the breath is blown) contains a drying agent such as
calcium chloride, to remove moisture from the breath. Without this,
the moisture from the breath and saliva would condense at the end of
the gun, drip onto the specimen and cause stains which might prove
indelible. The second tube contains a small amount of iodine crystals
which are vaporized by the heat of the breath, augmented by the warmth
of the hand cupped around the tube containing the iodine. This vapor
is blown onto the specimen (fig. 420). Glass wool serves to hold the
calcium chloride and iodine in place.
[Illustration: 420. Iodine fuming gun in use.]
Due to the amount of physical exertion involved, the gun is generally
limited to the examination of a few small specimens. Where a large
number of specimens are to be treated, the fuming cabinet, a
box-shaped wooden receptacle with a glass front and top permitting the
operator to control the amount of fumes in the cabinet and observe the
development of the latent impressions, is used (fig. 421). The fumes
are generated by placing a small alcohol burner under an evaporating
dish containing the iodine crystals. This is set in a hole cut in the
bottom of the cabinet. As soon as the fumes begin to appear in
sufficient amounts, the burner is removed. The specimens may be hung
in the cabinet by wooden clothes pins fastened to a removable stick
which is supported by wooden strips affixed near the top edges of the
cabinet. The top of the cabinet is removable to permit access.
Diagrams for the construction of the iodine gun or fuming cabinet will
be furnished on request to members of the law enforcement profession.
Many specimens bear small, greasy areas which, in addition to any
latent impressions of a greasy nature, will also appear
yellowish-brown after exposure to iodine fumes. All these stains will
eventually disappear if the specimen is placed in a current of air
from a fan or vent. All latent impressions on an object will not be
developed by the iodine process but only those containing fat or oil.
Due to this fact and the fact that iodine evaporates from the surface,
it is used prior to (it cannot be used afterward), and in conjunction
with, the silver nitrate process.
[Illustration: 421. Iodine fuming cabinet in use.]
No ill effects have been noted from contact with small amounts of
iodine vapor but prolonged or excessive contact will produce
irritation of the skin and respiratory passages. To prevent gradual
loss of the chemical through evaporation and the corrosion of
surrounding metal surfaces, iodine crystals should be kept in an
airtight container when not being used.
The development of latent impressions with silver nitrate is dependent
on the fact that the sodium chloride (the same substance as common
table salt) present in the perspiration which forms the ridges in most
latent impressions reacts with the silver nitrate solution to form
silver chloride. Silver chloride is white but is unstable on exposure
to light and breaks down into its components, silver and chlorine. The
ridges of the fingerprints developed in this manner appear
reddish-brown against the background. Immersion in the silver nitrate
solution will wash traces of fat and oil from the paper;
consequently, it is necessary to fume the specimen for latents of such
a nature prior to treatment with silver nitrate.
Once the requisite equipment is assembled, the steps in the process
Dip the specimen in the solution, blot and dry it, expose to light,
and photograph latents when contrast is good.
Chemically standardized solutions are not required for the successful
application of this process. It has been determined through long
practice that a 3-percent solution of silver nitrate is adequate for
the purpose, although concentrations up to 10 percent are sometimes
used. A solution of approximately 3 percent may be prepared by
dissolving 4 ounces of silver nitrate in 1 gallon of distilled water.
Smaller quantities of 3-percent solution are made by using the
components in the same proportion. For instance, one quart of water
will require 1 ounce of the crystals. For a 10-percent solution, use
13-1/3 ounces of crystals per gallon.
An alcohol solution may be preferred. This is prepared by mixing 4
ounces of silver nitrate crystals, 4 ounces of distilled water, and 1
gallon of grain alcohol, 190 proof. The alcoholic solution dries
faster, and when treating paper bearing writing in ink, it is less
likely to cause the ink to run. On the other hand, the alcoholic
solution is much more expensive and there is some loss by evaporation
while in use.
The solutions may be used several times before losing their strength
and when not in use should be kept in brown bottles in cupboards to
retard deterioration. If the strength of the solution is doubtful, the
operator should attempt to develop test latent impressions before
proceeding on evidence.
Silver nitrate crystals and distilled water in small amounts are
obtainable from druggists or in large amounts from chemical supply
houses. Dealers in distilled water are located in many communities.
Tap water should not be used in the preparation of the solution
because it generally contains chemicals which will partially
neutralize the silver nitrate.
