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Chemical Development Of Latent Impressions

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. Firs
, 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

are these:

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

bulb reflector.]

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.