Chemical and Physical Development Methods for Latent and Patent
Friction Ridge Impressions
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Friction Ridge Impression -  The skin over most of the body is relatively smooth.  Skin that consists of tiny raised lines or 'friction ridges', however, are found on the digits, palms and soles.  They are called 'friction ridges’ because of their biological function to assist in our ability to grasp and hold onto objects.  They have been compared to fine lines found in corduroy, however unlike corduroy, ridges vary in length and width, branch off, end suddenly and, for the most part, flow in concert with each other to form distinct patterns.  Present on the tops of the friction ridges are very minute sweat pores that are constantly exuding perspiration.  This perspiration adheres to the surface of the ridges.  When the friction ridges come into contact with a suitable surface, this perspiration is transferred to that surface, leaving a recording, or print, of the friction ridges.  This impression of friction ridge skin is comprised mostly of sweat but may also contain a variety of contaminates such as oil and dirt. 

Latent Impression -   This type of friction ridge impression is not readily visible.  The term 'latent' is commonly applied to all chance or unintentional impressions that are of evidentiary value.

Patent Impression -   This type of friction ridge impression is visible e.g. an impression found in blood.

Plastic Impression -  This type of friction ridge impression is moulded into the surface of material such as putty or modeling clay forming a three dimensional impression.

'Chemical' Development of Latent Friction Ridge Impressions -   A visual chemical reaction between the reagent and one or more of the constituents of human perspiration, to yield to a coloured, luminescent, or radioactive product.  In this manner the ridge detail becomes visible and the prints can be photographed and further manipulated.  Advances in Fingerprint Technology, 2nd Edition.  Editied by Henry C.Lee and R.E. Gaensslen pg. 178

'Physical' Development of Latent Friction Ridge Impressions -   relies on the mechanical adherence of a substance (such as fingerprint powder particles  to the moisture and oily components of friction skin deposits.
Before a friction ridge identification specialist can enhance a print, he/she must first find it.  In most cases this is easier said than done.  Recognition of the areas where one is most likely to find latent prints is an extremely important step in the examination of latent print evidence.

Once the potential area which is most likely to yield fingerprint evidence is selected, a variety of techniques such as physical and chemical methods can be used to process the surface and to develop friction ridge impressions.  The selection of methods should be determined from 3 important factors 1) nature of the surface e.g. porous or non-porous, wet, dirty etc. 2) the apparent composition of the fingerprint residue or matrix (e.g.moisture contaminated with oil and dirt and; 3) condition of the friction ridge impression. Advances in Fingerprint Technology, Chapter 2

Important - The examination of any potential latent print evidence should include the following steps:

1)  Visual examination (followed by photography if appropriate).
2)  Fluorescence examination (followed by photography if appropriate).
3)  Selection and application of an appropriate enhancement method(s) - physical and/or chemical and/or digital enhancement followed by photography after each process.