It is suggested that the solution be placed in a glass or enamelware
tray approximately 18 by 12 by 5 inches for use, a size used in
photographic development. Treatment with this solution is called
silvering. The specimen is immersed in the solution so that the
surfaces are completely moistened, then taken out, placed between
blotters to remove the excess solution, and dried. The drying is
readily accomplished with an electric hair dryer. Blotters may be
dried and used several times before discarding. It is not necessary to
work in a dark room. Work in an illuminated room but not in direct
sunlight. Soaking the specimen in the solution does not aid
development and is actually undesirable as it requires a longer drying
time. The specimen should be reasonably dry before exposing to the
light, otherwise the latent prints may be developed while the paper
is still wet, thus necessitating drying in subdued light to prevent
Development of the latent impressions occurs rapidly when the specimen
is exposed to a blue or violet light source. A 1,000-watt blue or
daylight photographer's lamp, a mercury arc (most ultraviolet lamps
are of this type), or carbon arc is excellent for the purpose (fig.
422). If a weaker light is used, a stronger mixture of the solution
should be prepared. For instance, if a 300-watt bulb is used, the
10-percent solution would be preferable. Direct sunlight will cause
the latent impressions to appear very rapidly and if several specimens
are exposed at once it is not possible for a single operator to
properly control the development. Sunlight coming through a window
pane will serve for development. Where fingerprints containing sodium
chloride (normally exuded from the sweat pores in the ridges) have
been deposited, the silver chloride formed will darken against the
[Illustration: 422. Developing silver nitrate prints using 1,000-watt
As soon as the ridge detail of the prints is clearly visible, the
paper should be removed from the light. Continued exposure will darken
the paper and the contrast will be lost.
Paper so treated should be kept in darkness; that is, in a heavy
envelope or drawer until ready to photograph.
Immediate photographing, as in the case of iodine prints, is not
always essential, since the prints are permanent and become illegible
only through eventual clouding of the background. Prompt photographing
is recommended, however, as, in exceptional instances, silver-nitrate
prints have become illegible in a matter of hours. Darkening
ordinarily will occur slowly if the paper is preserved in absolute
darkness, and silver-nitrate prints so preserved more than 10 years
have been observed to be quite clear.
Items such as cardboard cartons, newspapers, road maps, large pieces
of wrapping paper, or smooth, unpainted wood surfaces, too large for
dipping, may be treated by brushing the solution over the surface with
a paint brush (fig. 423). Brushing does not damage or destroy latent
impressions on surfaces of this type. Cardboard boxes may be slit down
the edges and flattened out to permit easy placement under the light.
[Illustration: 423. Silver nitrate solution being applied with paint
Wet paper should be handled with extreme care to prevent tearing. In
treating very thin types of paper the solution is best applied with a
cotton swab or brush.
Photographs, Photostats, and blueprints of any value should not be
treated with silver nitrate, since the developed prints or stains
cannot be removed without destroying them.
In working with silver nitrate, wear rubber gloves or handle all
specimens with tweezers; avoid spilling it on clothing. It will cause
dark brown stains on clothing, skin, and fingernails. Such stains are
not easily removed. Areas of the skin subjected to prolonged contact
are deadened, will turn black and peel.
If removal of silver nitrate prints (called de-silvering) is
desired, this may be accomplished by placing the specimen in a
2-percent solution of mercuric nitrate in a tray similar to that used
for the silver nitrate.
To prepare a small amount of this solution, dissolve two-thirds of an
ounce of mercuric nitrate crystals in 1 quart of distilled water and
add one-third of a fluid ounce of nitric acid. Shake well. This
solution, too, may be used several times before losing its strength
and is not necessarily discarded after each use. It is not necessary
to keep it in a dark bottle.
The specimen bearing silver nitrate prints is immersed in this
solution until all traces of the prints disappear. It should then be
rinsed thoroughly in water to remove all mercuric nitrate. If this is
not done the paper deteriorates, becoming brittle and crumbly. A tray
of distilled water may be used for rinsing or a tray of ordinary tap
water changed several times during the rinsing. The specimen is then
laid out flat to dry.
Wrinkles, such as are left in paper after ordinary drying, may be
prevented by ironing with a moderately hot iron. An electric iron with
a temperature control is desirable. If kept too hot it will scorch or
wrinkle the paper somewhat. The bottom of the iron should be clean so
that unremovable smudges will not be left on the paper.
No ill effects have been noted from working in the 2-percent mercuric
nitrate solution with bare hands for very short periods, but it is a
caustic solution and it is suggested that the specimens be handled
with tweezers or that rubber gloves be worn if contact is prolonged.