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.
[Fig. 420]
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.
[Fig. 421]
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.