velopment 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. 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
gloves.

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 m